有個場景就是一個頁面裡有多個VideoView播放視訊，然後每個視訊都有一個音量值，但是VideoView並不支援直接設定音量，而是要通過調節系統音量來實現，那麼這樣的話，就不能實現為每個視訊獨立調節音量了我們知道MediaPlayer+SurfaceView也能實現視訊的播放，並且MediaPlayer是可以直接通過setVolume來調節視訊音量的，但是因為這裡已經用了VideoView實現了播放，再改的話就太浪費時間和人力了。想到VideoView是繼承SurfaceView的，那會不會VideoView的視訊播放控制是由MediaPlayer實現的呢，檢視VideoView原始碼：

try 
 
{
    mMediaPlayer = new MediaPlayer();
// TODO: create SubtitleController in MediaPlayer, but we need
// a context for the subtitle renderers
final Context context = getContext();
    final SubtitleController controller = new SubtitleController(
            context, mMediaPlayer.getMediaTimeProvider(), 
 
mMediaPlayer);
controller.registerRenderer(new WebVttRenderer(context));
controller.registerRenderer(new TtmlRenderer(context));
controller.registerRenderer(new Cea708CaptionRenderer(context));
controller.registerRenderer(new ClosedCaptionRenderer(context));
mMediaPlayer.setSubtitleAnchor(controller, this
 
);
    if (mAudioSession != 0) {
        mMediaPlayer.setAudioSessionId(mAudioSession);
} else {
        mAudioSession = mMediaPlayer.getAudioSessionId();
}
    mMediaPlayer.setOnPreparedListener(mPreparedListener);
mMediaPlayer.setOnVideoSizeChangedListener(mSizeChangedListener);
mMediaPlayer.setOnCompletionListener(mCompletionListener);
mMediaPlayer.setOnErrorListener(mErrorListener);
mMediaPlayer.setOnInfoListener(mInfoListener);
mMediaPlayer.setOnBufferingUpdateListener(mBufferingUpdateListener);
mCurrentBufferPercentage = 0;
mMediaPlayer.setDataSource(mContext, mUri, mHeaders);
mMediaPlayer.setDisplay(mSurfaceHolder);
mMediaPlayer.setAudioStreamType(AudioManager.STREAM_MUSIC);
mMediaPlayer.setScreenOnWhilePlaying(true);
mMediaPlayer.prepareAsync();

/**
 * @param volume 音量大小
 * @param object VideoView例項
 * */
public void setVolume(float volume,Object object) {
    try {
        Class<?> forName = Class.forName("android.widget.VideoView");
Field field = forName.getDeclaredField("mMediaPlayer");
field.setAccessible(true);
MediaPlayer mMediaPlayer = (MediaPlayer) field.get(object);
mMediaPlayer.setVolume(volume, volume);
} catch (Exception e) {
    }
}

private static final int DEFAULT_OFFSCREEN_PAGES = 1;
private int mOffscreenPageLimit = DEFAULT_OFFSCREEN_PAGES;
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();
}
}

try {
    Class<?> forName = Class.forName("android.support.v4.view.ViewPager");
Field defauleField = forName.getDeclaredField("mOffscreenPageLimit");
defauleField.setAccessible(true);
defauleField.set(0,viewPager);
} catch (Exception e) {
}