/*
 * Copyright (C) 2006 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.Nullable;
import android.util.Log;

import java.lang.ref.WeakReference;
import java.util.HashMap;

Monitors files (using inotify) to fire an event after files are accessed or changed by by any process on the device (including this one). FileObserver is an abstract class; subclasses must implement the event handler onEvent(int, String). 
Each FileObserver instance monitors a single file or directory.
If a directory is monitored, events will be triggered for all files and
subdirectories inside the monitored directory.
An event mask is used to specify which changes or actions to report.
Event type constants are used to describe the possible changes in the
event mask as well as what actually happened in event callbacks.
Warning: If a FileObserver is garbage collected, it
will stop sending events.  To ensure you keep receiving events, you must
keep a reference to the FileObserver instance from some other live object.
/**
 * Monitors files (using <a href="http://en.wikipedia.org/wiki/Inotify">inotify</a>)
 * to fire an event after files are accessed or changed by by any process on
 * the device (including this one).  FileObserver is an abstract class;
 * subclasses must implement the event handler {@link #onEvent(int, String)}.
 *
 * <p>Each FileObserver instance monitors a single file or directory.
 * If a directory is monitored, events will be triggered for all files and
 * subdirectories inside the monitored directory.</p>
 *
 * <p>An event mask is used to specify which changes or actions to report.
 * Event type constants are used to describe the possible changes in the
 * event mask as well as what actually happened in event callbacks.</p>
 *
 * <p class="caution"><b>Warning</b>: If a FileObserver is garbage collected, it
 * will stop sending events.  To ensure you keep receiving events, you must
 * keep a reference to the FileObserver instance from some other live object.</p>
 */
public abstract class FileObserver {
    
Event type: Data was read from a file /** Event type: Data was read from a file */
    public static final int ACCESS = 0x00000001;
    
Event type: Data was written to a file /** Event type: Data was written to a file */
    public static final int MODIFY = 0x00000002;
    
Event type: Metadata (permissions, owner, timestamp) was changed explicitly /** Event type: Metadata (permissions, owner, timestamp) was changed explicitly */
    public static final int ATTRIB = 0x00000004;
    
Event type: Someone had a file or directory open for writing, and closed it /** Event type: Someone had a file or directory open for writing, and closed it */
    public static final int CLOSE_WRITE = 0x00000008;
    
Event type: Someone had a file or directory open read-only, and closed it /** Event type: Someone had a file or directory open read-only, and closed it */
    public static final int CLOSE_NOWRITE = 0x00000010;
    
Event type: A file or directory was opened /** Event type: A file or directory was opened */
    public static final int OPEN = 0x00000020;
    
Event type: A file or subdirectory was moved from the monitored directory /** Event type: A file or subdirectory was moved from the monitored directory */
    public static final int MOVED_FROM = 0x00000040;
    
Event type: A file or subdirectory was moved to the monitored directory /** Event type: A file or subdirectory was moved to the monitored directory */
    public static final int MOVED_TO = 0x00000080;
    
Event type: A new file or subdirectory was created under the monitored directory /** Event type: A new file or subdirectory was created under the monitored directory */
    public static final int CREATE = 0x00000100;
    
Event type: A file was deleted from the monitored directory /** Event type: A file was deleted from the monitored directory */
    public static final int DELETE = 0x00000200;
    
Event type: The monitored file or directory was deleted; monitoring effectively stops /** Event type: The monitored file or directory was deleted; monitoring effectively stops */
    public static final int DELETE_SELF = 0x00000400;
    
Event type: The monitored file or directory was moved; monitoring continues /** Event type: The monitored file or directory was moved; monitoring continues */
    public static final int MOVE_SELF = 0x00000800;

    
Event mask: All valid event types, combined /** Event mask: All valid event types, combined */
    public static final int ALL_EVENTS = ACCESS | MODIFY | ATTRIB | CLOSE_WRITE
            | CLOSE_NOWRITE | OPEN | MOVED_FROM | MOVED_TO | DELETE | CREATE
            | DELETE_SELF | MOVE_SELF;

    private static final String LOG_TAG = "FileObserver";

    private static class ObserverThread extends Thread {
        private HashMap<Integer, WeakReference> m_observers = new HashMap<Integer, WeakReference>();
        private int m_fd;

        public ObserverThread() {
            super("FileObserver");
            m_fd = init();
        }

        public void run() {
            observe(m_fd);
        }

        public int startWatching(String path, int mask, FileObserver observer) {
            int wfd = startWatching(m_fd, path, mask);

            Integer i = new Integer(wfd);
            if (wfd >= 0) {
                synchronized (m_observers) {
                    m_observers.put(i, new WeakReference(observer));
                }
            }

            return i;
        }

        public void stopWatching(int descriptor) {
            stopWatching(m_fd, descriptor);
        }

        public void onEvent(int wfd, int mask, String path) {
            // look up our observer, fixing up the map if necessary...
            FileObserver observer = null;

            synchronized (m_observers) {
                WeakReference weak = m_observers.get(wfd);
                if (weak != null) {  // can happen with lots of events from a dead wfd
                    observer = (FileObserver) weak.get();
                    if (observer == null) {
                        m_observers.remove(wfd);
                    }
                }
            }

            // ...then call out to the observer without the sync lock held
            if (observer != null) {
                try {
                    observer.onEvent(mask, path);
                } catch (Throwable throwable) {
                    Log.wtf(LOG_TAG, "Unhandled exception in FileObserver " + observer, throwable);
                }
            }
        }

        private native int init();
        private native void observe(int fd);
        private native int startWatching(int fd, String path, int mask);
        private native void stopWatching(int fd, int wfd);
    }

    private static ObserverThread s_observerThread;

    static {
        s_observerThread = new ObserverThread();
        s_observerThread.start();
    }

    // instance
    private String m_path;
    private Integer m_descriptor;
    private int m_mask;

    
Equivalent to FileObserver(path, FileObserver.ALL_EVENTS).
/**
     * Equivalent to FileObserver(path, FileObserver.ALL_EVENTS).
     */
    public FileObserver(String path) {
        this(path, ALL_EVENTS);
    }

    
Create a new file observer for a certain file or directory. Monitoring does not start on creation! You must call startWatching() before you will receive events. 
Params:
path – The file or directory to monitor
mask – The event or events (added together) to watch for/**
     * Create a new file observer for a certain file or directory.
     * Monitoring does not start on creation!  You must call
     * {@link #startWatching()} before you will receive events.
     *
     * @param path The file or directory to monitor
     * @param mask The event or events (added together) to watch for
     */
    public FileObserver(String path, int mask) {
        m_path = path;
        m_mask = mask;
        m_descriptor = -1;
    }

    protected void finalize() {
        stopWatching();
    }

    
Start watching for events.  The monitored file or directory must exist at
this time, or else no events will be reported (even if it appears later).
If monitoring is already started, this call has no effect.
/**
     * Start watching for events.  The monitored file or directory must exist at
     * this time, or else no events will be reported (even if it appears later).
     * If monitoring is already started, this call has no effect.
     */
    public void startWatching() {
        if (m_descriptor < 0) {
            m_descriptor = s_observerThread.startWatching(m_path, m_mask, this);
        }
    }

    
Stop watching for events.  Some events may be in process, so events
may continue to be reported even after this method completes.  If
monitoring is already stopped, this call has no effect.
/**
     * Stop watching for events.  Some events may be in process, so events
     * may continue to be reported even after this method completes.  If
     * monitoring is already stopped, this call has no effect.
     */
    public void stopWatching() {
        if (m_descriptor >= 0) {
            s_observerThread.stopWatching(m_descriptor);
            m_descriptor = -1;
        }
    }

    
The event handler, which must be implemented by subclasses.
This method is invoked on a special FileObserver thread. It runs independently of any threads, so take care to use appropriate synchronization! Consider using Handler.post(Runnable) to shift event handling work to the main thread to avoid concurrency problems.
Event handlers must not throw exceptions.
Params:
event – The type of event which happened
path – The path, relative to the main monitored file or directory, of the file or directory which triggered the event. This value can be null for certain events, such as MOVE_SELF./**
     * The event handler, which must be implemented by subclasses.
     *
     * <p class="note">This method is invoked on a special FileObserver thread.
     * It runs independently of any threads, so take care to use appropriate
     * synchronization!  Consider using {@link Handler#post(Runnable)} to shift
     * event handling work to the main thread to avoid concurrency problems.</p>
     *
     * <p>Event handlers must not throw exceptions.</p>
     *
     * @param event The type of event which happened
     * @param path The path, relative to the main monitored file or directory,
     *     of the file or directory which triggered the event.  This value can
     *     be {@code null} for certain events, such as {@link #MOVE_SELF}.
     */
    public abstract void onEvent(int event, @Nullable String path);
}