/*
 * Copyright (C) 2008 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 android.os;

import android.annotation.MainThread;
import android.annotation.Nullable;
import android.annotation.WorkerThread;
import java.util.ArrayDeque;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.Callable;
import java.util.concurrent.CancellationException;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Executor;
import java.util.concurrent.FutureTask;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.atomic.AtomicInteger;

AsyncTask enables proper and easy use of the UI thread. This class allows you
to perform background operations and publish results on the UI thread without
having to manipulate threads and/or handlers.
AsyncTask is designed to be a helper class around Thread and Handler and does not constitute a generic threading framework. AsyncTasks should ideally be used for short operations (a few seconds at the most.) If you need to keep threads running for long periods of time, it is highly recommended you use the various APIs provided by the java.util.concurrent package such as Executor, ThreadPoolExecutor and FutureTask.
An asynchronous task is defined by a computation that runs on a background thread and
whose result is published on the UI thread. An asynchronous task is defined by 3 generic
types, called Params, Progress and Result,
and 4 steps, called onPreExecute, doInBackground,
onProgressUpdate and onPostExecute.

Developer Guides
For more information about using tasks and threads, read the
Processes and
Threads developer guide.

Usage
AsyncTask must be subclassed to be used. The subclass will override at least one method (doInBackground), and most often will override a second one (onPostExecute.)
Here is an example of subclassing:
private class DownloadFilesTask extends AsyncTask<URL, Integer, Long> {
    protected Long doInBackground(URL... urls) {
        int count = urls.length;
        long totalSize = 0;
        for (int i = 0; i < count; i++) {
            totalSize += Downloader.downloadFile(urls[i]);
            publishProgress((int) ((i / (float) count) * 100));
            // Escape early if cancel() is called
            if (isCancelled()) break;
        }
        return totalSize;
    }
    protected void onProgressUpdate(Integer... progress) {
        setProgressPercent(progress[0]);
    }
    protected void onPostExecute(Long result) {
        showDialog("Downloaded " + result + " bytes");
    }
}

Once created, a task is executed very simply:
new DownloadFilesTask().execute(url1, url2, url3);

AsyncTask's generic types
The three types used by an asynchronous task are the following:

    
Params, the type of the parameters sent to the task upon
    execution.
    
Progress, the type of the progress units published during
    the background computation.
    
Result, the type of the result of the background
    computation.

Not all types are always used by an asynchronous task. To mark a type as unused, simply use the type Void:
private class MyTask extends AsyncTask<Void, Void, Void> { ... }

The 4 steps
When an asynchronous task is executed, the task goes through 4 steps:

    
onPreExecute(), invoked on the UI thread before the task is executed. This step is normally used to setup the task, for instance by showing a progress bar in the user interface.
    
doInBackground, invoked on the background thread immediately after onPreExecute() finishes executing. This step is used to perform background computation that can take a long time. The parameters of the asynchronous task are passed to this step. The result of the computation must be returned by this step and will be passed back to the last step. This step can also use publishProgress to publish one or more units of progress. These values are published on the UI thread, in the onProgressUpdate step.
    
onProgressUpdate, invoked on the UI thread after a call to publishProgress. The timing of the execution is undefined. This method is used to display any form of progress in the user interface while the background computation is still executing. For instance, it can be used to animate a progress bar or show logs in a text field.
    
onPostExecute, invoked on the UI thread after the background computation finishes. The result of the background computation is passed to this step as a parameter.

Cancelling a task
A task can be cancelled at any time by invoking cancel(boolean). Invoking this method will cause subsequent calls to isCancelled() to return true. After invoking this method, onCancelled(Object), instead of onPostExecute(Object) will be invoked after doInBackground(Object[]) returns. To ensure that a task is cancelled as quickly as possible, you should always check the return value of isCancelled() periodically from doInBackground(Object[]), if possible (inside a loop for instance.)
Threading rules
There are a few threading rules that must be followed for this class to
work properly:

    
The AsyncTask class must be loaded on the UI thread. This is done automatically as of VERSION_CODES.JELLY_BEAN.
    
The task instance must be created on the UI thread.
    
execute must be invoked on the UI thread.
    
Do not call onPreExecute(), onPostExecute, doInBackground, onProgressUpdate manually.
    
The task can be executed only once (an exception will be thrown if
    a second execution is attempted.)

Memory observability
AsyncTask guarantees that all callback calls are synchronized in such a way that the following
operations are safe without explicit synchronizations.

    
Set member fields in the constructor or onPreExecute, and refer to them in doInBackground. 
Set member fields in doInBackground, and refer to them in onProgressUpdate and onPostExecute. 
Order of execution
When first introduced, AsyncTasks were executed serially on a single background thread. Starting with VERSION_CODES.DONUT, this was changed to a pool of threads allowing multiple tasks to operate in parallel. Starting with VERSION_CODES.HONEYCOMB, tasks are executed on a single thread to avoid common application errors caused by parallel execution.
If you truly want parallel execution, you can invoke executeOnExecutor(Executor, Object[]) with AsyncTask<Params,Progress,Result>.THREAD_POOL_EXECUTOR.
/**
 * <p>AsyncTask enables proper and easy use of the UI thread. This class allows you
 * to perform background operations and publish results on the UI thread without
 * having to manipulate threads and/or handlers.</p>
 *
 * <p>AsyncTask is designed to be a helper class around {@link Thread} and {@link Handler}
 * and does not constitute a generic threading framework. AsyncTasks should ideally be
 * used for short operations (a few seconds at the most.) If you need to keep threads
 * running for long periods of time, it is highly recommended you use the various APIs
 * provided by the <code>java.util.concurrent</code> package such as {@link Executor},
 * {@link ThreadPoolExecutor} and {@link FutureTask}.</p>
 *
 * <p>An asynchronous task is defined by a computation that runs on a background thread and
 * whose result is published on the UI thread. An asynchronous task is defined by 3 generic
 * types, called <code>Params</code>, <code>Progress</code> and <code>Result</code>,
 * and 4 steps, called <code>onPreExecute</code>, <code>doInBackground</code>,
 * <code>onProgressUpdate</code> and <code>onPostExecute</code>.</p>
 *
 * <div class="special reference">
 * <h3>Developer Guides</h3>
 * <p>For more information about using tasks and threads, read the
 * <a href="{@docRoot}guide/components/processes-and-threads.html">Processes and
 * Threads</a> developer guide.</p>
 * </div>
 *
 * <h2>Usage</h2>
 * <p>AsyncTask must be subclassed to be used. The subclass will override at least
 * one method ({@link #doInBackground}), and most often will override a
 * second one ({@link #onPostExecute}.)</p>
 *
 * <p>Here is an example of subclassing:</p>
 * <pre class="prettyprint">
 * private class DownloadFilesTask extends AsyncTask&lt;URL, Integer, Long&gt; {
 *     protected Long doInBackground(URL... urls) {
 *         int count = urls.length;
 *         long totalSize = 0;
 *         for (int i = 0; i < count; i++) {
 *             totalSize += Downloader.downloadFile(urls[i]);
 *             publishProgress((int) ((i / (float) count) * 100));
 *             // Escape early if cancel() is called
 *             if (isCancelled()) break;
 *         }
 *         return totalSize;
 *     }
 *
 *     protected void onProgressUpdate(Integer... progress) {
 *         setProgressPercent(progress[0]);
 *     }
 *
 *     protected void onPostExecute(Long result) {
 *         showDialog("Downloaded " + result + " bytes");
 *     }
 * }
 * </pre>
 *
 * <p>Once created, a task is executed very simply:</p>
 * <pre class="prettyprint">
 * new DownloadFilesTask().execute(url1, url2, url3);
 * </pre>
 *
 * <h2>AsyncTask's generic types</h2>
 * <p>The three types used by an asynchronous task are the following:</p>
 * <ol>
 *     <li><code>Params</code>, the type of the parameters sent to the task upon
 *     execution.</li>
 *     <li><code>Progress</code>, the type of the progress units published during
 *     the background computation.</li>
 *     <li><code>Result</code>, the type of the result of the background
 *     computation.</li>
 * </ol>
 * <p>Not all types are always used by an asynchronous task. To mark a type as unused,
 * simply use the type {@link Void}:</p>
 * <pre>
 * private class MyTask extends AsyncTask&lt;Void, Void, Void&gt; { ... }
 * </pre>
 *
 * <h2>The 4 steps</h2>
 * <p>When an asynchronous task is executed, the task goes through 4 steps:</p>
 * <ol>
 *     <li>{@link #onPreExecute()}, invoked on the UI thread before the task
 *     is executed. This step is normally used to setup the task, for instance by
 *     showing a progress bar in the user interface.</li>
 *     <li>{@link #doInBackground}, invoked on the background thread
 *     immediately after {@link #onPreExecute()} finishes executing. This step is used
 *     to perform background computation that can take a long time. The parameters
 *     of the asynchronous task are passed to this step. The result of the computation must
 *     be returned by this step and will be passed back to the last step. This step
 *     can also use {@link #publishProgress} to publish one or more units
 *     of progress. These values are published on the UI thread, in the
 *     {@link #onProgressUpdate} step.</li>
 *     <li>{@link #onProgressUpdate}, invoked on the UI thread after a
 *     call to {@link #publishProgress}. The timing of the execution is
 *     undefined. This method is used to display any form of progress in the user
 *     interface while the background computation is still executing. For instance,
 *     it can be used to animate a progress bar or show logs in a text field.</li>
 *     <li>{@link #onPostExecute}, invoked on the UI thread after the background
 *     computation finishes. The result of the background computation is passed to
 *     this step as a parameter.</li>
 * </ol>
 * 
 * <h2>Cancelling a task</h2>
 * <p>A task can be cancelled at any time by invoking {@link #cancel(boolean)}. Invoking
 * this method will cause subsequent calls to {@link #isCancelled()} to return true.
 * After invoking this method, {@link #onCancelled(Object)}, instead of
 * {@link #onPostExecute(Object)} will be invoked after {@link #doInBackground(Object[])}
 * returns. To ensure that a task is cancelled as quickly as possible, you should always
 * check the return value of {@link #isCancelled()} periodically from
 * {@link #doInBackground(Object[])}, if possible (inside a loop for instance.)</p>
 *
 * <h2>Threading rules</h2>
 * <p>There are a few threading rules that must be followed for this class to
 * work properly:</p>
 * <ul>
 *     <li>The AsyncTask class must be loaded on the UI thread. This is done
 *     automatically as of {@link android.os.Build.VERSION_CODES#JELLY_BEAN}.</li>
 *     <li>The task instance must be created on the UI thread.</li>
 *     <li>{@link #execute} must be invoked on the UI thread.</li>
 *     <li>Do not call {@link #onPreExecute()}, {@link #onPostExecute},
 *     {@link #doInBackground}, {@link #onProgressUpdate} manually.</li>
 *     <li>The task can be executed only once (an exception will be thrown if
 *     a second execution is attempted.)</li>
 * </ul>
 *
 * <h2>Memory observability</h2>
 * <p>AsyncTask guarantees that all callback calls are synchronized in such a way that the following
 * operations are safe without explicit synchronizations.</p>
 * <ul>
 *     <li>Set member fields in the constructor or {@link #onPreExecute}, and refer to them
 *     in {@link #doInBackground}.
 *     <li>Set member fields in {@link #doInBackground}, and refer to them in
 *     {@link #onProgressUpdate} and {@link #onPostExecute}.
 * </ul>
 *
 * <h2>Order of execution</h2>
 * <p>When first introduced, AsyncTasks were executed serially on a single background
 * thread. Starting with {@link android.os.Build.VERSION_CODES#DONUT}, this was changed
 * to a pool of threads allowing multiple tasks to operate in parallel. Starting with
 * {@link android.os.Build.VERSION_CODES#HONEYCOMB}, tasks are executed on a single
 * thread to avoid common application errors caused by parallel execution.</p>
 * <p>If you truly want parallel execution, you can invoke
 * {@link #executeOnExecutor(java.util.concurrent.Executor, Object[])} with
 * {@link #THREAD_POOL_EXECUTOR}.</p>
 */
public abstract class AsyncTask<Params, Progress, Result> {
    private static final String LOG_TAG = "AsyncTask";

    private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors();
    // We want at least 2 threads and at most 4 threads in the core pool,
    // preferring to have 1 less than the CPU count to avoid saturating
    // the CPU with background work
    private static final int CORE_POOL_SIZE = Math.max(2, Math.min(CPU_COUNT - 1, 4));
    private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;
    private static final int KEEP_ALIVE_SECONDS = 30;

    private static final ThreadFactory sThreadFactory = new ThreadFactory() {
        private final AtomicInteger mCount = new AtomicInteger(1);

        public Thread newThread(Runnable r) {
            return new Thread(r, "AsyncTask #" + mCount.getAndIncrement());
        }
    };

    private static final BlockingQueue<Runnable> sPoolWorkQueue =
            new LinkedBlockingQueue<Runnable>(128);

    
An Executor that can be used to execute tasks in parallel. /**
     * An {@link Executor} that can be used to execute tasks in parallel.
     */
    public static final Executor THREAD_POOL_EXECUTOR;

    static {
        ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(
                CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE_SECONDS, TimeUnit.SECONDS,
                sPoolWorkQueue, sThreadFactory);
        threadPoolExecutor.allowCoreThreadTimeOut(true);
        THREAD_POOL_EXECUTOR = threadPoolExecutor;
    }

    
An Executor that executes tasks one at a time in serial order. This serialization is global to a particular process. /**
     * An {@link Executor} that executes tasks one at a time in serial
     * order.  This serialization is global to a particular process.
     */
    public static final Executor SERIAL_EXECUTOR = new SerialExecutor();

    private static final int MESSAGE_POST_RESULT = 0x1;
    private static final int MESSAGE_POST_PROGRESS = 0x2;

    private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;
    private static InternalHandler sHandler;

    private final WorkerRunnable<Params, Result> mWorker;
    private final FutureTask<Result> mFuture;

    private volatile Status mStatus = Status.PENDING;
    
    private final AtomicBoolean mCancelled = new AtomicBoolean();
    private final AtomicBoolean mTaskInvoked = new AtomicBoolean();

    private final Handler mHandler;

    private static class SerialExecutor implements Executor {
        final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>();
        Runnable mActive;

        public synchronized void execute(final Runnable r) {
            mTasks.offer(new Runnable() {
                public void run() {
                    try {
                        r.run();
                    } finally {
                        scheduleNext();
                    }
                }
            });
            if (mActive == null) {
                scheduleNext();
            }
        }

        protected synchronized void scheduleNext() {
            if ((mActive = mTasks.poll()) != null) {
                THREAD_POOL_EXECUTOR.execute(mActive);
            }
        }
    }

    
Indicates the current status of the task. Each status will be set only once
during the lifetime of a task.
/**
     * Indicates the current status of the task. Each status will be set only once
     * during the lifetime of a task.
     */
    public enum Status {
        
Indicates that the task has not been executed yet.
/**
         * Indicates that the task has not been executed yet.
         */
        PENDING,
        
Indicates that the task is running.
/**
         * Indicates that the task is running.
         */
        RUNNING,
        
Indicates that AsyncTask.onPostExecute has finished. /**
         * Indicates that {@link AsyncTask#onPostExecute} has finished.
         */
        FINISHED,
    }

    private static Handler getMainHandler() {
        synchronized (AsyncTask.class) {
            if (sHandler == null) {
                sHandler = new InternalHandler(Looper.getMainLooper());
            }
            return sHandler;
        }
    }

    private Handler getHandler() {
        return mHandler;
    }

    
@hide
/** @hide */
    public static void setDefaultExecutor(Executor exec) {
        sDefaultExecutor = exec;
    }

    
Creates a new asynchronous task. This constructor must be invoked on the UI thread.
/**
     * Creates a new asynchronous task. This constructor must be invoked on the UI thread.
     */
    public AsyncTask() {
        this((Looper) null);
    }

    
Creates a new asynchronous task. This constructor must be invoked on the UI thread.
@hide
/**
     * Creates a new asynchronous task. This constructor must be invoked on the UI thread.
     *
     * @hide
     */
    public AsyncTask(@Nullable Handler handler) {
        this(handler != null ? handler.getLooper() : null);
    }

    
Creates a new asynchronous task. This constructor must be invoked on the UI thread.
@hide
/**
     * Creates a new asynchronous task. This constructor must be invoked on the UI thread.
     *
     * @hide
     */
    public AsyncTask(@Nullable Looper callbackLooper) {
        mHandler = callbackLooper == null || callbackLooper == Looper.getMainLooper()
            ? getMainHandler()
            : new Handler(callbackLooper);

        mWorker = new WorkerRunnable<Params, Result>() {
            public Result call() throws Exception {
                mTaskInvoked.set(true);
                Result result = null;
                try {
                    Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
                    //noinspection unchecked
                    result = doInBackground(mParams);
                    Binder.flushPendingCommands();
                } catch (Throwable tr) {
                    mCancelled.set(true);
                    throw tr;
                } finally {
                    postResult(result);
                }
                return result;
            }
        };

        mFuture = new FutureTask<Result>(mWorker) {
            @Override
            protected void done() {
                try {
                    postResultIfNotInvoked(get());
                } catch (InterruptedException e) {
                    android.util.Log.w(LOG_TAG, e);
                } catch (ExecutionException e) {
                    throw new RuntimeException("An error occurred while executing doInBackground()",
                            e.getCause());
                } catch (CancellationException e) {
                    postResultIfNotInvoked(null);
                }
            }
        };
    }

    private void postResultIfNotInvoked(Result result) {
        final boolean wasTaskInvoked = mTaskInvoked.get();
        if (!wasTaskInvoked) {
            postResult(result);
        }
    }

    private Result postResult(Result result) {
        @SuppressWarnings("unchecked")
        Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT,
                new AsyncTaskResult<Result>(this, result));
        message.sendToTarget();
        return result;
    }

    
Returns the current status of this task.
Returns:
The current status./**
     * Returns the current status of this task.
     *
     * @return The current status.
     */
    public final Status getStatus() {
        return mStatus;
    }

    
Override this method to perform a computation on a background thread. The specified parameters are the parameters passed to execute by the caller of this task. This method can call publishProgress to publish updates on the UI thread. 
Params:
params – The parameters of the task.
See Also:
onPreExecute()
onPostExecute
publishProgress
Returns:
A result, defined by the subclass of this task./**
     * Override this method to perform a computation on a background thread. The
     * specified parameters are the parameters passed to {@link #execute}
     * by the caller of this task.
     *
     * This method can call {@link #publishProgress} to publish updates
     * on the UI thread.
     *
     * @param params The parameters of the task.
     *
     * @return A result, defined by the subclass of this task.
     *
     * @see #onPreExecute()
     * @see #onPostExecute
     * @see #publishProgress
     */
    @WorkerThread
    protected abstract Result doInBackground(Params... params);

    
Runs on the UI thread before doInBackground. 
See Also:
onPostExecute
doInBackground/**
     * Runs on the UI thread before {@link #doInBackground}.
     *
     * @see #onPostExecute
     * @see #doInBackground
     */
    @MainThread
    protected void onPreExecute() {
    }

    
Runs on the UI thread after doInBackground. The specified result is the value returned by doInBackground.
This method won't be invoked if the task was cancelled.
Params:
result – The result of the operation computed by doInBackground.
See Also:
onPreExecute
doInBackground
onCancelled(Object)/**
     * <p>Runs on the UI thread after {@link #doInBackground}. The
     * specified result is the value returned by {@link #doInBackground}.</p>
     * 
     * <p>This method won't be invoked if the task was cancelled.</p>
     *
     * @param result The result of the operation computed by {@link #doInBackground}.
     *
     * @see #onPreExecute
     * @see #doInBackground
     * @see #onCancelled(Object) 
     */
    @SuppressWarnings({"UnusedDeclaration"})
    @MainThread
    protected void onPostExecute(Result result) {
    }

    
Runs on the UI thread after publishProgress is invoked. The specified values are the values passed to publishProgress. 
Params:
values – The values indicating progress.
See Also:
publishProgress
doInBackground/**
     * Runs on the UI thread after {@link #publishProgress} is invoked.
     * The specified values are the values passed to {@link #publishProgress}.
     *
     * @param values The values indicating progress.
     *
     * @see #publishProgress
     * @see #doInBackground
     */
    @SuppressWarnings({"UnusedDeclaration"})
    @MainThread
    protected void onProgressUpdate(Progress... values) {
    }

    
Runs on the UI thread after cancel(boolean) is invoked and doInBackground(Object[]) has finished.
The default implementation simply invokes onCancelled() and ignores the result. If you write your own implementation, do not call super.onCancelled(result).
Params:
result – The result, if any, computed in doInBackground(Object[]), can be null
See Also:
cancel(boolean)
isCancelled()/**
     * <p>Runs on the UI thread after {@link #cancel(boolean)} is invoked and
     * {@link #doInBackground(Object[])} has finished.</p>
     * 
     * <p>The default implementation simply invokes {@link #onCancelled()} and
     * ignores the result. If you write your own implementation, do not call
     * <code>super.onCancelled(result)</code>.</p>
     *
     * @param result The result, if any, computed in
     *               {@link #doInBackground(Object[])}, can be null
     * 
     * @see #cancel(boolean)
     * @see #isCancelled()
     */
    @SuppressWarnings({"UnusedParameters"})
    @MainThread
    protected void onCancelled(Result result) {
        onCancelled();
    }    
    
    
Applications should preferably override onCancelled(Object). This method is invoked by the default implementation of onCancelled(Object).
Runs on the UI thread after cancel(boolean) is invoked and doInBackground(Object[]) has finished.
See Also:
onCancelled(Object)
cancel(boolean)
isCancelled()/**
     * <p>Applications should preferably override {@link #onCancelled(Object)}.
     * This method is invoked by the default implementation of
     * {@link #onCancelled(Object)}.</p>
     * 
     * <p>Runs on the UI thread after {@link #cancel(boolean)} is invoked and
     * {@link #doInBackground(Object[])} has finished.</p>
     *
     * @see #onCancelled(Object) 
     * @see #cancel(boolean)
     * @see #isCancelled()
     */
    @MainThread
    protected void onCancelled() {
    }

    
Returns true if this task was cancelled before it completed normally. If you are calling cancel(boolean) on the task, the value returned by this method should be checked periodically from doInBackground(Object[]) to end the task as soon as possible. 
See Also:
cancel(boolean)
Returns:
true if task was cancelled before it completed/**
     * Returns <tt>true</tt> if this task was cancelled before it completed
     * normally. If you are calling {@link #cancel(boolean)} on the task,
     * the value returned by this method should be checked periodically from
     * {@link #doInBackground(Object[])} to end the task as soon as possible.
     *
     * @return <tt>true</tt> if task was cancelled before it completed
     *
     * @see #cancel(boolean)
     */
    public final boolean isCancelled() {
        return mCancelled.get();
    }

    
Attempts to cancel execution of this task.  This attempt will
fail if the task has already completed, already been cancelled,
or could not be cancelled for some other reason. If successful,
and this task has not started when cancel is called,
this task should never run. If the task has already started,
then the mayInterruptIfRunning parameter determines
whether the thread executing this task should be interrupted in
an attempt to stop the task.
Calling this method will result in onCancelled(Object) being invoked on the UI thread after doInBackground(Object[]) returns. Calling this method guarantees that onPostExecute(Object) is never invoked. After invoking this method, you should check the value returned by isCancelled() periodically from doInBackground(Object[]) to finish the task as early as possible.
Params:
mayInterruptIfRunning – true if the thread executing this
       task should be interrupted; otherwise, in-progress tasks are allowed
       to complete.
See Also:
isCancelled()
onCancelled(Object)
Returns:
false if the task could not be cancelled,
        typically because it has already completed normally;
        true otherwise/**
     * <p>Attempts to cancel execution of this task.  This attempt will
     * fail if the task has already completed, already been cancelled,
     * or could not be cancelled for some other reason. If successful,
     * and this task has not started when <tt>cancel</tt> is called,
     * this task should never run. If the task has already started,
     * then the <tt>mayInterruptIfRunning</tt> parameter determines
     * whether the thread executing this task should be interrupted in
     * an attempt to stop the task.</p>
     * 
     * <p>Calling this method will result in {@link #onCancelled(Object)} being
     * invoked on the UI thread after {@link #doInBackground(Object[])}
     * returns. Calling this method guarantees that {@link #onPostExecute(Object)}
     * is never invoked. After invoking this method, you should check the
     * value returned by {@link #isCancelled()} periodically from
     * {@link #doInBackground(Object[])} to finish the task as early as
     * possible.</p>
     *
     * @param mayInterruptIfRunning <tt>true</tt> if the thread executing this
     *        task should be interrupted; otherwise, in-progress tasks are allowed
     *        to complete.
     *
     * @return <tt>false</tt> if the task could not be cancelled,
     *         typically because it has already completed normally;
     *         <tt>true</tt> otherwise
     *
     * @see #isCancelled()
     * @see #onCancelled(Object)
     */
    public final boolean cancel(boolean mayInterruptIfRunning) {
        mCancelled.set(true);
        return mFuture.cancel(mayInterruptIfRunning);
    }

    
Waits if necessary for the computation to complete, and then
retrieves its result.
Throws:
CancellationException – If the computation was cancelled.
ExecutionException – If the computation threw an exception.
InterruptedException – If the current thread was interrupted
        while waiting.
Returns:
The computed result./**
     * Waits if necessary for the computation to complete, and then
     * retrieves its result.
     *
     * @return The computed result.
     *
     * @throws CancellationException If the computation was cancelled.
     * @throws ExecutionException If the computation threw an exception.
     * @throws InterruptedException If the current thread was interrupted
     *         while waiting.
     */
    public final Result get() throws InterruptedException, ExecutionException {
        return mFuture.get();
    }

    
Waits if necessary for at most the given time for the computation
to complete, and then retrieves its result.
Params:
timeout – Time to wait before cancelling the operation.
unit – The time unit for the timeout.
Throws:
CancellationException – If the computation was cancelled.
ExecutionException – If the computation threw an exception.
InterruptedException – If the current thread was interrupted
        while waiting.
TimeoutException – If the wait timed out.
Returns:
The computed result./**
     * Waits if necessary for at most the given time for the computation
     * to complete, and then retrieves its result.
     *
     * @param timeout Time to wait before cancelling the operation.
     * @param unit The time unit for the timeout.
     *
     * @return The computed result.
     *
     * @throws CancellationException If the computation was cancelled.
     * @throws ExecutionException If the computation threw an exception.
     * @throws InterruptedException If the current thread was interrupted
     *         while waiting.
     * @throws TimeoutException If the wait timed out.
     */
    public final Result get(long timeout, TimeUnit unit) throws InterruptedException,
            ExecutionException, TimeoutException {
        return mFuture.get(timeout, unit);
    }

    
Executes the task with the specified parameters. The task returns
itself (this) so that the caller can keep a reference to it.
Note: this function schedules the task on a queue for a single background thread or pool of threads depending on the platform version. When first introduced, AsyncTasks were executed serially on a single background thread. Starting with VERSION_CODES.DONUT, this was changed to a pool of threads allowing multiple tasks to operate in parallel. Starting VERSION_CODES.HONEYCOMB, tasks are back to being executed on a single thread to avoid common application errors caused by parallel execution. If you truly want parallel execution, you can use the executeOnExecutor version of this method with AsyncTask<Params,Progress,Result>.THREAD_POOL_EXECUTOR; however, see commentary there for warnings on its use. 
This method must be invoked on the UI thread.
Params:
params – The parameters of the task.
Throws:
IllegalStateException – If getStatus() returns either Status.RUNNING or Status.FINISHED.
See Also:
executeOnExecutor(Executor, Object[])
execute(Runnable)
Returns:
This instance of AsyncTask./**
     * Executes the task with the specified parameters. The task returns
     * itself (this) so that the caller can keep a reference to it.
     * 
     * <p>Note: this function schedules the task on a queue for a single background
     * thread or pool of threads depending on the platform version.  When first
     * introduced, AsyncTasks were executed serially on a single background thread.
     * Starting with {@link android.os.Build.VERSION_CODES#DONUT}, this was changed
     * to a pool of threads allowing multiple tasks to operate in parallel. Starting
     * {@link android.os.Build.VERSION_CODES#HONEYCOMB}, tasks are back to being
     * executed on a single thread to avoid common application errors caused
     * by parallel execution.  If you truly want parallel execution, you can use
     * the {@link #executeOnExecutor} version of this method
     * with {@link #THREAD_POOL_EXECUTOR}; however, see commentary there for warnings
     * on its use.
     *
     * <p>This method must be invoked on the UI thread.
     *
     * @param params The parameters of the task.
     *
     * @return This instance of AsyncTask.
     *
     * @throws IllegalStateException If {@link #getStatus()} returns either
     *         {@link AsyncTask.Status#RUNNING} or {@link AsyncTask.Status#FINISHED}.
     *
     * @see #executeOnExecutor(java.util.concurrent.Executor, Object[])
     * @see #execute(Runnable)
     */
    @MainThread
    public final AsyncTask<Params, Progress, Result> execute(Params... params) {
        return executeOnExecutor(sDefaultExecutor, params);
    }

    
Executes the task with the specified parameters. The task returns
itself (this) so that the caller can keep a reference to it.
This method is typically used with AsyncTask<Params,Progress,Result>.THREAD_POOL_EXECUTOR to allow multiple tasks to run in parallel on a pool of threads managed by AsyncTask, however you can also use your own Executor for custom behavior. 
Warning: Allowing multiple tasks to run in parallel from
a thread pool is generally not what one wants, because the order of their operation is not defined. For example, if these tasks are used to modify any state in common (such as writing a file due to a button click), there are no guarantees on the order of the modifications. Without careful work it is possible in rare cases for the newer version of the data to be over-written by an older one, leading to obscure data loss and stability issues. Such changes are best executed in serial; to guarantee such work is serialized regardless of platform version you can use this function with AsyncTask<Params,Progress,Result>.SERIAL_EXECUTOR. 
This method must be invoked on the UI thread.
Params:
exec – The executor to use. AsyncTask<Params,Progress,Result>.THREAD_POOL_EXECUTOR is available as a convenient process-wide thread pool for tasks that are loosely coupled.
params – The parameters of the task.
Throws:
IllegalStateException – If getStatus() returns either Status.RUNNING or Status.FINISHED.
See Also:
execute(Object[])
Returns:
This instance of AsyncTask./**
     * Executes the task with the specified parameters. The task returns
     * itself (this) so that the caller can keep a reference to it.
     * 
     * <p>This method is typically used with {@link #THREAD_POOL_EXECUTOR} to
     * allow multiple tasks to run in parallel on a pool of threads managed by
     * AsyncTask, however you can also use your own {@link Executor} for custom
     * behavior.
     * 
     * <p><em>Warning:</em> Allowing multiple tasks to run in parallel from
     * a thread pool is generally <em>not</em> what one wants, because the order
     * of their operation is not defined.  For example, if these tasks are used
     * to modify any state in common (such as writing a file due to a button click),
     * there are no guarantees on the order of the modifications.
     * Without careful work it is possible in rare cases for the newer version
     * of the data to be over-written by an older one, leading to obscure data
     * loss and stability issues.  Such changes are best
     * executed in serial; to guarantee such work is serialized regardless of
     * platform version you can use this function with {@link #SERIAL_EXECUTOR}.
     *
     * <p>This method must be invoked on the UI thread.
     *
     * @param exec The executor to use.  {@link #THREAD_POOL_EXECUTOR} is available as a
     *              convenient process-wide thread pool for tasks that are loosely coupled.
     * @param params The parameters of the task.
     *
     * @return This instance of AsyncTask.
     *
     * @throws IllegalStateException If {@link #getStatus()} returns either
     *         {@link AsyncTask.Status#RUNNING} or {@link AsyncTask.Status#FINISHED}.
     *
     * @see #execute(Object[])
     */
    @MainThread
    public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec,
            Params... params) {
        if (mStatus != Status.PENDING) {
            switch (mStatus) {
                case RUNNING:
                    throw new IllegalStateException("Cannot execute task:"
                            + " the task is already running.");
                case FINISHED:
                    throw new IllegalStateException("Cannot execute task:"
                            + " the task has already been executed "
                            + "(a task can be executed only once)");
            }
        }

        mStatus = Status.RUNNING;

        onPreExecute();

        mWorker.mParams = params;
        exec.execute(mFuture);

        return this;
    }

    
Convenience version of execute(Object...) for use with a simple Runnable object. See execute(Object[]) for more information on the order of execution. 
See Also:
execute(Object[])
executeOnExecutor(Executor, Object[])/**
     * Convenience version of {@link #execute(Object...)} for use with
     * a simple Runnable object. See {@link #execute(Object[])} for more
     * information on the order of execution.
     *
     * @see #execute(Object[])
     * @see #executeOnExecutor(java.util.concurrent.Executor, Object[])
     */
    @MainThread
    public static void execute(Runnable runnable) {
        sDefaultExecutor.execute(runnable);
    }

    
This method can be invoked from doInBackground to publish updates on the UI thread while the background computation is still running. Each call to this method will trigger the execution of onProgressUpdate on the UI thread. onProgressUpdate will not be called if the task has been canceled. 
Params:
values – The progress values to update the UI with.
See Also:
onProgressUpdate
doInBackground/**
     * This method can be invoked from {@link #doInBackground} to
     * publish updates on the UI thread while the background computation is
     * still running. Each call to this method will trigger the execution of
     * {@link #onProgressUpdate} on the UI thread.
     *
     * {@link #onProgressUpdate} will not be called if the task has been
     * canceled.
     *
     * @param values The progress values to update the UI with.
     *
     * @see #onProgressUpdate
     * @see #doInBackground
     */
    @WorkerThread
    protected final void publishProgress(Progress... values) {
        if (!isCancelled()) {
            getHandler().obtainMessage(MESSAGE_POST_PROGRESS,
                    new AsyncTaskResult<Progress>(this, values)).sendToTarget();
        }
    }

    private void finish(Result result) {
        if (isCancelled()) {
            onCancelled(result);
        } else {
            onPostExecute(result);
        }
        mStatus = Status.FINISHED;
    }

    private static class InternalHandler extends Handler {
        public InternalHandler(Looper looper) {
            super(looper);
        }

        @SuppressWarnings({"unchecked", "RawUseOfParameterizedType"})
        @Override
        public void handleMessage(Message msg) {
            AsyncTaskResult<?> result = (AsyncTaskResult<?>) msg.obj;
            switch (msg.what) {
                case MESSAGE_POST_RESULT:
                    // There is only one result
                    result.mTask.finish(result.mData[0]);
                    break;
                case MESSAGE_POST_PROGRESS:
                    result.mTask.onProgressUpdate(result.mData);
                    break;
            }
        }
    }

    private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> {
        Params[] mParams;
    }

    @SuppressWarnings({"RawUseOfParameterizedType"})
    private static class AsyncTaskResult<Data> {
        final AsyncTask mTask;
        final Data[] mData;

        AsyncTaskResult(AsyncTask task, Data... data) {
            mTask = task;
            mData = data;
        }
    }
}