/*
 * Copyright (C) 2015 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.android.internal.widget;

import android.annotation.DrawableRes;
import android.annotation.NonNull;
import android.content.Context;
import android.content.res.Resources;
import android.content.res.TypedArray;
import android.database.DataSetObserver;
import android.graphics.Canvas;
import android.graphics.Rect;
import android.graphics.drawable.Drawable;
import android.os.Bundle;
import android.os.Parcel;
import android.os.Parcelable;
import android.util.AttributeSet;
import android.util.Log;
import android.util.MathUtils;
import android.view.AbsSavedState;
import android.view.FocusFinder;
import android.view.Gravity;
import android.view.KeyEvent;
import android.view.MotionEvent;
import android.view.SoundEffectConstants;
import android.view.VelocityTracker;
import android.view.View;
import android.view.ViewConfiguration;
import android.view.ViewGroup;
import android.view.ViewParent;
import android.view.accessibility.AccessibilityEvent;
import android.view.accessibility.AccessibilityNodeInfo;
import android.view.accessibility.AccessibilityNodeInfo.AccessibilityAction;
import android.view.animation.Interpolator;
import android.widget.EdgeEffect;
import android.widget.Scroller;

import com.android.internal.R;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;

Framework copy of the support-v4 ViewPager class.
/**
 * Framework copy of the support-v4 ViewPager class.
 */
public class ViewPager extends ViewGroup {
    private static final String TAG = "ViewPager";
    private static final boolean DEBUG = false;

    private static final int MAX_SCROLL_X = 2 << 23;
    private static final boolean USE_CACHE = false;

    private static final int DEFAULT_OFFSCREEN_PAGES = 1;
    private static final int MAX_SETTLE_DURATION = 600; // ms
    private static final int MIN_DISTANCE_FOR_FLING = 25; // dips

    private static final int DEFAULT_GUTTER_SIZE = 16; // dips

    private static final int MIN_FLING_VELOCITY = 400; // dips

    private static final int[] LAYOUT_ATTRS = new int[] {
        com.android.internal.R.attr.layout_gravity
    };

    
Used to track what the expected number of items in the adapter should be.
If the app changes this when we don't expect it, we'll throw a big obnoxious exception.
/**
     * Used to track what the expected number of items in the adapter should be.
     * If the app changes this when we don't expect it, we'll throw a big obnoxious exception.
     */
    private int mExpectedAdapterCount;

    static class ItemInfo {
        Object object;
        boolean scrolling;
        float widthFactor;

        
Logical position of the item within the pager adapter. /** Logical position of the item within the pager adapter. */
        int position;

        
Offset between the starting edges of the item and its container. /** Offset between the starting edges of the item and its container. */
        float offset;
    }

    private static final Comparator<ItemInfo> COMPARATOR = new Comparator<ItemInfo>(){
        @Override
        public int compare(ItemInfo lhs, ItemInfo rhs) {
            return lhs.position - rhs.position;
        }
    };

    private static final Interpolator sInterpolator = new Interpolator() {
        public float getInterpolation(float t) {
            t -= 1.0f;
            return t * t * t * t * t + 1.0f;
        }
    };

    private final ArrayList<ItemInfo> mItems = new ArrayList<ItemInfo>();
    private final ItemInfo mTempItem = new ItemInfo();

    private final Rect mTempRect = new Rect();

    private PagerAdapter mAdapter;
    private int mCurItem;   // Index of currently displayed page.
    private int mRestoredCurItem = -1;
    private Parcelable mRestoredAdapterState = null;
    private ClassLoader mRestoredClassLoader = null;
    private final Scroller mScroller;
    private PagerObserver mObserver;

    private int mPageMargin;
    private Drawable mMarginDrawable;
    private int mTopPageBounds;
    private int mBottomPageBounds;

    
The increment used to move in the "left" direction. Dependent on layout
direction.
/**
     * The increment used to move in the "left" direction. Dependent on layout
     * direction.
     */
    private int mLeftIncr = -1;

    // Offsets of the first and last items, if known.
    // Set during population, used to determine if we are at the beginning
    // or end of the pager data set during touch scrolling.
    private float mFirstOffset = -Float.MAX_VALUE;
    private float mLastOffset = Float.MAX_VALUE;

    private int mChildWidthMeasureSpec;
    private int mChildHeightMeasureSpec;
    private boolean mInLayout;

    private boolean mScrollingCacheEnabled;

    private boolean mPopulatePending;
    private int mOffscreenPageLimit = DEFAULT_OFFSCREEN_PAGES;

    private boolean mIsBeingDragged;
    private boolean mIsUnableToDrag;
    private final int mDefaultGutterSize;
    private int mGutterSize;
    private final int mTouchSlop;
    
Position of the last motion event.
/**
     * Position of the last motion event.
     */
    private float mLastMotionX;
    private float mLastMotionY;
    private float mInitialMotionX;
    private float mInitialMotionY;
    
ID of the active pointer. This is used to retain consistency during
drags/flings if multiple pointers are used.
/**
     * ID of the active pointer. This is used to retain consistency during
     * drags/flings if multiple pointers are used.
     */
    private int mActivePointerId = INVALID_POINTER;
    
Sentinel value for no current active pointer. Used by mActivePointerId. /**
     * Sentinel value for no current active pointer.
     * Used by {@link #mActivePointerId}.
     */
    private static final int INVALID_POINTER = -1;

    
Determines speed during touch scrolling
/**
     * Determines speed during touch scrolling
     */
    private VelocityTracker mVelocityTracker;
    private final int mMinimumVelocity;
    private final int mMaximumVelocity;
    private final int mFlingDistance;
    private final int mCloseEnough;

    // If the pager is at least this close to its final position, complete the scroll
    // on touch down and let the user interact with the content inside instead of
    // "catching" the flinging pager.
    private static final int CLOSE_ENOUGH = 2; // dp

    private final EdgeEffect mLeftEdge;
    private final EdgeEffect mRightEdge;

    private boolean mFirstLayout = true;
    private boolean mCalledSuper;
    private int mDecorChildCount;

    private OnPageChangeListener mOnPageChangeListener;
    private OnPageChangeListener mInternalPageChangeListener;
    private OnAdapterChangeListener mAdapterChangeListener;
    private PageTransformer mPageTransformer;

    private static final int DRAW_ORDER_DEFAULT = 0;
    private static final int DRAW_ORDER_FORWARD = 1;
    private static final int DRAW_ORDER_REVERSE = 2;
    private int mDrawingOrder;
    private ArrayList<View> mDrawingOrderedChildren;
    private static final ViewPositionComparator sPositionComparator = new ViewPositionComparator();

    
Indicates that the pager is in an idle, settled state. The current page
is fully in view and no animation is in progress.
/**
     * Indicates that the pager is in an idle, settled state. The current page
     * is fully in view and no animation is in progress.
     */
    public static final int SCROLL_STATE_IDLE = 0;

    
Indicates that the pager is currently being dragged by the user.
/**
     * Indicates that the pager is currently being dragged by the user.
     */
    public static final int SCROLL_STATE_DRAGGING = 1;

    
Indicates that the pager is in the process of settling to a final position.
/**
     * Indicates that the pager is in the process of settling to a final position.
     */
    public static final int SCROLL_STATE_SETTLING = 2;

    private final Runnable mEndScrollRunnable = new Runnable() {
        public void run() {
            setScrollState(SCROLL_STATE_IDLE);
            populate();
        }
    };

    private int mScrollState = SCROLL_STATE_IDLE;

    
Callback interface for responding to changing state of the selected page.
/**
     * Callback interface for responding to changing state of the selected page.
     */
    public interface OnPageChangeListener {

        
This method will be invoked when the current page is scrolled, either as part
of a programmatically initiated smooth scroll or a user initiated touch scroll.
Params:
position – Position index of the first page currently being displayed.
                Page position+1 will be visible if positionOffset is nonzero.
positionOffset – Value from [0, 1) indicating the offset from the page at position.
positionOffsetPixels – Value in pixels indicating the offset from position./**
         * This method will be invoked when the current page is scrolled, either as part
         * of a programmatically initiated smooth scroll or a user initiated touch scroll.
         *
         * @param position Position index of the first page currently being displayed.
         *                 Page position+1 will be visible if positionOffset is nonzero.
         * @param positionOffset Value from [0, 1) indicating the offset from the page at position.
         * @param positionOffsetPixels Value in pixels indicating the offset from position.
         */
        public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels);

        
This method will be invoked when a new page becomes selected. Animation is not
necessarily complete.
Params:
position – Position index of the new selected page./**
         * This method will be invoked when a new page becomes selected. Animation is not
         * necessarily complete.
         *
         * @param position Position index of the new selected page.
         */
        public void onPageSelected(int position);

        
Called when the scroll state changes. Useful for discovering when the user
begins dragging, when the pager is automatically settling to the current page,
or when it is fully stopped/idle.
Params:
state – The new scroll state.
See Also:
ViewPager.SCROLL_STATE_IDLE
ViewPager.SCROLL_STATE_DRAGGING
ViewPager.SCROLL_STATE_SETTLING/**
         * Called when the scroll state changes. Useful for discovering when the user
         * begins dragging, when the pager is automatically settling to the current page,
         * or when it is fully stopped/idle.
         *
         * @param state The new scroll state.
         * @see com.android.internal.widget.ViewPager#SCROLL_STATE_IDLE
         * @see com.android.internal.widget.ViewPager#SCROLL_STATE_DRAGGING
         * @see com.android.internal.widget.ViewPager#SCROLL_STATE_SETTLING
         */
        public void onPageScrollStateChanged(int state);
    }

    
Simple implementation of the OnPageChangeListener interface with stub implementations of each method. Extend this if you do not intend to override every method of OnPageChangeListener. /**
     * Simple implementation of the {@link OnPageChangeListener} interface with stub
     * implementations of each method. Extend this if you do not intend to override
     * every method of {@link OnPageChangeListener}.
     */
    public static class SimpleOnPageChangeListener implements OnPageChangeListener {
        @Override
        public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
            // This space for rent
        }

        @Override
        public void onPageSelected(int position) {
            // This space for rent
        }

        @Override
        public void onPageScrollStateChanged(int state) {
            // This space for rent
        }
    }

    
A PageTransformer is invoked whenever a visible/attached page is scrolled.
This offers an opportunity for the application to apply a custom transformation
to the page views using animation properties.
As property animation is only supported as of Android 3.0 and forward,
setting a PageTransformer on a ViewPager on earlier platform versions will
be ignored.
/**
     * A PageTransformer is invoked whenever a visible/attached page is scrolled.
     * This offers an opportunity for the application to apply a custom transformation
     * to the page views using animation properties.
     *
     * <p>As property animation is only supported as of Android 3.0 and forward,
     * setting a PageTransformer on a ViewPager on earlier platform versions will
     * be ignored.</p>
     */
    public interface PageTransformer {
        
Apply a property transformation to the given page.
Params:
page – Apply the transformation to this page
position – Position of page relative to the current front-and-center
                position of the pager. 0 is front and center. 1 is one full
                page position to the right, and -1 is one page position to the left./**
         * Apply a property transformation to the given page.
         *
         * @param page Apply the transformation to this page
         * @param position Position of page relative to the current front-and-center
         *                 position of the pager. 0 is front and center. 1 is one full
         *                 page position to the right, and -1 is one page position to the left.
         */
        public void transformPage(View page, float position);
    }

    
Used internally to monitor when adapters are switched.
/**
     * Used internally to monitor when adapters are switched.
     */
    interface OnAdapterChangeListener {
        public void onAdapterChanged(PagerAdapter oldAdapter, PagerAdapter newAdapter);
    }

    
Used internally to tag special types of child views that should be added as
pager decorations by default.
/**
     * Used internally to tag special types of child views that should be added as
     * pager decorations by default.
     */
    interface Decor {}

    public ViewPager(Context context) {
        this(context, null);
    }

    public ViewPager(Context context, AttributeSet attrs) {
        this(context, attrs, 0);
    }

    public ViewPager(Context context, AttributeSet attrs, int defStyleAttr) {
        this(context, attrs, defStyleAttr, 0);
    }

    public ViewPager(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
        super(context, attrs, defStyleAttr, defStyleRes);

        setWillNotDraw(false);
        setDescendantFocusability(FOCUS_AFTER_DESCENDANTS);
        setFocusable(true);

        mScroller = new Scroller(context, sInterpolator);
        final ViewConfiguration configuration = ViewConfiguration.get(context);
        final float density = context.getResources().getDisplayMetrics().density;

        mTouchSlop = configuration.getScaledPagingTouchSlop();
        mMinimumVelocity = (int) (MIN_FLING_VELOCITY * density);
        mMaximumVelocity = configuration.getScaledMaximumFlingVelocity();
        mLeftEdge = new EdgeEffect(context);
        mRightEdge = new EdgeEffect(context);

        mFlingDistance = (int) (MIN_DISTANCE_FOR_FLING * density);
        mCloseEnough = (int) (CLOSE_ENOUGH * density);
        mDefaultGutterSize = (int) (DEFAULT_GUTTER_SIZE * density);

        if (getImportantForAccessibility() == IMPORTANT_FOR_ACCESSIBILITY_AUTO) {
            setImportantForAccessibility(IMPORTANT_FOR_ACCESSIBILITY_YES);
        }
    }

    @Override
    protected void onDetachedFromWindow() {
        removeCallbacks(mEndScrollRunnable);
        super.onDetachedFromWindow();
    }

    private void setScrollState(int newState) {
        if (mScrollState == newState) {
            return;
        }

        mScrollState = newState;
        if (mPageTransformer != null) {
            // PageTransformers can do complex things that benefit from hardware layers.
            enableLayers(newState != SCROLL_STATE_IDLE);
        }
        if (mOnPageChangeListener != null) {
            mOnPageChangeListener.onPageScrollStateChanged(newState);
        }
    }

    
Set a PagerAdapter that will supply views for this pager as needed.
Params:
adapter – Adapter to use/**
     * Set a PagerAdapter that will supply views for this pager as needed.
     *
     * @param adapter Adapter to use
     */
    public void setAdapter(PagerAdapter adapter) {
        if (mAdapter != null) {
            mAdapter.unregisterDataSetObserver(mObserver);
            mAdapter.startUpdate(this);
            for (int i = 0; i < mItems.size(); i++) {
                final ItemInfo ii = mItems.get(i);
                mAdapter.destroyItem(this, ii.position, ii.object);
            }
            mAdapter.finishUpdate(this);
            mItems.clear();
            removeNonDecorViews();
            mCurItem = 0;
            scrollTo(0, 0);
        }

        final PagerAdapter oldAdapter = mAdapter;
        mAdapter = adapter;
        mExpectedAdapterCount = 0;

        if (mAdapter != null) {
            if (mObserver == null) {
                mObserver = new PagerObserver();
            }
            mAdapter.registerDataSetObserver(mObserver);
            mPopulatePending = false;
            final boolean wasFirstLayout = mFirstLayout;
            mFirstLayout = true;
            mExpectedAdapterCount = mAdapter.getCount();
            if (mRestoredCurItem >= 0) {
                mAdapter.restoreState(mRestoredAdapterState, mRestoredClassLoader);
                setCurrentItemInternal(mRestoredCurItem, false, true);
                mRestoredCurItem = -1;
                mRestoredAdapterState = null;
                mRestoredClassLoader = null;
            } else if (!wasFirstLayout) {
                populate();
            } else {
                requestLayout();
            }
        }

        if (mAdapterChangeListener != null && oldAdapter != adapter) {
            mAdapterChangeListener.onAdapterChanged(oldAdapter, adapter);
        }
    }

    private void removeNonDecorViews() {
        for (int i = 0; i < getChildCount(); i++) {
            final View child = getChildAt(i);
            final LayoutParams lp = (LayoutParams) child.getLayoutParams();
            if (!lp.isDecor) {
                removeViewAt(i);
                i--;
            }
        }
    }

    
Retrieve the current adapter supplying pages.
Returns:
The currently registered PagerAdapter/**
     * Retrieve the current adapter supplying pages.
     *
     * @return The currently registered PagerAdapter
     */
    public PagerAdapter getAdapter() {
        return mAdapter;
    }

    void setOnAdapterChangeListener(OnAdapterChangeListener listener) {
        mAdapterChangeListener = listener;
    }

    private int getPaddedWidth() {
        return getMeasuredWidth() - getPaddingLeft() - getPaddingRight();
    }

    
Set the currently selected page. If the ViewPager has already been through its first
layout with its current adapter there will be a smooth animated transition between
the current item and the specified item.
Params:
item – Item index to select/**
     * Set the currently selected page. If the ViewPager has already been through its first
     * layout with its current adapter there will be a smooth animated transition between
     * the current item and the specified item.
     *
     * @param item Item index to select
     */
    public void setCurrentItem(int item) {
        mPopulatePending = false;
        setCurrentItemInternal(item, !mFirstLayout, false);
    }

    
Set the currently selected page.
Params:
item – Item index to select
smoothScroll – True to smoothly scroll to the new item, false to transition immediately/**
     * Set the currently selected page.
     *
     * @param item Item index to select
     * @param smoothScroll True to smoothly scroll to the new item, false to transition immediately
     */
    public void setCurrentItem(int item, boolean smoothScroll) {
        mPopulatePending = false;
        setCurrentItemInternal(item, smoothScroll, false);
    }

    public int getCurrentItem() {
        return mCurItem;
    }

    boolean setCurrentItemInternal(int item, boolean smoothScroll, boolean always) {
        return setCurrentItemInternal(item, smoothScroll, always, 0);
    }

    boolean setCurrentItemInternal(int item, boolean smoothScroll, boolean always, int velocity) {
        if (mAdapter == null || mAdapter.getCount() <= 0) {
            setScrollingCacheEnabled(false);
            return false;
        }

        item = MathUtils.constrain(item, 0, mAdapter.getCount() - 1);
        if (!always && mCurItem == item && mItems.size() != 0) {
            setScrollingCacheEnabled(false);
            return false;
        }

        final int pageLimit = mOffscreenPageLimit;
        if (item > (mCurItem + pageLimit) || item < (mCurItem - pageLimit)) {
            // We are doing a jump by more than one page.  To avoid
            // glitches, we want to keep all current pages in the view
            // until the scroll ends.
            for (int i = 0; i < mItems.size(); i++) {
                mItems.get(i).scrolling = true;
            }
        }

        final boolean dispatchSelected = mCurItem != item;
        if (mFirstLayout) {
            // We don't have any idea how big we are yet and shouldn't have any pages either.
            // Just set things up and let the pending layout handle things.
            mCurItem = item;
            if (dispatchSelected && mOnPageChangeListener != null) {
                mOnPageChangeListener.onPageSelected(item);
            }
            if (dispatchSelected && mInternalPageChangeListener != null) {
                mInternalPageChangeListener.onPageSelected(item);
            }
            requestLayout();
        } else {
            populate(item);
            scrollToItem(item, smoothScroll, velocity, dispatchSelected);
        }

        return true;
    }

    private void scrollToItem(int position, boolean smoothScroll, int velocity,
            boolean dispatchSelected) {
        final int destX = getLeftEdgeForItem(position);

        if (smoothScroll) {
            smoothScrollTo(destX, 0, velocity);

            if (dispatchSelected && mOnPageChangeListener != null) {
                mOnPageChangeListener.onPageSelected(position);
            }
            if (dispatchSelected && mInternalPageChangeListener != null) {
                mInternalPageChangeListener.onPageSelected(position);
            }
        } else {
            if (dispatchSelected && mOnPageChangeListener != null) {
                mOnPageChangeListener.onPageSelected(position);
            }
            if (dispatchSelected && mInternalPageChangeListener != null) {
                mInternalPageChangeListener.onPageSelected(position);
            }

            completeScroll(false);
            scrollTo(destX, 0);
            pageScrolled(destX);
        }
    }

    private int getLeftEdgeForItem(int position) {
        final ItemInfo info = infoForPosition(position);
        if (info == null) {
            return 0;
        }

        final int width = getPaddedWidth();
        final int scaledOffset = (int) (width * MathUtils.constrain(
                info.offset, mFirstOffset, mLastOffset));

        if (isLayoutRtl()) {
            final int itemWidth = (int) (width * info.widthFactor + 0.5f);
            return MAX_SCROLL_X - itemWidth - scaledOffset;
        } else {
            return scaledOffset;
        }
    }

    
Set a listener that will be invoked whenever the page changes or is incrementally scrolled. See OnPageChangeListener. 
Params:
listener – Listener to set/**
     * Set a listener that will be invoked whenever the page changes or is incrementally
     * scrolled. See {@link OnPageChangeListener}.
     *
     * @param listener Listener to set
     */
    public void setOnPageChangeListener(OnPageChangeListener listener) {
        mOnPageChangeListener = listener;
    }

    
Set a PageTransformer that will be called for each attached page whenever the scroll position is changed. This allows the application to apply custom property transformations to each page, overriding the default sliding look and feel. 
Note: Prior to Android 3.0 the property animation APIs did not exist.
As a result, setting a PageTransformer prior to Android 3.0 (API 11) will have no effect.
Params:
reverseDrawingOrder – true if the supplied PageTransformer requires page views
                           to be drawn from last to first instead of first to last.
transformer – PageTransformer that will modify each page's animation properties/**
     * Set a {@link PageTransformer} that will be called for each attached page whenever
     * the scroll position is changed. This allows the application to apply custom property
     * transformations to each page, overriding the default sliding look and feel.
     *
     * <p><em>Note:</em> Prior to Android 3.0 the property animation APIs did not exist.
     * As a result, setting a PageTransformer prior to Android 3.0 (API 11) will have no effect.</p>
     *
     * @param reverseDrawingOrder true if the supplied PageTransformer requires page views
     *                            to be drawn from last to first instead of first to last.
     * @param transformer PageTransformer that will modify each page's animation properties
     */
    public void setPageTransformer(boolean reverseDrawingOrder, PageTransformer transformer) {
        final boolean hasTransformer = transformer != null;
        final boolean needsPopulate = hasTransformer != (mPageTransformer != null);
        mPageTransformer = transformer;
        setChildrenDrawingOrderEnabled(hasTransformer);
        if (hasTransformer) {
            mDrawingOrder = reverseDrawingOrder ? DRAW_ORDER_REVERSE : DRAW_ORDER_FORWARD;
        } else {
            mDrawingOrder = DRAW_ORDER_DEFAULT;
        }
        if (needsPopulate) populate();
    }

    @Override
    protected int getChildDrawingOrder(int childCount, int i) {
        final int index = mDrawingOrder == DRAW_ORDER_REVERSE ? childCount - 1 - i : i;
        final int result = ((LayoutParams) mDrawingOrderedChildren.get(index).getLayoutParams()).childIndex;
        return result;
    }

    
Set a separate OnPageChangeListener for internal use by the support library.
Params:
listener – Listener to set
Returns:
The old listener that was set, if any./**
     * Set a separate OnPageChangeListener for internal use by the support library.
     *
     * @param listener Listener to set
     * @return The old listener that was set, if any.
     */
    OnPageChangeListener setInternalPageChangeListener(OnPageChangeListener listener) {
        OnPageChangeListener oldListener = mInternalPageChangeListener;
        mInternalPageChangeListener = listener;
        return oldListener;
    }

    
Returns the number of pages that will be retained to either side of the
current page in the view hierarchy in an idle state. Defaults to 1.
See Also:
setOffscreenPageLimit(int)
Returns:
How many pages will be kept offscreen on either side/**
     * Returns the number of pages that will be retained to either side of the
     * current page in the view hierarchy in an idle state. Defaults to 1.
     *
     * @return How many pages will be kept offscreen on either side
     * @see #setOffscreenPageLimit(int)
     */
    public int getOffscreenPageLimit() {
        return mOffscreenPageLimit;
    }

    
Set the number of pages that should be retained to either side of the
current page in the view hierarchy in an idle state. Pages beyond this
limit will be recreated from the adapter when needed.
This is offered as an optimization. If you know in advance the number
of pages you will need to support or have lazy-loading mechanisms in place
on your pages, tweaking this setting can have benefits in perceived smoothness
of paging animations and interaction. If you have a small number of pages (3-4)
that you can keep active all at once, less time will be spent in layout for
newly created view subtrees as the user pages back and forth.
You should keep this limit low, especially if your pages have complex layouts.
This setting defaults to 1.
Params:
limit – How many pages will be kept offscreen in an idle state./**
     * Set the number of pages that should be retained to either side of the
     * current page in the view hierarchy in an idle state. Pages beyond this
     * limit will be recreated from the adapter when needed.
     *
     * <p>This is offered as an optimization. If you know in advance the number
     * of pages you will need to support or have lazy-loading mechanisms in place
     * on your pages, tweaking this setting can have benefits in perceived smoothness
     * of paging animations and interaction. If you have a small number of pages (3-4)
     * that you can keep active all at once, less time will be spent in layout for
     * newly created view subtrees as the user pages back and forth.</p>
     *
     * <p>You should keep this limit low, especially if your pages have complex layouts.
     * This setting defaults to 1.</p>
     *
     * @param limit How many pages will be kept offscreen in an idle state.
     */
    public void setOffscreenPageLimit(int limit) {
        if (limit < DEFAULT_OFFSCREEN_PAGES) {
            Log.w(TAG, "Requested offscreen page limit " + limit + " too small; defaulting to " +
                    DEFAULT_OFFSCREEN_PAGES);
            limit = DEFAULT_OFFSCREEN_PAGES;
        }
        if (limit != mOffscreenPageLimit) {
            mOffscreenPageLimit = limit;
            populate();
        }
    }

    
Set the margin between pages.
Params:
marginPixels – Distance between adjacent pages in pixels
See Also:
getPageMargin()
setPageMarginDrawable(Drawable)
setPageMarginDrawable(int)/**
     * Set the margin between pages.
     *
     * @param marginPixels Distance between adjacent pages in pixels
     * @see #getPageMargin()
     * @see #setPageMarginDrawable(android.graphics.drawable.Drawable)
     * @see #setPageMarginDrawable(int)
     */
    public void setPageMargin(int marginPixels) {
        final int oldMargin = mPageMargin;
        mPageMargin = marginPixels;

        final int width = getWidth();
        recomputeScrollPosition(width, width, marginPixels, oldMargin);

        requestLayout();
    }

    
Return the margin between pages.
Returns:
The size of the margin in pixels/**
     * Return the margin between pages.
     *
     * @return The size of the margin in pixels
     */
    public int getPageMargin() {
        return mPageMargin;
    }

    
Set a drawable that will be used to fill the margin between pages.
Params:
d – Drawable to display between pages/**
     * Set a drawable that will be used to fill the margin between pages.
     *
     * @param d Drawable to display between pages
     */
    public void setPageMarginDrawable(Drawable d) {
        mMarginDrawable = d;
        if (d != null) refreshDrawableState();
        setWillNotDraw(d == null);
        invalidate();
    }

    
Set a drawable that will be used to fill the margin between pages.
Params:
resId – Resource ID of a drawable to display between pages/**
     * Set a drawable that will be used to fill the margin between pages.
     *
     * @param resId Resource ID of a drawable to display between pages
     */
    public void setPageMarginDrawable(@DrawableRes int resId) {
        setPageMarginDrawable(getContext().getDrawable(resId));
    }

    @Override
    protected boolean verifyDrawable(@NonNull Drawable who) {
        return super.verifyDrawable(who) || who == mMarginDrawable;
    }

    @Override
    protected void drawableStateChanged() {
        super.drawableStateChanged();
        final Drawable marginDrawable = mMarginDrawable;
        if (marginDrawable != null && marginDrawable.isStateful()
                && marginDrawable.setState(getDrawableState())) {
            invalidateDrawable(marginDrawable);
        }
    }

    // We want the duration of the page snap animation to be influenced by the distance that
    // the screen has to travel, however, we don't want this duration to be effected in a
    // purely linear fashion. Instead, we use this method to moderate the effect that the distance
    // of travel has on the overall snap duration.
    float distanceInfluenceForSnapDuration(float f) {
        f -= 0.5f; // center the values about 0.
        f *= 0.3f * Math.PI / 2.0f;
        return (float) Math.sin(f);
    }

    
Like View.scrollBy, but scroll smoothly instead of immediately. 
Params:
x – the number of pixels to scroll by on the X axis
y – the number of pixels to scroll by on the Y axis/**
     * Like {@link android.view.View#scrollBy}, but scroll smoothly instead of immediately.
     *
     * @param x the number of pixels to scroll by on the X axis
     * @param y the number of pixels to scroll by on the Y axis
     */
    void smoothScrollTo(int x, int y) {
        smoothScrollTo(x, y, 0);
    }

    
Like View.scrollBy, but scroll smoothly instead of immediately. 
Params:
x – the number of pixels to scroll by on the X axis
y – the number of pixels to scroll by on the Y axis
velocity – the velocity associated with a fling, if applicable. (0 otherwise)/**
     * Like {@link android.view.View#scrollBy}, but scroll smoothly instead of immediately.
     *
     * @param x the number of pixels to scroll by on the X axis
     * @param y the number of pixels to scroll by on the Y axis
     * @param velocity the velocity associated with a fling, if applicable. (0 otherwise)
     */
    void smoothScrollTo(int x, int y, int velocity) {
        if (getChildCount() == 0) {
            // Nothing to do.
            setScrollingCacheEnabled(false);
            return;
        }
        int sx = getScrollX();
        int sy = getScrollY();
        int dx = x - sx;
        int dy = y - sy;
        if (dx == 0 && dy == 0) {
            completeScroll(false);
            populate();
            setScrollState(SCROLL_STATE_IDLE);
            return;
        }

        setScrollingCacheEnabled(true);
        setScrollState(SCROLL_STATE_SETTLING);

        final int width = getPaddedWidth();
        final int halfWidth = width / 2;
        final float distanceRatio = Math.min(1f, 1.0f * Math.abs(dx) / width);
        final float distance = halfWidth + halfWidth *
                distanceInfluenceForSnapDuration(distanceRatio);

        int duration = 0;
        velocity = Math.abs(velocity);
        if (velocity > 0) {
            duration = 4 * Math.round(1000 * Math.abs(distance / velocity));
        } else {
            final float pageWidth = width * mAdapter.getPageWidth(mCurItem);
            final float pageDelta = (float) Math.abs(dx) / (pageWidth + mPageMargin);
            duration = (int) ((pageDelta + 1) * 100);
        }
        duration = Math.min(duration, MAX_SETTLE_DURATION);

        mScroller.startScroll(sx, sy, dx, dy, duration);
        postInvalidateOnAnimation();
    }

    ItemInfo addNewItem(int position, int index) {
        ItemInfo ii = new ItemInfo();
        ii.position = position;
        ii.object = mAdapter.instantiateItem(this, position);
        ii.widthFactor = mAdapter.getPageWidth(position);
        if (index < 0 || index >= mItems.size()) {
            mItems.add(ii);
        } else {
            mItems.add(index, ii);
        }
        return ii;
    }

    void dataSetChanged() {
        // This method only gets called if our observer is attached, so mAdapter is non-null.

        final int adapterCount = mAdapter.getCount();
        mExpectedAdapterCount = adapterCount;
        boolean needPopulate = mItems.size() < mOffscreenPageLimit * 2 + 1 &&
                mItems.size() < adapterCount;
        int newCurrItem = mCurItem;

        boolean isUpdating = false;
        for (int i = 0; i < mItems.size(); i++) {
            final ItemInfo ii = mItems.get(i);
            final int newPos = mAdapter.getItemPosition(ii.object);

            if (newPos == PagerAdapter.POSITION_UNCHANGED) {
                continue;
            }

            if (newPos == PagerAdapter.POSITION_NONE) {
                mItems.remove(i);
                i--;

                if (!isUpdating) {
                    mAdapter.startUpdate(this);
                    isUpdating = true;
                }

                mAdapter.destroyItem(this, ii.position, ii.object);
                needPopulate = true;

                if (mCurItem == ii.position) {
                    // Keep the current item in the valid range
                    newCurrItem = Math.max(0, Math.min(mCurItem, adapterCount - 1));
                    needPopulate = true;
                }
                continue;
            }

            if (ii.position != newPos) {
                if (ii.position == mCurItem) {
                    // Our current item changed position. Follow it.
                    newCurrItem = newPos;
                }

                ii.position = newPos;
                needPopulate = true;
            }
        }

        if (isUpdating) {
            mAdapter.finishUpdate(this);
        }

        Collections.sort(mItems, COMPARATOR);

        if (needPopulate) {
            // Reset our known page widths; populate will recompute them.
            final int childCount = getChildCount();
            for (int i = 0; i < childCount; i++) {
                final View child = getChildAt(i);
                final LayoutParams lp = (LayoutParams) child.getLayoutParams();
                if (!lp.isDecor) {
                    lp.widthFactor = 0.f;
                }
            }

            setCurrentItemInternal(newCurrItem, false, true);
            requestLayout();
        }
    }

    public void populate() {
        populate(mCurItem);
    }

    void populate(int newCurrentItem) {
        ItemInfo oldCurInfo = null;
        int focusDirection = View.FOCUS_FORWARD;
        if (mCurItem != newCurrentItem) {
            focusDirection = mCurItem < newCurrentItem ? View.FOCUS_RIGHT : View.FOCUS_LEFT;
            oldCurInfo = infoForPosition(mCurItem);
            mCurItem = newCurrentItem;
        }

        if (mAdapter == null) {
            sortChildDrawingOrder();
            return;
        }

        // Bail now if we are waiting to populate.  This is to hold off
        // on creating views from the time the user releases their finger to
        // fling to a new position until we have finished the scroll to
        // that position, avoiding glitches from happening at that point.
        if (mPopulatePending) {
            if (DEBUG) Log.i(TAG, "populate is pending, skipping for now...");
            sortChildDrawingOrder();
            return;
        }

        // Also, don't populate until we are attached to a window.  This is to
        // avoid trying to populate before we have restored our view hierarchy
        // state and conflicting with what is restored.
        if (getWindowToken() == null) {
            return;
        }

        mAdapter.startUpdate(this);

        final int pageLimit = mOffscreenPageLimit;
        final int startPos = Math.max(0, mCurItem - pageLimit);
        final int N = mAdapter.getCount();
        final int endPos = Math.min(N-1, mCurItem + pageLimit);

        if (N != mExpectedAdapterCount) {
            String resName;
            try {
                resName = getResources().getResourceName(getId());
            } catch (Resources.NotFoundException e) {
                resName = Integer.toHexString(getId());
            }
            throw new IllegalStateException("The application's PagerAdapter changed the adapter's" +
                    " contents without calling PagerAdapter#notifyDataSetChanged!" +
                    " Expected adapter item count: " + mExpectedAdapterCount + ", found: " + N +
                    " Pager id: " + resName +
                    " Pager class: " + getClass() +
                    " Problematic adapter: " + mAdapter.getClass());
        }

        // Locate the currently focused item or add it if needed.
        int curIndex = -1;
        ItemInfo curItem = null;
        for (curIndex = 0; curIndex < mItems.size(); curIndex++) {
            final ItemInfo ii = mItems.get(curIndex);
            if (ii.position >= mCurItem) {
                if (ii.position == mCurItem) curItem = ii;
                break;
            }
        }

        if (curItem == null && N > 0) {
            curItem = addNewItem(mCurItem, curIndex);
        }

        // Fill 3x the available width or up to the number of offscreen
        // pages requested to either side, whichever is larger.
        // If we have no current item we have no work to do.
        if (curItem != null) {
            float extraWidthLeft = 0.f;
            int itemIndex = curIndex - 1;
            ItemInfo ii = itemIndex >= 0 ? mItems.get(itemIndex) : null;
            final int clientWidth = getPaddedWidth();
            final float leftWidthNeeded = clientWidth <= 0 ? 0 :
                    2.f - curItem.widthFactor + (float) getPaddingLeft() / (float) clientWidth;
            for (int pos = mCurItem - 1; pos >= 0; pos--) {
                if (extraWidthLeft >= leftWidthNeeded && pos < startPos) {
                    if (ii == null) {
                        break;
                    }
                    if (pos == ii.position && !ii.scrolling) {
                        mItems.remove(itemIndex);
                        mAdapter.destroyItem(this, pos, ii.object);
                        if (DEBUG) {
                            Log.i(TAG, "populate() - destroyItem() with pos: " + pos +
                                    " view: " + ii.object);
                        }
                        itemIndex--;
                        curIndex--;
                        ii = itemIndex >= 0 ? mItems.get(itemIndex) : null;
                    }
                } else if (ii != null && pos == ii.position) {
                    extraWidthLeft += ii.widthFactor;
                    itemIndex--;
                    ii = itemIndex >= 0 ? mItems.get(itemIndex) : null;
                } else {
                    ii = addNewItem(pos, itemIndex + 1);
                    extraWidthLeft += ii.widthFactor;
                    curIndex++;
                    ii = itemIndex >= 0 ? mItems.get(itemIndex) : null;
                }
            }

            float extraWidthRight = curItem.widthFactor;
            itemIndex = curIndex + 1;
            if (extraWidthRight < 2.f) {
                ii = itemIndex < mItems.size() ? mItems.get(itemIndex) : null;
                final float rightWidthNeeded = clientWidth <= 0 ? 0 :
                        (float) getPaddingRight() / (float) clientWidth + 2.f;
                for (int pos = mCurItem + 1; pos < N; pos++) {
                    if (extraWidthRight >= rightWidthNeeded && pos > endPos) {
                        if (ii == null) {
                            break;
                        }
                        if (pos == ii.position && !ii.scrolling) {
                            mItems.remove(itemIndex);
                            mAdapter.destroyItem(this, pos, ii.object);
                            if (DEBUG) {
                                Log.i(TAG, "populate() - destroyItem() with pos: " + pos +
                                        " view: " + ii.object);
                            }
                            ii = itemIndex < mItems.size() ? mItems.get(itemIndex) : null;
                        }
                    } else if (ii != null && pos == ii.position) {
                        extraWidthRight += ii.widthFactor;
                        itemIndex++;
                        ii = itemIndex < mItems.size() ? mItems.get(itemIndex) : null;
                    } else {
                        ii = addNewItem(pos, itemIndex);
                        itemIndex++;
                        extraWidthRight += ii.widthFactor;
                        ii = itemIndex < mItems.size() ? mItems.get(itemIndex) : null;
                    }
                }
            }

            calculatePageOffsets(curItem, curIndex, oldCurInfo);
        }

        if (DEBUG) {
            Log.i(TAG, "Current page list:");
            for (int i=0; i<mItems.size(); i++) {
                Log.i(TAG, "#" + i + ": page " + mItems.get(i).position);
            }
        }

        mAdapter.setPrimaryItem(this, mCurItem, curItem != null ? curItem.object : null);

        mAdapter.finishUpdate(this);

        // Check width measurement of current pages and drawing sort order.
        // Update LayoutParams as needed.
        final int childCount = getChildCount();
        for (int i = 0; i < childCount; i++) {
            final View child = getChildAt(i);
            final LayoutParams lp = (LayoutParams) child.getLayoutParams();
            lp.childIndex = i;
            if (!lp.isDecor && lp.widthFactor == 0.f) {
                // 0 means requery the adapter for this, it doesn't have a valid width.
                final ItemInfo ii = infoForChild(child);
                if (ii != null) {
                    lp.widthFactor = ii.widthFactor;
                    lp.position = ii.position;
                }
            }
        }
        sortChildDrawingOrder();

        if (hasFocus()) {
            View currentFocused = findFocus();
            ItemInfo ii = currentFocused != null ? infoForAnyChild(currentFocused) : null;
            if (ii == null || ii.position != mCurItem) {
                for (int i=0; i<getChildCount(); i++) {
                    View child = getChildAt(i);
                    ii = infoForChild(child);
                    if (ii != null && ii.position == mCurItem) {
                        final Rect focusRect;
                        if (currentFocused == null) {
                            focusRect = null;
                        } else {
                            focusRect = mTempRect;
                            currentFocused.getFocusedRect(mTempRect);
                            offsetDescendantRectToMyCoords(currentFocused, mTempRect);
                            offsetRectIntoDescendantCoords(child, mTempRect);
                        }
                        if (child.requestFocus(focusDirection, focusRect)) {
                            break;
                        }
                    }
                }
            }
        }
    }

    private void sortChildDrawingOrder() {
        if (mDrawingOrder != DRAW_ORDER_DEFAULT) {
            if (mDrawingOrderedChildren == null) {
                mDrawingOrderedChildren = new ArrayList<View>();
            } else {
                mDrawingOrderedChildren.clear();
            }
            final int childCount = getChildCount();
            for (int i = 0; i < childCount; i++) {
                final View child = getChildAt(i);
                mDrawingOrderedChildren.add(child);
            }
            Collections.sort(mDrawingOrderedChildren, sPositionComparator);
        }
    }

    private void calculatePageOffsets(ItemInfo curItem, int curIndex, ItemInfo oldCurInfo) {
        final int N = mAdapter.getCount();
        final int width = getPaddedWidth();
        final float marginOffset = width > 0 ? (float) mPageMargin / width : 0;

        // Fix up offsets for later layout.
        if (oldCurInfo != null) {
            final int oldCurPosition = oldCurInfo.position;

            // Base offsets off of oldCurInfo.
            if (oldCurPosition < curItem.position) {
                int itemIndex = 0;
                float offset = oldCurInfo.offset + oldCurInfo.widthFactor + marginOffset;
                for (int pos = oldCurPosition + 1; pos <= curItem.position && itemIndex < mItems.size(); pos++) {
                    ItemInfo ii = mItems.get(itemIndex);
                    while (pos > ii.position && itemIndex < mItems.size() - 1) {
                        itemIndex++;
                        ii = mItems.get(itemIndex);
                    }

                    while (pos < ii.position) {
                        // We don't have an item populated for this,
                        // ask the adapter for an offset.
                        offset += mAdapter.getPageWidth(pos) + marginOffset;
                        pos++;
                    }

                    ii.offset = offset;
                    offset += ii.widthFactor + marginOffset;
                }
            } else if (oldCurPosition > curItem.position) {
                int itemIndex = mItems.size() - 1;
                float offset = oldCurInfo.offset;
                for (int pos = oldCurPosition - 1; pos >= curItem.position && itemIndex >= 0; pos--) {
                    ItemInfo ii = mItems.get(itemIndex);
                    while (pos < ii.position && itemIndex > 0) {
                        itemIndex--;
                        ii = mItems.get(itemIndex);
                    }

                    while (pos > ii.position) {
                        // We don't have an item populated for this,
                        // ask the adapter for an offset.
                        offset -= mAdapter.getPageWidth(pos) + marginOffset;
                        pos--;
                    }

                    offset -= ii.widthFactor + marginOffset;
                    ii.offset = offset;
                }
            }
        }

        // Base all offsets off of curItem.
        final int itemCount = mItems.size();
        float offset = curItem.offset;
        int pos = curItem.position - 1;
        mFirstOffset = curItem.position == 0 ? curItem.offset : -Float.MAX_VALUE;
        mLastOffset = curItem.position == N - 1 ?
                curItem.offset + curItem.widthFactor - 1 : Float.MAX_VALUE;

        // Previous pages
        for (int i = curIndex - 1; i >= 0; i--, pos--) {
            final ItemInfo ii = mItems.get(i);
            while (pos > ii.position) {
                offset -= mAdapter.getPageWidth(pos--) + marginOffset;
            }
            offset -= ii.widthFactor + marginOffset;
            ii.offset = offset;
            if (ii.position == 0) mFirstOffset = offset;
        }

        offset = curItem.offset + curItem.widthFactor + marginOffset;
        pos = curItem.position + 1;

        // Next pages
        for (int i = curIndex + 1; i < itemCount; i++, pos++) {
            final ItemInfo ii = mItems.get(i);
            while (pos < ii.position) {
                offset += mAdapter.getPageWidth(pos++) + marginOffset;
            }
            if (ii.position == N - 1) {
                mLastOffset = offset + ii.widthFactor - 1;
            }
            ii.offset = offset;
            offset += ii.widthFactor + marginOffset;
        }
    }

    
This is the persistent state that is saved by ViewPager.  Only needed
if you are creating a sublass of ViewPager that must save its own
state, in which case it should implement a subclass of this which
contains that state.
/**
     * This is the persistent state that is saved by ViewPager.  Only needed
     * if you are creating a sublass of ViewPager that must save its own
     * state, in which case it should implement a subclass of this which
     * contains that state.
     */
    public static class SavedState extends AbsSavedState {
        int position;
        Parcelable adapterState;
        ClassLoader loader;

        public SavedState(@NonNull Parcelable superState) {
            super(superState);
        }

        @Override
        public void writeToParcel(Parcel out, int flags) {
            super.writeToParcel(out, flags);
            out.writeInt(position);
            out.writeParcelable(adapterState, flags);
        }

        @Override
        public String toString() {
            return "FragmentPager.SavedState{"
                    + Integer.toHexString(System.identityHashCode(this))
                    + " position=" + position + "}";
        }

        public static final Creator<SavedState> CREATOR = new ClassLoaderCreator<SavedState>() {
            @Override
            public SavedState createFromParcel(Parcel in, ClassLoader loader) {
                return new SavedState(in, loader);
            }

            @Override
            public SavedState createFromParcel(Parcel in) {
                return new SavedState(in, null);
            }
            @Override
            public SavedState[] newArray(int size) {
                return new SavedState[size];
            }
        };

        SavedState(Parcel in, ClassLoader loader) {
            super(in, loader);
            if (loader == null) {
                loader = getClass().getClassLoader();
            }
            position = in.readInt();
            adapterState = in.readParcelable(loader);
            this.loader = loader;
        }
    }

    @Override
    public Parcelable onSaveInstanceState() {
        Parcelable superState = super.onSaveInstanceState();
        SavedState ss = new SavedState(superState);
        ss.position = mCurItem;
        if (mAdapter != null) {
            ss.adapterState = mAdapter.saveState();
        }
        return ss;
    }

    @Override
    public void onRestoreInstanceState(Parcelable state) {
        if (!(state instanceof SavedState)) {
            super.onRestoreInstanceState(state);
            return;
        }

        SavedState ss = (SavedState)state;
        super.onRestoreInstanceState(ss.getSuperState());

        if (mAdapter != null) {
            mAdapter.restoreState(ss.adapterState, ss.loader);
            setCurrentItemInternal(ss.position, false, true);
        } else {
            mRestoredCurItem = ss.position;
            mRestoredAdapterState = ss.adapterState;
            mRestoredClassLoader = ss.loader;
        }
    }

    @Override
    public void addView(View child, int index, ViewGroup.LayoutParams params) {
        if (!checkLayoutParams(params)) {
            params = generateLayoutParams(params);
        }
        final LayoutParams lp = (LayoutParams) params;
        lp.isDecor |= child instanceof Decor;
        if (mInLayout) {
            if (lp != null && lp.isDecor) {
                throw new IllegalStateException("Cannot add pager decor view during layout");
            }
            lp.needsMeasure = true;
            addViewInLayout(child, index, params);
        } else {
            super.addView(child, index, params);
        }

        if (USE_CACHE) {
            if (child.getVisibility() != GONE) {
                child.setDrawingCacheEnabled(mScrollingCacheEnabled);
            } else {
                child.setDrawingCacheEnabled(false);
            }
        }
    }

    public Object getCurrent() {
        final ItemInfo itemInfo = infoForPosition(getCurrentItem());
        return itemInfo == null ? null : itemInfo.object;
    }

    @Override
    public void removeView(View view) {
        if (mInLayout) {
            removeViewInLayout(view);
        } else {
            super.removeView(view);
        }
    }

    ItemInfo infoForChild(View child) {
        for (int i=0; i<mItems.size(); i++) {
            ItemInfo ii = mItems.get(i);
            if (mAdapter.isViewFromObject(child, ii.object)) {
                return ii;
            }
        }
        return null;
    }

    ItemInfo infoForAnyChild(View child) {
        ViewParent parent;
        while ((parent=child.getParent()) != this) {
            if (parent == null || !(parent instanceof View)) {
                return null;
            }
            child = (View)parent;
        }
        return infoForChild(child);
    }

    ItemInfo infoForPosition(int position) {
        for (int i = 0; i < mItems.size(); i++) {
            ItemInfo ii = mItems.get(i);
            if (ii.position == position) {
                return ii;
            }
        }
        return null;
    }

    @Override
    protected void onAttachedToWindow() {
        super.onAttachedToWindow();
        mFirstLayout = true;
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        // For simple implementation, our internal size is always 0.
        // We depend on the container to specify the layout size of
        // our view.  We can't really know what it is since we will be
        // adding and removing different arbitrary views and do not
        // want the layout to change as this happens.
        setMeasuredDimension(getDefaultSize(0, widthMeasureSpec),
                getDefaultSize(0, heightMeasureSpec));

        final int measuredWidth = getMeasuredWidth();
        final int maxGutterSize = measuredWidth / 10;
        mGutterSize = Math.min(maxGutterSize, mDefaultGutterSize);

        // Children are just made to fill our space.
        int childWidthSize = measuredWidth - getPaddingLeft() - getPaddingRight();
        int childHeightSize = getMeasuredHeight() - getPaddingTop() - getPaddingBottom();

        /*
         * Make sure all children have been properly measured. Decor views first.
         * Right now we cheat and make this less complicated by assuming decor
         * views won't intersect. We will pin to edges based on gravity.
         */
        int size = getChildCount();
        for (int i = 0; i < size; ++i) {
            final View child = getChildAt(i);
            if (child.getVisibility() != GONE) {
                final LayoutParams lp = (LayoutParams) child.getLayoutParams();
                if (lp != null && lp.isDecor) {
                    final int hgrav = lp.gravity & Gravity.HORIZONTAL_GRAVITY_MASK;
                    final int vgrav = lp.gravity & Gravity.VERTICAL_GRAVITY_MASK;
                    int widthMode = MeasureSpec.AT_MOST;
                    int heightMode = MeasureSpec.AT_MOST;
                    boolean consumeVertical = vgrav == Gravity.TOP || vgrav == Gravity.BOTTOM;
                    boolean consumeHorizontal = hgrav == Gravity.LEFT || hgrav == Gravity.RIGHT;

                    if (consumeVertical) {
                        widthMode = MeasureSpec.EXACTLY;
                    } else if (consumeHorizontal) {
                        heightMode = MeasureSpec.EXACTLY;
                    }

                    int widthSize = childWidthSize;
                    int heightSize = childHeightSize;
                    if (lp.width != LayoutParams.WRAP_CONTENT) {
                        widthMode = MeasureSpec.EXACTLY;
                        if (lp.width != LayoutParams.FILL_PARENT) {
                            widthSize = lp.width;
                        }
                    }
                    if (lp.height != LayoutParams.WRAP_CONTENT) {
                        heightMode = MeasureSpec.EXACTLY;
                        if (lp.height != LayoutParams.FILL_PARENT) {
                            heightSize = lp.height;
                        }
                    }
                    final int widthSpec = MeasureSpec.makeMeasureSpec(widthSize, widthMode);
                    final int heightSpec = MeasureSpec.makeMeasureSpec(heightSize, heightMode);
                    child.measure(widthSpec, heightSpec);

                    if (consumeVertical) {
                        childHeightSize -= child.getMeasuredHeight();
                    } else if (consumeHorizontal) {
                        childWidthSize -= child.getMeasuredWidth();
                    }
                }
            }
        }

        mChildWidthMeasureSpec = MeasureSpec.makeMeasureSpec(childWidthSize, MeasureSpec.EXACTLY);
        mChildHeightMeasureSpec = MeasureSpec.makeMeasureSpec(childHeightSize, MeasureSpec.EXACTLY);

        // Make sure we have created all fragments that we need to have shown.
        mInLayout = true;
        populate();
        mInLayout = false;

        // Page views next.
        size = getChildCount();
        for (int i = 0; i < size; ++i) {
            final View child = getChildAt(i);
            if (child.getVisibility() != GONE) {
                if (DEBUG) Log.v(TAG, "Measuring #" + i + " " + child
                        + ": " + mChildWidthMeasureSpec);

                final LayoutParams lp = (LayoutParams) child.getLayoutParams();
                if (lp == null || !lp.isDecor) {
                    final int widthSpec = MeasureSpec.makeMeasureSpec(
                            (int) (childWidthSize * lp.widthFactor), MeasureSpec.EXACTLY);
                    child.measure(widthSpec, mChildHeightMeasureSpec);
                }
            }
        }
    }

    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        super.onSizeChanged(w, h, oldw, oldh);

        // Make sure scroll position is set correctly.
        if (w != oldw) {
            recomputeScrollPosition(w, oldw, mPageMargin, mPageMargin);
        }
    }

    private void recomputeScrollPosition(int width, int oldWidth, int margin, int oldMargin) {
        if (oldWidth > 0 && !mItems.isEmpty()) {
            final int widthWithMargin = width - getPaddingLeft() - getPaddingRight() + margin;
            final int oldWidthWithMargin = oldWidth - getPaddingLeft() - getPaddingRight()
                                           + oldMargin;
            final int xpos = getScrollX();
            final float pageOffset = (float) xpos / oldWidthWithMargin;
            final int newOffsetPixels = (int) (pageOffset * widthWithMargin);

            scrollTo(newOffsetPixels, getScrollY());
            if (!mScroller.isFinished()) {
                // We now return to your regularly scheduled scroll, already in progress.
                final int newDuration = mScroller.getDuration() - mScroller.timePassed();
                ItemInfo targetInfo = infoForPosition(mCurItem);
                mScroller.startScroll(newOffsetPixels, 0,
                        (int) (targetInfo.offset * width), 0, newDuration);
            }
        } else {
            final ItemInfo ii = infoForPosition(mCurItem);
            final float scrollOffset = ii != null ? Math.min(ii.offset, mLastOffset) : 0;
            final int scrollPos = (int) (scrollOffset *
                                         (width - getPaddingLeft() - getPaddingRight()));
            if (scrollPos != getScrollX()) {
                completeScroll(false);
                scrollTo(scrollPos, getScrollY());
            }
        }
    }

    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        final int count = getChildCount();
        int width = r - l;
        int height = b - t;
        int paddingLeft = getPaddingLeft();
        int paddingTop = getPaddingTop();
        int paddingRight = getPaddingRight();
        int paddingBottom = getPaddingBottom();
        final int scrollX = getScrollX();

        int decorCount = 0;

        // First pass - decor views. We need to do this in two passes so that
        // we have the proper offsets for non-decor views later.
        for (int i = 0; i < count; i++) {
            final View child = getChildAt(i);
            if (child.getVisibility() != GONE) {
                final LayoutParams lp = (LayoutParams) child.getLayoutParams();
                int childLeft = 0;
                int childTop = 0;
                if (lp.isDecor) {
                    final int hgrav = lp.gravity & Gravity.HORIZONTAL_GRAVITY_MASK;
                    final int vgrav = lp.gravity & Gravity.VERTICAL_GRAVITY_MASK;
                    switch (hgrav) {
                        default:
                            childLeft = paddingLeft;
                            break;
                        case Gravity.LEFT:
                            childLeft = paddingLeft;
                            paddingLeft += child.getMeasuredWidth();
                            break;
                        case Gravity.CENTER_HORIZONTAL:
                            childLeft = Math.max((width - child.getMeasuredWidth()) / 2,
                                    paddingLeft);
                            break;
                        case Gravity.RIGHT:
                            childLeft = width - paddingRight - child.getMeasuredWidth();
                            paddingRight += child.getMeasuredWidth();
                            break;
                    }
                    switch (vgrav) {
                        default:
                            childTop = paddingTop;
                            break;
                        case Gravity.TOP:
                            childTop = paddingTop;
                            paddingTop += child.getMeasuredHeight();
                            break;
                        case Gravity.CENTER_VERTICAL:
                            childTop = Math.max((height - child.getMeasuredHeight()) / 2,
                                    paddingTop);
                            break;
                        case Gravity.BOTTOM:
                            childTop = height - paddingBottom - child.getMeasuredHeight();
                            paddingBottom += child.getMeasuredHeight();
                            break;
                    }
                    childLeft += scrollX;
                    child.layout(childLeft, childTop,
                            childLeft + child.getMeasuredWidth(),
                            childTop + child.getMeasuredHeight());
                    decorCount++;
                }
            }
        }

        final int childWidth = width - paddingLeft - paddingRight;
        // Page views. Do this once we have the right padding offsets from above.
        for (int i = 0; i < count; i++) {
            final View child = getChildAt(i);
            if (child.getVisibility() == GONE) {
                continue;
            }

            final LayoutParams lp = (LayoutParams) child.getLayoutParams();
            if (lp.isDecor) {
                continue;
            }

            final ItemInfo ii = infoForChild(child);
            if (ii == null) {
                continue;
            }

            if (lp.needsMeasure) {
                // This was added during layout and needs measurement.
                // Do it now that we know what we're working with.
                lp.needsMeasure = false;
                final int widthSpec = MeasureSpec.makeMeasureSpec(
                        (int) (childWidth * lp.widthFactor),
                        MeasureSpec.EXACTLY);
                final int heightSpec = MeasureSpec.makeMeasureSpec(
                        (int) (height - paddingTop - paddingBottom),
                        MeasureSpec.EXACTLY);
                child.measure(widthSpec, heightSpec);
            }

            final int childMeasuredWidth = child.getMeasuredWidth();
            final int startOffset = (int) (childWidth * ii.offset);
            final int childLeft;
            if (isLayoutRtl()) {
                childLeft = MAX_SCROLL_X - paddingRight - startOffset - childMeasuredWidth;
            } else {
                childLeft = paddingLeft + startOffset;
            }

            final int childTop = paddingTop;
            child.layout(childLeft, childTop, childLeft + childMeasuredWidth,
                    childTop + child.getMeasuredHeight());
        }

        mTopPageBounds = paddingTop;
        mBottomPageBounds = height - paddingBottom;
        mDecorChildCount = decorCount;

        if (mFirstLayout) {
            scrollToItem(mCurItem, false, 0, false);
        }
        mFirstLayout = false;
    }

    @Override
    public void computeScroll() {
        if (!mScroller.isFinished() && mScroller.computeScrollOffset()) {
            final int oldX = getScrollX();
            final int oldY = getScrollY();
            final int x = mScroller.getCurrX();
            final int y = mScroller.getCurrY();

            if (oldX != x || oldY != y) {
                scrollTo(x, y);

                if (!pageScrolled(x)) {
                    mScroller.abortAnimation();
                    scrollTo(0, y);
                }
            }

            // Keep on drawing until the animation has finished.
            postInvalidateOnAnimation();
            return;
        }

        // Done with scroll, clean up state.
        completeScroll(true);
    }

    private boolean pageScrolled(int scrollX) {
        if (mItems.size() == 0) {
            mCalledSuper = false;
            onPageScrolled(0, 0, 0);
            if (!mCalledSuper) {
                throw new IllegalStateException(
                        "onPageScrolled did not call superclass implementation");
            }
            return false;
        }

        // Translate to scrollX to scrollStart for RTL.
        final int scrollStart;
        if (isLayoutRtl()) {
            scrollStart = MAX_SCROLL_X - scrollX;
        } else {
            scrollStart = scrollX;
        }

        final ItemInfo ii = infoForFirstVisiblePage();
        final int width = getPaddedWidth();
        final int widthWithMargin = width + mPageMargin;
        final float marginOffset = (float) mPageMargin / width;
        final int currentPage = ii.position;
        final float pageOffset = (((float) scrollStart / width) - ii.offset) /
                (ii.widthFactor + marginOffset);
        final int offsetPixels = (int) (pageOffset * widthWithMargin);

        mCalledSuper = false;
        onPageScrolled(currentPage, pageOffset, offsetPixels);
        if (!mCalledSuper) {
            throw new IllegalStateException(
                    "onPageScrolled did not call superclass implementation");
        }
        return true;
    }

    
This method will be invoked when the current page is scrolled, either as part
of a programmatically initiated smooth scroll or a user initiated touch scroll.
If you override this method you must call through to the superclass implementation
(e.g. super.onPageScrolled(position, offset, offsetPixels)) before onPageScrolled
returns.
Params:
position – Position index of the first page currently being displayed.
                Page position+1 will be visible if positionOffset is nonzero.
offset – Value from [0, 1) indicating the offset from the page at position.
offsetPixels – Value in pixels indicating the offset from position./**
     * This method will be invoked when the current page is scrolled, either as part
     * of a programmatically initiated smooth scroll or a user initiated touch scroll.
     * If you override this method you must call through to the superclass implementation
     * (e.g. super.onPageScrolled(position, offset, offsetPixels)) before onPageScrolled
     * returns.
     *
     * @param position Position index of the first page currently being displayed.
     *                 Page position+1 will be visible if positionOffset is nonzero.
     * @param offset Value from [0, 1) indicating the offset from the page at position.
     * @param offsetPixels Value in pixels indicating the offset from position.
     */
    protected void onPageScrolled(int position, float offset, int offsetPixels) {
        // Offset any decor views if needed - keep them on-screen at all times.
        if (mDecorChildCount > 0) {
            final int scrollX = getScrollX();
            int paddingLeft = getPaddingLeft();
            int paddingRight = getPaddingRight();
            final int width = getWidth();
            final int childCount = getChildCount();
            for (int i = 0; i < childCount; i++) {
                final View child = getChildAt(i);
                final LayoutParams lp = (LayoutParams) child.getLayoutParams();
                if (!lp.isDecor) continue;

                final int hgrav = lp.gravity & Gravity.HORIZONTAL_GRAVITY_MASK;
                int childLeft = 0;
                switch (hgrav) {
                    default:
                        childLeft = paddingLeft;
                        break;
                    case Gravity.LEFT:
                        childLeft = paddingLeft;
                        paddingLeft += child.getWidth();
                        break;
                    case Gravity.CENTER_HORIZONTAL:
                        childLeft = Math.max((width - child.getMeasuredWidth()) / 2,
                                paddingLeft);
                        break;
                    case Gravity.RIGHT:
                        childLeft = width - paddingRight - child.getMeasuredWidth();
                        paddingRight += child.getMeasuredWidth();
                        break;
                }
                childLeft += scrollX;

                final int childOffset = childLeft - child.getLeft();
                if (childOffset != 0) {
                    child.offsetLeftAndRight(childOffset);
                }
            }
        }

        if (mOnPageChangeListener != null) {
            mOnPageChangeListener.onPageScrolled(position, offset, offsetPixels);
        }
        if (mInternalPageChangeListener != null) {
            mInternalPageChangeListener.onPageScrolled(position, offset, offsetPixels);
        }

        if (mPageTransformer != null) {
            final int scrollX = getScrollX();
            final int childCount = getChildCount();
            for (int i = 0; i < childCount; i++) {
                final View child = getChildAt(i);
                final LayoutParams lp = (LayoutParams) child.getLayoutParams();

                if (lp.isDecor) continue;

                final float transformPos = (float) (child.getLeft() - scrollX) / getPaddedWidth();
                mPageTransformer.transformPage(child, transformPos);
            }
        }

        mCalledSuper = true;
    }

    private void completeScroll(boolean postEvents) {
        boolean needPopulate = mScrollState == SCROLL_STATE_SETTLING;
        if (needPopulate) {
            // Done with scroll, no longer want to cache view drawing.
            setScrollingCacheEnabled(false);
            mScroller.abortAnimation();
            int oldX = getScrollX();
            int oldY = getScrollY();
            int x = mScroller.getCurrX();
            int y = mScroller.getCurrY();
            if (oldX != x || oldY != y) {
                scrollTo(x, y);
            }
        }
        mPopulatePending = false;
        for (int i=0; i<mItems.size(); i++) {
            ItemInfo ii = mItems.get(i);
            if (ii.scrolling) {
                needPopulate = true;
                ii.scrolling = false;
            }
        }
        if (needPopulate) {
            if (postEvents) {
                postOnAnimation(mEndScrollRunnable);
            } else {
                mEndScrollRunnable.run();
            }
        }
    }

    private boolean isGutterDrag(float x, float dx) {
        return (x < mGutterSize && dx > 0) || (x > getWidth() - mGutterSize && dx < 0);
    }

    private void enableLayers(boolean enable) {
        final int childCount = getChildCount();
        for (int i = 0; i < childCount; i++) {
            final int layerType = enable ? LAYER_TYPE_HARDWARE : LAYER_TYPE_NONE;
            getChildAt(i).setLayerType(layerType, null);
        }
    }

    @Override
    public boolean onInterceptTouchEvent(MotionEvent ev) {
        /*
         * This method JUST determines whether we want to intercept the motion.
         * If we return true, onMotionEvent will be called and we do the actual
         * scrolling there.
         */

        final int action = ev.getAction() & MotionEvent.ACTION_MASK;

        // Always take care of the touch gesture being complete.
        if (action == MotionEvent.ACTION_CANCEL || action == MotionEvent.ACTION_UP) {
            // Release the drag.
            if (DEBUG) Log.v(TAG, "Intercept done!");
            mIsBeingDragged = false;
            mIsUnableToDrag = false;
            mActivePointerId = INVALID_POINTER;
            if (mVelocityTracker != null) {
                mVelocityTracker.recycle();
                mVelocityTracker = null;
            }
            return false;
        }

        // Nothing more to do here if we have decided whether or not we
        // are dragging.
        if (action != MotionEvent.ACTION_DOWN) {
            if (mIsBeingDragged) {
                if (DEBUG) Log.v(TAG, "Being dragged, intercept returning true!");
                return true;
            }
            if (mIsUnableToDrag) {
                if (DEBUG) Log.v(TAG, "Unable to drag, intercept returning false!");
                return false;
            }
        }

        switch (action) {
            case MotionEvent.ACTION_MOVE: {
                /*
                 * mIsBeingDragged == false, otherwise the shortcut would have caught it. Check
                 * whether the user has moved far enough from his original down touch.
                 */

                /*
                * Locally do absolute value. mLastMotionY is set to the y value
                * of the down event.
                */
                final int activePointerId = mActivePointerId;
                if (activePointerId == INVALID_POINTER) {
                    // If we don't have a valid id, the touch down wasn't on content.
                    break;
                }

                final int pointerIndex = ev.findPointerIndex(activePointerId);
                final float x = ev.getX(pointerIndex);
                final float dx = x - mLastMotionX;
                final float xDiff = Math.abs(dx);
                final float y = ev.getY(pointerIndex);
                final float yDiff = Math.abs(y - mInitialMotionY);
                if (DEBUG) Log.v(TAG, "Moved x to " + x + "," + y + " diff=" + xDiff + "," + yDiff);

                if (dx != 0 && !isGutterDrag(mLastMotionX, dx) &&
                        canScroll(this, false, (int) dx, (int) x, (int) y)) {
                    // Nested view has scrollable area under this point. Let it be handled there.
                    mLastMotionX = x;
                    mLastMotionY = y;
                    mIsUnableToDrag = true;
                    return false;
                }
                if (xDiff > mTouchSlop && xDiff * 0.5f > yDiff) {
                    if (DEBUG) Log.v(TAG, "Starting drag!");
                    mIsBeingDragged = true;
                    requestParentDisallowInterceptTouchEvent(true);
                    setScrollState(SCROLL_STATE_DRAGGING);
                    mLastMotionX = dx > 0 ? mInitialMotionX + mTouchSlop :
                            mInitialMotionX - mTouchSlop;
                    mLastMotionY = y;
                    setScrollingCacheEnabled(true);
                } else if (yDiff > mTouchSlop) {
                    // The finger has moved enough in the vertical
                    // direction to be counted as a drag...  abort
                    // any attempt to drag horizontally, to work correctly
                    // with children that have scrolling containers.
                    if (DEBUG) Log.v(TAG, "Starting unable to drag!");
                    mIsUnableToDrag = true;
                }
                if (mIsBeingDragged) {
                    // Scroll to follow the motion event
                    if (performDrag(x)) {
                        postInvalidateOnAnimation();
                    }
                }
                break;
            }

            case MotionEvent.ACTION_DOWN: {
                /*
                 * Remember location of down touch.
                 * ACTION_DOWN always refers to pointer index 0.
                 */
                mLastMotionX = mInitialMotionX = ev.getX();
                mLastMotionY = mInitialMotionY = ev.getY();
                mActivePointerId = ev.getPointerId(0);
                mIsUnableToDrag = false;

                mScroller.computeScrollOffset();
                if (mScrollState == SCROLL_STATE_SETTLING &&
                        Math.abs(mScroller.getFinalX() - mScroller.getCurrX()) > mCloseEnough) {
                    // Let the user 'catch' the pager as it animates.
                    mScroller.abortAnimation();
                    mPopulatePending = false;
                    populate();
                    mIsBeingDragged = true;
                    requestParentDisallowInterceptTouchEvent(true);
                    setScrollState(SCROLL_STATE_DRAGGING);
                } else {
                    completeScroll(false);
                    mIsBeingDragged = false;
                }

                if (DEBUG) Log.v(TAG, "Down at " + mLastMotionX + "," + mLastMotionY
                        + " mIsBeingDragged=" + mIsBeingDragged
                        + "mIsUnableToDrag=" + mIsUnableToDrag);
                break;
            }

            case MotionEvent.ACTION_POINTER_UP:
                onSecondaryPointerUp(ev);
                break;
        }

        if (mVelocityTracker == null) {
            mVelocityTracker = VelocityTracker.obtain();
        }
        mVelocityTracker.addMovement(ev);

        /*
         * The only time we want to intercept motion events is if we are in the
         * drag mode.
         */
        return mIsBeingDragged;
    }

    @Override
    public boolean onTouchEvent(MotionEvent ev) {
        if (ev.getAction() == MotionEvent.ACTION_DOWN && ev.getEdgeFlags() != 0) {
            // Don't handle edge touches immediately -- they may actually belong to one of our
            // descendants.
            return false;
        }

        if (mAdapter == null || mAdapter.getCount() == 0) {
            // Nothing to present or scroll; nothing to touch.
            return false;
        }

        if (mVelocityTracker == null) {
            mVelocityTracker = VelocityTracker.obtain();
        }
        mVelocityTracker.addMovement(ev);

        final int action = ev.getAction();
        boolean needsInvalidate = false;

        switch (action & MotionEvent.ACTION_MASK) {
            case MotionEvent.ACTION_DOWN: {
                mScroller.abortAnimation();
                mPopulatePending = false;
                populate();

                // Remember where the motion event started
                mLastMotionX = mInitialMotionX = ev.getX();
                mLastMotionY = mInitialMotionY = ev.getY();
                mActivePointerId = ev.getPointerId(0);
                break;
            }
            case MotionEvent.ACTION_MOVE:
                if (!mIsBeingDragged) {
                    final int pointerIndex = ev.findPointerIndex(mActivePointerId);
                    final float x = ev.getX(pointerIndex);
                    final float xDiff = Math.abs(x - mLastMotionX);
                    final float y = ev.getY(pointerIndex);
                    final float yDiff = Math.abs(y - mLastMotionY);
                    if (DEBUG) Log.v(TAG, "Moved x to " + x + "," + y + " diff=" + xDiff + "," + yDiff);
                    if (xDiff > mTouchSlop && xDiff > yDiff) {
                        if (DEBUG) Log.v(TAG, "Starting drag!");
                        mIsBeingDragged = true;
                        requestParentDisallowInterceptTouchEvent(true);
                        mLastMotionX = x - mInitialMotionX > 0 ? mInitialMotionX + mTouchSlop :
                                mInitialMotionX - mTouchSlop;
                        mLastMotionY = y;
                        setScrollState(SCROLL_STATE_DRAGGING);
                        setScrollingCacheEnabled(true);

                        // Disallow Parent Intercept, just in case
                        ViewParent parent = getParent();
                        if (parent != null) {
                            parent.requestDisallowInterceptTouchEvent(true);
                        }
                    }
                }
                // Not else! Note that mIsBeingDragged can be set above.
                if (mIsBeingDragged) {
                    // Scroll to follow the motion event
                    final int activePointerIndex = ev.findPointerIndex(mActivePointerId);
                    final float x = ev.getX(activePointerIndex);
                    needsInvalidate |= performDrag(x);
                }
                break;
            case MotionEvent.ACTION_UP:
                if (mIsBeingDragged) {
                    final VelocityTracker velocityTracker = mVelocityTracker;
                    velocityTracker.computeCurrentVelocity(1000, mMaximumVelocity);
                    final int initialVelocity = (int) velocityTracker.getXVelocity(mActivePointerId);

                    mPopulatePending = true;

                    final float scrollStart = getScrollStart();
                    final float scrolledPages = scrollStart / getPaddedWidth();
                    final ItemInfo ii = infoForFirstVisiblePage();
                    final int currentPage = ii.position;
                    final float nextPageOffset;
                    if (isLayoutRtl()) {
                        nextPageOffset = (ii.offset - scrolledPages) / ii.widthFactor;
                    }  else {
                        nextPageOffset = (scrolledPages - ii.offset) / ii.widthFactor;
                    }

                    final int activePointerIndex = ev.findPointerIndex(mActivePointerId);
                    final float x = ev.getX(activePointerIndex);
                    final int totalDelta = (int) (x - mInitialMotionX);
                    final int nextPage = determineTargetPage(
                            currentPage, nextPageOffset, initialVelocity, totalDelta);
                    setCurrentItemInternal(nextPage, true, true, initialVelocity);

                    mActivePointerId = INVALID_POINTER;
                    endDrag();
                    mLeftEdge.onRelease();
                    mRightEdge.onRelease();
                    needsInvalidate = true;
                }
                break;
            case MotionEvent.ACTION_CANCEL:
                if (mIsBeingDragged) {
                    scrollToItem(mCurItem, true, 0, false);
                    mActivePointerId = INVALID_POINTER;
                    endDrag();
                    mLeftEdge.onRelease();
                    mRightEdge.onRelease();
                    needsInvalidate = true;
                }
                break;
            case MotionEvent.ACTION_POINTER_DOWN: {
                final int index = ev.getActionIndex();
                final float x = ev.getX(index);
                mLastMotionX = x;
                mActivePointerId = ev.getPointerId(index);
                break;
            }
            case MotionEvent.ACTION_POINTER_UP:
                onSecondaryPointerUp(ev);
                mLastMotionX = ev.getX(ev.findPointerIndex(mActivePointerId));
                break;
        }
        if (needsInvalidate) {
            postInvalidateOnAnimation();
        }
        return true;
    }

    private void requestParentDisallowInterceptTouchEvent(boolean disallowIntercept) {
        final ViewParent parent = getParent();
        if (parent != null) {
            parent.requestDisallowInterceptTouchEvent(disallowIntercept);
        }
    }

    private boolean performDrag(float x) {
        boolean needsInvalidate = false;

        final int width = getPaddedWidth();
        final float deltaX = mLastMotionX - x;
        mLastMotionX = x;

        final EdgeEffect startEdge;
        final EdgeEffect endEdge;
        if (isLayoutRtl()) {
            startEdge = mRightEdge;
            endEdge = mLeftEdge;
        } else {
            startEdge = mLeftEdge;
            endEdge = mRightEdge;
        }

        // Translate scroll to relative coordinates.
        final float nextScrollX = getScrollX() + deltaX;
        final float scrollStart;
        if (isLayoutRtl()) {
            scrollStart = MAX_SCROLL_X - nextScrollX;
        } else {
            scrollStart = nextScrollX;
        }

        final float startBound;
        final ItemInfo startItem = mItems.get(0);
        final boolean startAbsolute = startItem.position == 0;
        if (startAbsolute) {
            startBound = startItem.offset * width;
        } else {
            startBound = width * mFirstOffset;
        }

        final float endBound;
        final ItemInfo endItem = mItems.get(mItems.size() - 1);
        final boolean endAbsolute = endItem.position == mAdapter.getCount() - 1;
        if (endAbsolute) {
            endBound = endItem.offset * width;
        } else {
            endBound = width * mLastOffset;
        }

        final float clampedScrollStart;
        if (scrollStart < startBound) {
            if (startAbsolute) {
                final float over = startBound - scrollStart;
                startEdge.onPull(Math.abs(over) / width);
                needsInvalidate = true;
            }
            clampedScrollStart = startBound;
        } else if (scrollStart > endBound) {
            if (endAbsolute) {
                final float over = scrollStart - endBound;
                endEdge.onPull(Math.abs(over) / width);
                needsInvalidate = true;
            }
            clampedScrollStart = endBound;
        } else {
            clampedScrollStart = scrollStart;
        }

        // Translate back to absolute coordinates.
        final float targetScrollX;
        if (isLayoutRtl()) {
            targetScrollX = MAX_SCROLL_X - clampedScrollStart;
        } else {
            targetScrollX = clampedScrollStart;
        }

        // Don't lose the rounded component.
        mLastMotionX += targetScrollX - (int) targetScrollX;

        scrollTo((int) targetScrollX, getScrollY());
        pageScrolled((int) targetScrollX);

        return needsInvalidate;
    }

    
Returns:
Info about the page at the current scroll position.
        This can be synthetic for a missing middle page; the 'object' field can be null./**
     * @return Info about the page at the current scroll position.
     *         This can be synthetic for a missing middle page; the 'object' field can be null.
     */
    private ItemInfo infoForFirstVisiblePage() {
        final int startOffset = getScrollStart();
        final int width = getPaddedWidth();
        final float scrollOffset = width > 0 ? (float) startOffset / width : 0;
        final float marginOffset = width > 0 ? (float) mPageMargin / width : 0;

        int lastPos = -1;
        float lastOffset = 0.f;
        float lastWidth = 0.f;
        boolean first = true;
        ItemInfo lastItem = null;

        final int N = mItems.size();
        for (int i = 0; i < N; i++) {
            ItemInfo ii = mItems.get(i);

            // Seek to position.
            if (!first && ii.position != lastPos + 1) {
                // Create a synthetic item for a missing page.
                ii = mTempItem;
                ii.offset = lastOffset + lastWidth + marginOffset;
                ii.position = lastPos + 1;
                ii.widthFactor = mAdapter.getPageWidth(ii.position);
                i--;
            }

            final float offset = ii.offset;
            final float startBound = offset;
            if (first || scrollOffset >= startBound) {
                final float endBound = offset + ii.widthFactor + marginOffset;
                if (scrollOffset < endBound || i == mItems.size() - 1) {
                    return ii;
                }
            } else {
                return lastItem;
            }

            first = false;
            lastPos = ii.position;
            lastOffset = offset;
            lastWidth = ii.widthFactor;
            lastItem = ii;
        }

        return lastItem;
    }

    private int getScrollStart() {
        if (isLayoutRtl()) {
            return MAX_SCROLL_X - getScrollX();
        } else {
            return getScrollX();
        }
    }

    
Params:
currentPage – the position of the page with the first visible starting edge
pageOffset – the fraction of the right-hand page that's visible
velocity – the velocity of the touch event stream
deltaX – the distance of the touch event stream
Returns:
the position of the target page/**
     * @param currentPage the position of the page with the first visible starting edge
     * @param pageOffset the fraction of the right-hand page that's visible
     * @param velocity the velocity of the touch event stream
     * @param deltaX the distance of the touch event stream
     * @return the position of the target page
     */
    private int determineTargetPage(int currentPage, float pageOffset, int velocity, int deltaX) {
        int targetPage;
        if (Math.abs(deltaX) > mFlingDistance && Math.abs(velocity) > mMinimumVelocity) {
            targetPage = currentPage - (velocity < 0 ? mLeftIncr : 0);
        } else {
            final float truncator = currentPage >= mCurItem ? 0.4f : 0.6f;
            targetPage = (int) (currentPage - mLeftIncr * (pageOffset + truncator));
        }

        if (mItems.size() > 0) {
            final ItemInfo firstItem = mItems.get(0);
            final ItemInfo lastItem = mItems.get(mItems.size() - 1);

            // Only let the user target pages we have items for
            targetPage = MathUtils.constrain(targetPage, firstItem.position, lastItem.position);
        }

        return targetPage;
    }

    @Override
    public void draw(Canvas canvas) {
        super.draw(canvas);
        boolean needsInvalidate = false;

        final int overScrollMode = getOverScrollMode();
        if (overScrollMode == View.OVER_SCROLL_ALWAYS ||
                (overScrollMode == View.OVER_SCROLL_IF_CONTENT_SCROLLS &&
                        mAdapter != null && mAdapter.getCount() > 1)) {
            if (!mLeftEdge.isFinished()) {
                final int restoreCount = canvas.save();
                final int height = getHeight() - getPaddingTop() - getPaddingBottom();
                final int width = getWidth();

                canvas.rotate(270);
                canvas.translate(-height + getPaddingTop(), mFirstOffset * width);
                mLeftEdge.setSize(height, width);
                needsInvalidate |= mLeftEdge.draw(canvas);
                canvas.restoreToCount(restoreCount);
            }
            if (!mRightEdge.isFinished()) {
                final int restoreCount = canvas.save();
                final int width = getWidth();
                final int height = getHeight() - getPaddingTop() - getPaddingBottom();

                canvas.rotate(90);
                canvas.translate(-getPaddingTop(), -(mLastOffset + 1) * width);
                mRightEdge.setSize(height, width);
                needsInvalidate |= mRightEdge.draw(canvas);
                canvas.restoreToCount(restoreCount);
            }
        } else {
            mLeftEdge.finish();
            mRightEdge.finish();
        }

        if (needsInvalidate) {
            // Keep animating
            postInvalidateOnAnimation();
        }
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);

        // Draw the margin drawable between pages if needed.
        if (mPageMargin > 0 && mMarginDrawable != null && mItems.size() > 0 && mAdapter != null) {
            final int scrollX = getScrollX();
            final int width = getWidth();

            final float marginOffset = (float) mPageMargin / width;
            int itemIndex = 0;
            ItemInfo ii = mItems.get(0);
            float offset = ii.offset;

            final int itemCount = mItems.size();
            final int firstPos = ii.position;
            final int lastPos = mItems.get(itemCount - 1).position;
            for (int pos = firstPos; pos < lastPos; pos++) {
                while (pos > ii.position && itemIndex < itemCount) {
                    ii = mItems.get(++itemIndex);
                }

                final float itemOffset;
                final float widthFactor;
                if (pos == ii.position) {
                    itemOffset = ii.offset;
                    widthFactor = ii.widthFactor;
                } else {
                    itemOffset = offset;
                    widthFactor = mAdapter.getPageWidth(pos);
                }

                final float left;
                final float scaledOffset = itemOffset * width;
                if (isLayoutRtl()) {
                    left = MAX_SCROLL_X - scaledOffset;
                } else {
                    left = scaledOffset + widthFactor * width;
                }

                offset = itemOffset + widthFactor + marginOffset;

                if (left + mPageMargin > scrollX) {
                    mMarginDrawable.setBounds((int) left, mTopPageBounds,
                            (int) (left + mPageMargin + 0.5f), mBottomPageBounds);
                    mMarginDrawable.draw(canvas);
                }

                if (left > scrollX + width) {
                    break; // No more visible, no sense in continuing
                }
            }
        }
    }

    private void onSecondaryPointerUp(MotionEvent ev) {
        final int pointerIndex = ev.getActionIndex();
        final int pointerId = ev.getPointerId(pointerIndex);
        if (pointerId == mActivePointerId) {
            // This was our active pointer going up. Choose a new
            // active pointer and adjust accordingly.
            final int newPointerIndex = pointerIndex == 0 ? 1 : 0;
            mLastMotionX = ev.getX(newPointerIndex);
            mActivePointerId = ev.getPointerId(newPointerIndex);
            if (mVelocityTracker != null) {
                mVelocityTracker.clear();
            }
        }
    }

    private void endDrag() {
        mIsBeingDragged = false;
        mIsUnableToDrag = false;

        if (mVelocityTracker != null) {
            mVelocityTracker.recycle();
            mVelocityTracker = null;
        }
    }

    private void setScrollingCacheEnabled(boolean enabled) {
        if (mScrollingCacheEnabled != enabled) {
            mScrollingCacheEnabled = enabled;
            if (USE_CACHE) {
                final int size = getChildCount();
                for (int i = 0; i < size; ++i) {
                    final View child = getChildAt(i);
                    if (child.getVisibility() != GONE) {
                        child.setDrawingCacheEnabled(enabled);
                    }
                }
            }
        }
    }

    public boolean canScrollHorizontally(int direction) {
        if (mAdapter == null) {
            return false;
        }

        final int width = getPaddedWidth();
        final int scrollX = getScrollX();
        if (direction < 0) {
            return (scrollX > (int) (width * mFirstOffset));
        } else if (direction > 0) {
            return (scrollX < (int) (width * mLastOffset));
        } else {
            return false;
        }
    }

    
Tests scrollability within child views of v given a delta of dx.
Params:
v – View to test for horizontal scrollability
checkV – Whether the view v passed should itself be checked for scrollability (true),
              or just its children (false).
dx – Delta scrolled in pixels
x – X coordinate of the active touch point
y – Y coordinate of the active touch point
Returns:
true if child views of v can be scrolled by delta of dx./**
     * Tests scrollability within child views of v given a delta of dx.
     *
     * @param v View to test for horizontal scrollability
     * @param checkV Whether the view v passed should itself be checked for scrollability (true),
     *               or just its children (false).
     * @param dx Delta scrolled in pixels
     * @param x X coordinate of the active touch point
     * @param y Y coordinate of the active touch point
     * @return true if child views of v can be scrolled by delta of dx.
     */
    protected boolean canScroll(View v, boolean checkV, int dx, int x, int y) {
        if (v instanceof ViewGroup) {
            final ViewGroup group = (ViewGroup) v;
            final int scrollX = v.getScrollX();
            final int scrollY = v.getScrollY();
            final int count = group.getChildCount();
            // Count backwards - let topmost views consume scroll distance first.
            for (int i = count - 1; i >= 0; i--) {
                // TODO: Add support for transformed views.
                final View child = group.getChildAt(i);
                if (x + scrollX >= child.getLeft() && x + scrollX < child.getRight()
                        && y + scrollY >= child.getTop() && y + scrollY < child.getBottom()
                        && canScroll(child, true, dx, x + scrollX - child.getLeft(),
                                y + scrollY - child.getTop())) {
                    return true;
                }
            }
        }

        return checkV && v.canScrollHorizontally(-dx);
    }

    @Override
    public boolean dispatchKeyEvent(KeyEvent event) {
        // Let the focused view and/or our descendants get the key first
        return super.dispatchKeyEvent(event) || executeKeyEvent(event);
    }

    
You can call this function yourself to have the scroll view perform
scrolling from a key event, just as if the event had been dispatched to
it by the view hierarchy.
Params:
event – The key event to execute.
Returns:
Return true if the event was handled, else false./**
     * You can call this function yourself to have the scroll view perform
     * scrolling from a key event, just as if the event had been dispatched to
     * it by the view hierarchy.
     *
     * @param event The key event to execute.
     * @return Return true if the event was handled, else false.
     */
    public boolean executeKeyEvent(KeyEvent event) {
        boolean handled = false;
        if (event.getAction() == KeyEvent.ACTION_DOWN) {
            switch (event.getKeyCode()) {
                case KeyEvent.KEYCODE_DPAD_LEFT:
                    handled = arrowScroll(FOCUS_LEFT);
                    break;
                case KeyEvent.KEYCODE_DPAD_RIGHT:
                    handled = arrowScroll(FOCUS_RIGHT);
                    break;
                case KeyEvent.KEYCODE_TAB:
                    if (event.hasNoModifiers()) {
                        handled = arrowScroll(FOCUS_FORWARD);
                    } else if (event.hasModifiers(KeyEvent.META_SHIFT_ON)) {
                        handled = arrowScroll(FOCUS_BACKWARD);
                    }
                    break;
            }
        }
        return handled;
    }

    public boolean arrowScroll(int direction) {
        View currentFocused = findFocus();
        if (currentFocused == this) {
            currentFocused = null;
        } else if (currentFocused != null) {
            boolean isChild = false;
            for (ViewParent parent = currentFocused.getParent(); parent instanceof ViewGroup;
                    parent = parent.getParent()) {
                if (parent == this) {
                    isChild = true;
                    break;
                }
            }
            if (!isChild) {
                // This would cause the focus search down below to fail in fun ways.
                final StringBuilder sb = new StringBuilder();
                sb.append(currentFocused.getClass().getSimpleName());
                for (ViewParent parent = currentFocused.getParent(); parent instanceof ViewGroup;
                        parent = parent.getParent()) {
                    sb.append(" => ").append(parent.getClass().getSimpleName());
                }
                Log.e(TAG, "arrowScroll tried to find focus based on non-child " +
                        "current focused view " + sb.toString());
                currentFocused = null;
            }
        }

        boolean handled = false;

        View nextFocused = FocusFinder.getInstance().findNextFocus(this, currentFocused,
                direction);
        if (nextFocused != null && nextFocused != currentFocused) {
            if (direction == View.FOCUS_LEFT) {
                // If there is nothing to the left, or this is causing us to
                // jump to the right, then what we really want to do is page left.
                final int nextLeft = getChildRectInPagerCoordinates(mTempRect, nextFocused).left;
                final int currLeft = getChildRectInPagerCoordinates(mTempRect, currentFocused).left;
                if (currentFocused != null && nextLeft >= currLeft) {
                    handled = pageLeft();
                } else {
                    handled = nextFocused.requestFocus();
                }
            } else if (direction == View.FOCUS_RIGHT) {
                // If there is nothing to the right, or this is causing us to
                // jump to the left, then what we really want to do is page right.
                final int nextLeft = getChildRectInPagerCoordinates(mTempRect, nextFocused).left;
                final int currLeft = getChildRectInPagerCoordinates(mTempRect, currentFocused).left;
                if (currentFocused != null && nextLeft <= currLeft) {
                    handled = pageRight();
                } else {
                    handled = nextFocused.requestFocus();
                }
            }
        } else if (direction == FOCUS_LEFT || direction == FOCUS_BACKWARD) {
            // Trying to move left and nothing there; try to page.
            handled = pageLeft();
        } else if (direction == FOCUS_RIGHT || direction == FOCUS_FORWARD) {
            // Trying to move right and nothing there; try to page.
            handled = pageRight();
        }
        if (handled) {
            playSoundEffect(SoundEffectConstants.getContantForFocusDirection(direction));
        }
        return handled;
    }

    private Rect getChildRectInPagerCoordinates(Rect outRect, View child) {
        if (outRect == null) {
            outRect = new Rect();
        }
        if (child == null) {
            outRect.set(0, 0, 0, 0);
            return outRect;
        }
        outRect.left = child.getLeft();
        outRect.right = child.getRight();
        outRect.top = child.getTop();
        outRect.bottom = child.getBottom();

        ViewParent parent = child.getParent();
        while (parent instanceof ViewGroup && parent != this) {
            final ViewGroup group = (ViewGroup) parent;
            outRect.left += group.getLeft();
            outRect.right += group.getRight();
            outRect.top += group.getTop();
            outRect.bottom += group.getBottom();

            parent = group.getParent();
        }
        return outRect;
    }

    boolean pageLeft() {
        return setCurrentItemInternal(mCurItem + mLeftIncr, true, false);
    }

    boolean pageRight() {
        return setCurrentItemInternal(mCurItem - mLeftIncr, true, false);
    }

    @Override
    public void onRtlPropertiesChanged(@ResolvedLayoutDir int layoutDirection) {
        super.onRtlPropertiesChanged(layoutDirection);

        if (layoutDirection == LAYOUT_DIRECTION_LTR) {
            mLeftIncr = -1;
        } else {
            mLeftIncr = 1;
        }
    }

    
We only want the current page that is being shown to be focusable.
/**
     * We only want the current page that is being shown to be focusable.
     */
    @Override
    public void addFocusables(ArrayList<View> views, int direction, int focusableMode) {
        final int focusableCount = views.size();

        final int descendantFocusability = getDescendantFocusability();

        if (descendantFocusability != FOCUS_BLOCK_DESCENDANTS) {
            for (int i = 0; i < getChildCount(); i++) {
                final View child = getChildAt(i);
                if (child.getVisibility() == VISIBLE) {
                    ItemInfo ii = infoForChild(child);
                    if (ii != null && ii.position == mCurItem) {
                        child.addFocusables(views, direction, focusableMode);
                    }
                }
            }
        }

        // we add ourselves (if focusable) in all cases except for when we are
        // FOCUS_AFTER_DESCENDANTS and there are some descendants focusable.  this is
        // to avoid the focus search finding layouts when a more precise search
        // among the focusable children would be more interesting.
        if (
            descendantFocusability != FOCUS_AFTER_DESCENDANTS ||
                // No focusable descendants
                (focusableCount == views.size())) {
            // Note that we can't call the superclass here, because it will
            // add all views in.  So we need to do the same thing View does.
            if (!isFocusable()) {
                return;
            }
            if ((focusableMode & FOCUSABLES_TOUCH_MODE) == FOCUSABLES_TOUCH_MODE &&
                    isInTouchMode() && !isFocusableInTouchMode()) {
                return;
            }
            if (views != null) {
                views.add(this);
            }
        }
    }

    
We only want the current page that is being shown to be touchable.
/**
     * We only want the current page that is being shown to be touchable.
     */
    @Override
    public void addTouchables(ArrayList<View> views) {
        // Note that we don't call super.addTouchables(), which means that
        // we don't call View.addTouchables().  This is okay because a ViewPager
        // is itself not touchable.
        for (int i = 0; i < getChildCount(); i++) {
            final View child = getChildAt(i);
            if (child.getVisibility() == VISIBLE) {
                ItemInfo ii = infoForChild(child);
                if (ii != null && ii.position == mCurItem) {
                    child.addTouchables(views);
                }
            }
        }
    }

    
We only want the current page that is being shown to be focusable.
/**
     * We only want the current page that is being shown to be focusable.
     */
    @Override
    protected boolean onRequestFocusInDescendants(int direction,
            Rect previouslyFocusedRect) {
        int index;
        int increment;
        int end;
        int count = getChildCount();
        if ((direction & FOCUS_FORWARD) != 0) {
            index = 0;
            increment = 1;
            end = count;
        } else {
            index = count - 1;
            increment = -1;
            end = -1;
        }
        for (int i = index; i != end; i += increment) {
            View child = getChildAt(i);
            if (child.getVisibility() == VISIBLE) {
                ItemInfo ii = infoForChild(child);
                if (ii != null && ii.position == mCurItem) {
                    if (child.requestFocus(direction, previouslyFocusedRect)) {
                        return true;
                    }
                }
            }
        }
        return false;
    }

    @Override
    protected ViewGroup.LayoutParams generateDefaultLayoutParams() {
        return new LayoutParams();
    }

    @Override
    protected ViewGroup.LayoutParams generateLayoutParams(ViewGroup.LayoutParams p) {
        return generateDefaultLayoutParams();
    }

    @Override
    protected boolean checkLayoutParams(ViewGroup.LayoutParams p) {
        return p instanceof LayoutParams && super.checkLayoutParams(p);
    }

    @Override
    public ViewGroup.LayoutParams generateLayoutParams(AttributeSet attrs) {
        return new LayoutParams(getContext(), attrs);
    }


    @Override
    public void onInitializeAccessibilityEvent(AccessibilityEvent event) {
        super.onInitializeAccessibilityEvent(event);

        event.setClassName(ViewPager.class.getName());
        event.setScrollable(canScroll());

        if (event.getEventType() == AccessibilityEvent.TYPE_VIEW_SCROLLED && mAdapter != null) {
            event.setItemCount(mAdapter.getCount());
            event.setFromIndex(mCurItem);
            event.setToIndex(mCurItem);
        }
    }

    @Override
    public void onInitializeAccessibilityNodeInfo(AccessibilityNodeInfo info) {
        super.onInitializeAccessibilityNodeInfo(info);

        info.setClassName(ViewPager.class.getName());
        info.setScrollable(canScroll());

        if (canScrollHorizontally(1)) {
            info.addAction(AccessibilityAction.ACTION_SCROLL_FORWARD);
            info.addAction(AccessibilityAction.ACTION_SCROLL_RIGHT);
        }

        if (canScrollHorizontally(-1)) {
            info.addAction(AccessibilityAction.ACTION_SCROLL_BACKWARD);
            info.addAction(AccessibilityAction.ACTION_SCROLL_LEFT);
        }
    }

    @Override
    public boolean performAccessibilityAction(int action, Bundle args) {
        if (super.performAccessibilityAction(action, args)) {
            return true;
        }

        switch (action) {
            case AccessibilityNodeInfo.ACTION_SCROLL_FORWARD:
            case R.id.accessibilityActionScrollRight:
                if (canScrollHorizontally(1)) {
                    setCurrentItem(mCurItem + 1);
                    return true;
                }
                return false;
            case AccessibilityNodeInfo.ACTION_SCROLL_BACKWARD:
            case R.id.accessibilityActionScrollLeft:
                if (canScrollHorizontally(-1)) {
                    setCurrentItem(mCurItem - 1);
                    return true;
                }
                return false;
        }

        return false;
    }

    private boolean canScroll() {
        return mAdapter != null && mAdapter.getCount() > 1;
    }

    private class PagerObserver extends DataSetObserver {
        @Override
        public void onChanged() {
            dataSetChanged();
        }
        @Override
        public void onInvalidated() {
            dataSetChanged();
        }
    }

    
Layout parameters that should be supplied for views added to a
ViewPager.
/**
     * Layout parameters that should be supplied for views added to a
     * ViewPager.
     */
    public static class LayoutParams extends ViewGroup.LayoutParams {
        
true if this view is a decoration on the pager itself and not
a view supplied by the adapter.
/**
         * true if this view is a decoration on the pager itself and not
         * a view supplied by the adapter.
         */
        public boolean isDecor;

        
Gravity setting for use on decor views only: Where to position the view page within the overall ViewPager container; constants are defined in Gravity. /**
         * Gravity setting for use on decor views only:
         * Where to position the view page within the overall ViewPager
         * container; constants are defined in {@link android.view.Gravity}.
         */
        public int gravity;

        
Width as a 0-1 multiplier of the measured pager width
/**
         * Width as a 0-1 multiplier of the measured pager width
         */
        float widthFactor = 0.f;

        
true if this view was added during layout and needs to be measured
before being positioned.
/**
         * true if this view was added during layout and needs to be measured
         * before being positioned.
         */
        boolean needsMeasure;

        
Adapter position this view is for if !isDecor
/**
         * Adapter position this view is for if !isDecor
         */
        int position;

        
Current child index within the ViewPager that this view occupies
/**
         * Current child index within the ViewPager that this view occupies
         */
        int childIndex;

        public LayoutParams() {
            super(FILL_PARENT, FILL_PARENT);
        }

        public LayoutParams(Context context, AttributeSet attrs) {
            super(context, attrs);

            final TypedArray a = context.obtainStyledAttributes(attrs, LAYOUT_ATTRS);
            gravity = a.getInteger(0, Gravity.TOP);
            a.recycle();
        }
    }

    static class ViewPositionComparator implements Comparator<View> {
        @Override
        public int compare(View lhs, View rhs) {
            final LayoutParams llp = (LayoutParams) lhs.getLayoutParams();
            final LayoutParams rlp = (LayoutParams) rhs.getLayoutParams();
            if (llp.isDecor != rlp.isDecor) {
                return llp.isDecor ? 1 : -1;
            }
            return llp.position - rlp.position;
        }
    }
}