spydroid-ipcamera源码分析（一）：Session的内部机制与构建方式

Session类

Session对象主要用于存储某个会话对象所需的属性以及配置信息，并且使用同步或者异步的方法对Session内部的流媒体对象进行操作和控制。

    private String mOrigin; 
    private String mDestination;
    private int mTimeToLive = 64;
    private long mTimestamp;

    private AudioStream mAudioStream = null;
    private VideoStream mVideoStream = null;

    private Callback mCallback;
    private Handler mMainHandler;
    
    private static CountDownLatch sSignal;
    private static Handler sHandler;

mMainHandler：在主线程执行方法时用到的handler，其中所有的callback回调方法都是在主线程中回调的。

sHandler：在子线程执行方法用到的handler，大部分对Stream的操作都是在子线程中执行。

mAudioStream和mVideoStream：音频流媒体和视频流媒体对象，Session对其进行操作。

    static {
        // Creates the Thread that will be used when asynchronous methods of a Session are called
        sSignal = new CountDownLatch(1);
        new HandlerThread("net.majorkernelpanic.streaming.Session"){
            @Override
            protected void onLooperPrepared() {
                sHandler = new Handler();
                sSignal.countDown();
            }
        }.start();
    }

在静态块中创建一个子线程，作为各个异步方法的执行线程。

/** 
     * Creates a streaming session that can be customized by adding tracks.
     */
    public Session() {
        long uptime = System.currentTimeMillis();
        mMainHandler = new Handler(Looper.getMainLooper());
        mTimestamp = (uptime/1000)<<32 & (((uptime-((uptime/1000)*1000))>>32)/1000); // NTP timestamp
        mOrigin = "127.0.0.1";
        
        // Me make sure that we won't send Runnables to a non existing thread
        try {
            sSignal.await();
        } catch (InterruptedException e) {}
    }

构造函数中sSignal.await();是等待上面的子线程Looper创建完毕后，再往下执行主线程。

/** Deletes all existing tracks & release associated resources. */
    public void release() {
        removeAudioTrack();
        removeVideoTrack();
        sHandler.getLooper().quit();
    }

在最后release时，释放音频流和视频流，并且需要将sHandler的looper释放掉，这个方法是在主线程中执行。

对于Stream进行操作的一系列方法，由于篇幅原因不在这里展开列举，其实大多数的操作动作都是由sHandler.post发出消息，在子线程中执行的；而动作执行完成的回调都是由mMainHandler.post发出消息，在主线程中执行回调。

SessionBuilder类

SessionBuilder类就是用来构建Session对象。SessionBuilder采用了单例模式，需要使用getInstance()方法来获取到实例对象。

// Default configuration
    private VideoQuality mVideoQuality = VideoQuality.DEFAULT_VIDEO_QUALITY;
    private AudioQuality mAudioQuality = AudioQuality.DEFAULT_AUDIO_QUALITY;
    private Context mContext;
    private int mVideoEncoder = VIDEO_H263; 
    private int mAudioEncoder = AUDIO_AMRNB;
    private int mCamera = CameraInfo.CAMERA_FACING_BACK;
    private int mTimeToLive = 64;
    private int mOrientation = 0;
    private boolean mFlash = false;
    private SurfaceView mSurfaceView = null;
    private String mOrigin = null;
    private String mDestination = null;
    private Session.Callback mCallback = null;

构建Session时默认的配置参数，可以通过set方法设置参数值。

/**
     * Creates a new {@link Session}.
     * @return The new Session
     * @throws IOException 
     */
    public Session build() {
        Session session;

        session = new Session();
        session.setOrigin(mOrigin);
        session.setDestination(mDestination);
        session.setTimeToLive(mTimeToLive);
        session.setCallback(mCallback);

        switch (mAudioEncoder) {
        case AUDIO_AAC:
            AACStream stream = new AACStream();
            session.addAudioTrack(stream);
            if (mContext!=null) 
                stream.setPreferences(PreferenceManager.getDefaultSharedPreferences(mContext));
            break;
        case AUDIO_AMRNB:
            session.addAudioTrack(new AMRNBStream());
            break;
        }

        switch (mVideoEncoder) {
        case VIDEO_H263:
            session.addVideoTrack(new H263Stream(mCamera));
            break;
        case VIDEO_H264:
            H264Stream stream = new H264Stream(mCamera);
            if (mContext!=null) 
                stream.setPreferences(PreferenceManager.getDefaultSharedPreferences(mContext));
            session.addVideoTrack(stream);
            break;
        }

        if (session.getVideoTrack()!=null) {
            VideoStream video = session.getVideoTrack();
            video.setFlashState(mFlash);
            video.setVideoQuality(mVideoQuality);
            video.setSurfaceView(mSurfaceView);
            video.setPreviewOrientation(mOrientation);
            video.setDestinationPorts(5006);
        }

        if (session.getAudioTrack()!=null) {
            AudioStream audio = session.getAudioTrack();
            audio.setAudioQuality(mAudioQuality);
            audio.setDestinationPorts(5004);
        }

        return session;

    }

SessionBuilder使用了build模式来构建Session，build()方法创建Session对象并对其附上已设置完成的参数值，mAudioEncoder和mVideoEncoder用于设置音频和视频的编码格式。

这篇文章我们大致了解了Session内部机制（两个handler分别负责同步操作和异步操作）和构建方式（SessionBuilder的build()方法），下一步我们将逐步分析Stream类和MediaStream类。