android媒体--图库与API层MediaPlayer的交互

众所周知一个媒体播放器新建的几个步骤：

Mediaplayer mp  = new MediaPlayer(0
mp.setDatasource(xxx);
mp.setDispalyer(xxx);
mp.setAudioStreamType(xxx);
mp.prepareAsync();
mp.start

这个和activity该如何绑定呢？本文则是通过图库分析，来说明此中明细。

打开图库显得的是一个名字叫MovieActivity的Actiivity，具有activity的的生命周期，而媒体的操作是如何和他们关联起来呢，详见下图（图片长，上传的时候出现了背景黑色的情况，不知道为什么，此处分开贴图，真实图片上传资源那边，0积分下载）：

资源下载地址 ：http://download.csdn.net/detail/loovejava/6272059

从代码看到的和媒体关联的就这这几个生命周期的时候，onCreate() , onResume() ,onPause() ,onDestory()。

onPause在android媒体——图库pause时与服务端断开连接 中有讲述。

onCreate只是进行了初始化，无其他特殊操作

onDestory中则是停止了框架层mediaplayer的播放，并且对其进行状态反初始化的操作。

主要是在两个onResume和onKeyDown方法:

简明的说下，onResume是新建了一个媒体播放器并设置监听。

onKeyDown则是通过监听点击时间，来控制媒体的播放和暂停。

======================================= 下面附上关键代码=========================================================

1、onResume初始化媒体：

package\apps\Gallery2\src\com\android\gallery3d\app\MovieActivity.java

    public void onResume() {
        mPlayer.onResume();
        super.onResume();
    }

package\apps\Gallery2\src\com\android\gallery3d\app\MoviePlayer.java

   public void onResume() {
        if (mHasPaused) {
            mVideoView.seekTo(mVideoPosition);
            mVideoView.resume();

            // If we have slept for too long, pause the play
            if (System.currentTimeMillis() > mResumeableTime) {
                pauseVideo();
            }
        }
        mHandler.post(mProgressChecker);
    }

framework\base\core\java\android\widget\ViewView.java

    public void resume() {
        openVideo();
    }

       ...  ...

 private void openVideo() {
        if (mUri == null || mSurfaceHolder == null) {
            // not ready for playback just yet, will try again later
            return;
        }
        // Tell the music playback service to pause
        // TODO: these constants need to be published somewhere in the framework.
        Intent i = new Intent("com.android.music.musicservicecommand");
        i.putExtra("command", "pause");
        mContext.sendBroadcast(i);

        // we shouldn't clear the target state, because somebody might have
        // called start() previously
        release(false);
        try {
            mMediaPlayer = new MediaPlayer();
            if (mAudioSession != 0) {
                mMediaPlayer.setAudioSessionId(mAudioSession);
            } else {
                mAudioSession = mMediaPlayer.getAudioSessionId();
            }
            mMediaPlayer.setOnPreparedListener(mPreparedListener);
            mMediaPlayer.setOnVideoSizeChangedListener(mSizeChangedListener);
            mMediaPlayer.setOnCompletionListener(mCompletionListener);
            mMediaPlayer.setOnErrorListener(mErrorListener);
            mMediaPlayer.setOnInfoListener(mOnInfoListener);
            mMediaPlayer.setOnBufferingUpdateListener(mBufferingUpdateListener);
            mCurrentBufferPercentage = 0;
            mMediaPlayer.setDataSource(mContext, mUri, mHeaders);
            mMediaPlayer.setDisplay(mSurfaceHolder);
            mMediaPlayer.setAudioStreamType(AudioManager.STREAM_MUSIC);
            mMediaPlayer.setScreenOnWhilePlaying(true);
            mMediaPlayer.prepareAsync();
            // we don't set the target state here either, but preserve the
            // target state that was there before.
            mCurrentState = STATE_PREPARING;
            attachMediaController();
        } catch (IOException ex) {
            Log.w(TAG, "Unable to open content: " + mUri, ex);
            mCurrentState = STATE_ERROR;
            mTargetState = STATE_ERROR;
            mErrorListener.onError(mMediaPlayer, MediaPlayer.MEDIA_ERROR_UNKNOWN, 0);
            return;
        } catch (IllegalArgumentException ex) {
            Log.w(TAG, "Unable to open content: " + mUri, ex);
            mCurrentState = STATE_ERROR;
            mTargetState = STATE_ERROR;
            mErrorListener.onError(mMediaPlayer, MediaPlayer.MEDIA_ERROR_UNKNOWN, 0);
            return;
        }
    }

致此已经初始化完毕

2、onKeyDown的如何操控状态改变呢？

package\apps\Gallery2\src\com\android\gallery3d\app\MoviePlayer.java

    private void playVideo() {
        mVideoView.start();
        mController.showPlaying();
        setProgress();
    }

    private void pauseVideo() {
        mVideoView.pause();
        mController.showPaused();
    } 

            ...  ...

    public boolean onKeyDown(int keyCode, KeyEvent event) {

        // Some headsets will fire off 7-10 events on a single click
        if (event.getRepeatCount() > 0) {
            return isMediaKey(keyCode);
        }

        switch (keyCode) {
            case KeyEvent.KEYCODE_HEADSETHOOK:
            case KeyEvent.KEYCODE_MEDIA_PLAY_PAUSE:
                if (mVideoView.isPlaying()) {
                    pauseVideo();
                } else {
                    playVideo();
                }
                return true;
            case KEYCODE_MEDIA_PAUSE:
                if (mVideoView.isPlaying()) {
                    pauseVideo();
                }
                return true;
            case KEYCODE_MEDIA_PLAY:
                if (!mVideoView.isPlaying()) {
                    playVideo();
                }
                return true;
            case KeyEvent.KEYCODE_MEDIA_PREVIOUS:
            case KeyEvent.KEYCODE_MEDIA_NEXT:
                // TODO: Handle next / previous accordingly, for now we're
                // just consuming the events.
                return true;
        }
        return false;
    }

framework\base\core\java\android\widget\ViewView.java

 @Override
    public void start() {
        if (isInPlaybackState()) {
            mMediaPlayer.start();
            mCurrentState = STATE_PLAYING;
        }
        mTargetState = STATE_PLAYING;
    }

    @Override
    public void pause() {
        if (isInPlaybackState()) {
            if (mMediaPlayer.isPlaying()) {
                mMediaPlayer.pause();
                mCurrentState = STATE_PAUSED;
            }
        }
        mTargetState = STATE_PAUSED;
    }

至此已经把想操作媒体的意愿发送至mediaPlayer，mediaplayer会进行先关的处理，如何处理，后面播客会详细的讲述。

-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

看了本文一定有一个疑问，为什么moviePlayer为什么不直接调用mediaplayer的方法呢，为什么中间要隔着一个“第三者”呢?

从上图可以看出videoView继承了SurfaceView，实现了MediaPlayerControl接口，还依赖MediaPlayer

显而易见，我们看到了VideoView就是把媒体，媒体控制和显示的组合体，也可以理解成一个调度类。

我们写如果不使用VideoView的话，自己还得去实现媒体控制的相关操作和显示的相关操作。

here over.