android之VideoView和視頻播放View的擴展

1.概念及擴展
　　VideoView 是android 系統提供的一個媒體播放顯示和控制的控件。其結構層次如下：
　　原型：VideoView extends SurfaceView implements MediaController.MediaPlayerControl
　　類結構：
　　　　　　java.lang.Object
　　　　　　　　↳ android.view.View
　　　　　　　　　　↳ android.view.SurfaceView
　　　　　　　　　　　　↳ android.widget.VideoView
　　通過VideoView 的原型可知：如果構建更為復雜和有特色個性的視頻View，需要繼承SurfaceView 和實現MediaPlayerControl接口。其中SurfaceView 為顯示提供支持，MediaPlayerControl則為媒體控制提供了支持。
2.案例
1)VideoView案例
(我們沒有管理MediaPalyer的各種狀態，這些狀態都讓VideoView給封裝了，並且，當VideoView創建的時候，MediaPalyer對象將會創建，當VideoView對象銷毀的時候，MediaPlayer對象將會釋放。)
布局文件
 
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:orientation="vertical" android:layout_width="fill_parent"
    android:layout_height="fill_parent">
<VideoView android:id="@+id/video_view" android:layout_width="match_parent" android:layout_height="match_parent"
    android:layout_centerInParent="true" />
</LinearLayout>
主程序：
public class VideoPlayer extends Activity implements MediaPlayer.OnErrorListener, MediaPlayer.OnCompletionListener {
    public static final String TAG = "VideoPlayer";
    private VideoView mVideoView;
    private Uri mUri;
    private int mPositionWhenPaused = -1;

    private MediaController mMediaController;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.main);

        //Set the screen to landscape.
        this.setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_LANDSCAPE);

        mVideoView = (VideoView)findViewById(R.id.video_view);

        //Video file
        mUri = Uri.parse(Environment.getExternalStorageDirectory() + "/1.3gp");

        //Create media controller，組件可以控制視頻的播放，暫停，回復，seek等操作，不需要你實現
        mMediaController = new MediaController(this);
        mVideoView.setMediaController(mMediaController);
    }

    public void onStart() {
        // Play Video
        mVideoView.setVideoURI(mUri);
        mVideoView.start();

        super.onStart();
    }

    public void onPause() {
        // Stop video when the activity is pause.
        mPositionWhenPaused = mVideoView.getCurrentPosition();
        mVideoView.stopPlayback();

        super.onPause();
    }

    public void onResume() {
        // Resume video player
        if(mPositionWhenPaused >= 0) {
            mVideoView.seekTo(mPositionWhenPaused);
            mPositionWhenPaused = -1;
        }

        super.onResume();
    }

    public boolean onError(MediaPlayer player, int arg1, int arg2) {
        return false;
    }

    public void onCompletion(MediaPlayer mp) {
        this.finish();
    }
}
2)自定義VideoView
和VideoView實現類似，繼承了SurfaceView並且實現了MediaPlayerControl。
public class CustomerVideoView extends SurfaceView implements
        MediaPlayerControl {
    private static String TAG = "customer.videoplayer";
    private boolean pause;
    private boolean seekBackward;
    private boolean seekForward;
    private Uri videoUri;
    private MediaPlayer mediaPlayer;
    private Context context;
    private OnPreparedListener onPreparedListener;
    private int videoWidth;
    private int videoHeight;
    private MediaController mediaController;
    protected SurfaceHolder surfaceHolder;
    private Callback surfaceHolderCallback = new SurfaceHolder.Callback() {
        public void surfaceChanged(SurfaceHolder holder, int format, int w,
                int h) {
        }
        public void surfaceCreated(SurfaceHolder holder) {
            surfaceHolder = holder;
            if (mediaPlayer != null) {
                mediaPlayer.setDisplay(surfaceHolder);
                resume();
            } else {
                openVideo();
            }
        }
        public void surfaceDestroyed(SurfaceHolder holder) {
            surfaceHolder = null;
            if (mediaController != null) {
                mediaController.hide();
            }
            release(true);
        }
    };
    private void release(boolean cleartargetstate) {
        if (mediaPlayer != null) {
            mediaPlayer.reset();
            mediaPlayer.release();
            mediaPlayer = null;
        }
    }
    public void resume() {
        if (surfaceHolder == null) {
            return;
        }
        if (mediaPlayer != null) {
            return;
        }
        openVideo();
    }
    public CustomerVideoView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
        this.context = context;
        this.initVideoView();
    }
    public CustomerVideoView(Context context, AttributeSet attrs) {
        super(context, attrs);
        this.context = context;
        this.initVideoView();
    }
    public CustomerVideoView(Context context) {
        super(context);
        this.context = context;
        this.initVideoView();
    }
    @Override
    public boolean canPause() {
        return this.pause;
    }
    @Override
    public boolean canSeekBackward() {
        return this.seekBackward;
    }
    @Override
    public boolean canSeekForward() {
        return this.seekForward;
    }
    @Override
    public int getBufferPercentage() {
        return 0;
    }
    @Override
    public int getCurrentPosition() {
        return mediaPlayer!=null?mediaPlayer.getCurrentPosition():0;
    }
    @Override
    public int getDuration() {
        return mediaPlayer!=null?mediaPlayer.getDuration():0;
    }
    @Override
    public boolean isPlaying() {
        return false;
    }
    @Override
    public void pause() {
    }
    @Override
    public void seekTo(int mSec) {
    }
    @Override
    public void start() {
    }
    public void setVideoURI(Uri uri) {
        this.videoUri = uri;
        openVideo();
        requestLayout();
        invalidate();
    }
    private void openVideo() {
        this.mediaPlayer = new MediaPlayer();
        try {
            this.mediaPlayer.setDataSource(this.context, this.videoUri);
        } catch (Exception e) {
            Log.e(TAG, e.getMessage());
            throw new RuntimeException(e);
        }
        this.mediaPlayer.prepareAsync();
        this.mediaPlayer.setAudioStreamType(AudioManager.STREAM_MUSIC);
        this.mediaPlayer.setOnPreparedListener(onPreparedListener);
        attachMediaController();
    }
    private void attachMediaController() {
        if (mediaPlayer != null && mediaController != null) {
            mediaController.setMediaPlayer(this);
            View anchorView = this.getParent() instanceof View ? (View) this
                    .getParent() : this;
            mediaController.setAnchorView(anchorView);
            mediaController.setEnabled(true);
        }
    }
    public void setMediaController(MediaController controller) {
        if (mediaController != null) {
            mediaController.hide();
        }
        mediaController = controller;
        attachMediaController();
    }
    public void setOnPreparedListener(OnPreparedListener onPreparedListener) {
        this.onPreparedListener = onPreparedListener;
    }
    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        int width = getDefaultSize(videoWidth, widthMeasureSpec);
        int height = getDefaultSize(videoHeight, heightMeasureSpec);
        if (videoWidth > 0 && videoHeight > 0) {
            if (videoWidth * height > width * videoHeight) {
                height = width * videoHeight / videoWidth;
            } else if (videoWidth * height < width * videoHeight) {
                width = height * videoWidth / videoHeight;
            }
        }
        Log.i(TAG, "setting size: " + width + ‘x’ + height);
        setMeasuredDimension(width, height);
    }
    private void initVideoView() {
        videoWidth = 0;
        videoHeight = 0;
        getHolder().addCallback(surfaceHolderCallback);
        getHolder().setType(SurfaceHolder.SURFACE_TYPE_PUSH_BUFFERS);
        setFocusable(true);
        setFocusableInTouchMode(true);
        requestFocus();
    }
}
　一般情況下，android界面的繪制和更新，要交給主ui線程來操作，通過Handler機制。但是播放視頻，需要比較優先和實時的改變和繪制界面。android提供了使用單獨線程繪制UI的機制，就是SurfaceView。使用SurfaceView，需要實現SurfaceHolder.Callback接口：
surfaceCreated，在Surface（SurfaceView內部包含一個Surface實例）創建後，會立即調用該方法，可在該方法中做繪制界面相關的初始化工作；
surfaceChanged，當Surface的狀態發生變化，比如大小，會調用該方法，在surfaceCreated方法調用過至少會調用一次該方法；
surfaceDestroyed，當銷毀Surface的時候調用。
　　開發者不能直接操作Surface實例，要通過SurfaceHandler，在SurfaceView中可以通過getHandler方法獲取到SurfaceHandler實例。
SurfaceHander有一些類型，用來標識Surface實例界面數據來源，可以通過setType來操作：
SURFACE_TYPE_NORMAL：RAM緩存的原生數據
SURFACE_TYPE_HARDWARE：通過DMA，direct memory access，就是直接寫屏技術獲取到的數據，或者其他硬件加速的數據
SURFACE_TYPE_GPU：通過GPU加速的數據
SURFACE_TYPE_PUSH_BUFFERS：標識數據來源於其他對象，比如照相機，比如視頻播放服務器（android內部有視頻播放的服務器，所有播放視頻相當於客戶端）

　　CustomerVideoView的構造方法，使用超類的構造方法。都會執行initVideoView()方法用來初始化界面和參數。另外一個主要的內容是openVideo()方法：
mediaPlayer.prepareAsync()，用來異步准備播放，另外還有個prepare()方法，是同步的，也就是全部下載完畢才能播放，顯然，在播放網上視頻的時候需要用前者；
通過attachMediaController()方法，把控制條附加到播放視頻的SurfaceView上，這裡實現的不完全，因此還不能使用，僅僅是把MediaPlayerControl實例通過setMediaPlayer方法設置一下，供OnPreparedListener用來得到加載成功的回調，另外供外面代碼調用得到視頻的時長和當前時長。