前言

本文是基于PineAppRtc開源項目https://github.com/thfhongfeng/PineAppRtc

因為一個需求，我們需要將一個視頻流通過WebRtc發送出去，所以就研究一下WebRtc是如何采集視頻數據并進行處理發送的，于是有了這篇文章。

采集發送

在使用webrtc進行即時通話時，雙方連接上后，會根據參數創建一個PeerConnection連接對象，具體代碼在PeerConnectionClient類中，這個是需要自己來實現的。這個連接的作用來進行推拉流的。

然后創建一個MediaStream對象，并添加給PeerConnection

mPeerConnection.addStream(mMediaStream);

這個MediaStream就是處理流的，可以給MediaStream對象添加多個軌道，比如聲音軌道、視頻軌道

mMediaStream.addTrack(createVideoTrack(mVideoCapturer));

這里mVideoCapturer是一個VideoCapturer對象，用來處理視頻收集的，實際上就是封裝了相機

VideoCapturer是一個接口，有很多實現類。這里以CameraCapturer及子類Camera1Capturer為例子

繼續看createVideoTrack這個函數

private VideoTrack createVideoTrack(VideoCapturer capturer) {mVideoSource = mFactory.createVideoSource(capturer);capturer.startCapture(mVideoWidth, mVideoHeight, mVideoFps);mLocalVideoTrack = mFactory.createVideoTrack(VIDEO_TRACK_ID, mVideoSource);mLocalVideoTrack.setEnabled(mRenderVideo);mLocalVideoTrack.addRenderer(new VideoRenderer(mLocalRender));return mLocalVideoTrack; }

可以看到通過createVideoSource函數將VideoCapturer封裝到VideoSource對象中，然后利用VideoSource創建出軌道的VideoTrack。

來看看createVideoSource函數

public VideoSource createVideoSource(VideoCapturer capturer) {org.webrtc.EglBase.Context eglContext = this.localEglbase == null ? null : this.localEglbase.getEglBaseContext();SurfaceTextureHelper surfaceTextureHelper = SurfaceTextureHelper.create("VideoCapturerThread", eglContext);long nativeAndroidVideoTrackSource = nativeCreateVideoSource(this.nativeFactory, surfaceTextureHelper, capturer.isScreencast());CapturerObserver capturerObserver = new AndroidVideoTrackSourceObserver(nativeAndroidVideoTrackSource);capturer.initialize(surfaceTextureHelper, ContextUtils.getApplicationContext(), capturerObserver);return new VideoSource(nativeAndroidVideoTrackSource); }

可以看到這里新建了一個AndroidVideoTrackSourceObserver對象，它是CaptureObserver接口的實現，然后調用了VideoCapturer的initialize函數
在CameraCapturer實現的initialize函數中將AndroidVideoTrackSourceObserver對象賦值給了VideoCapturer的capturerObserver屬性。

回過頭再看看PeerConnectionClient類中，還調用了VideoCapturer的startCapture函數，看看它在CameraCapturer中的實現

public void startCapture(int width, int height, int framerate) {Logging.d("CameraCapturer", "startCapture: " + width + "x" + height + "@" + framerate);if (this.applicationContext == null) {throw new RuntimeException("CameraCapturer must be initialized before calling startCapture.");} else {Object var4 = this.stateLock;synchronized(this.stateLock) {if (!this.sessionOpening && this.currentSession == null) {this.width = width;this.height = height;this.framerate = framerate;this.sessionOpening = true;this.openAttemptsRemaining = 3;this.createSessionInternal(0, (MediaRecorder)null);} else {Logging.w("CameraCapturer", "Session already open");}}} }

最后執行了createSessionInternal

private void createSessionInternal(int delayMs, final MediaRecorder mediaRecorder) {this.uiThreadHandler.postDelayed(this.openCameraTimeoutRunnable, (long)(delayMs + 10000));this.cameraThreadHandler.postDelayed(new Runnable() {public void run() {CameraCapturer.this.createCameraSession(CameraCapturer.this.createSessionCallback, CameraCapturer.this.cameraSessionEventsHandler, CameraCapturer.this.applicationContext, CameraCapturer.this.surfaceHelper, mediaRecorder, CameraCapturer.this.cameraName, CameraCapturer.this.width, CameraCapturer.this.height, CameraCapturer.this.framerate);}}, (long)delayMs); }

又執行了createCameraSession，在Camera1Capturer中該函數代碼如下

protected void createCameraSession(CreateSessionCallback createSessionCallback, Events events, Context applicationContext, SurfaceTextureHelper surfaceTextureHelper, MediaRecorder mediaRecorder, String cameraName, int width, int height, int framerate) {Camera1Session.create(createSessionCallback, events, this.captureToTexture || mediaRecorder != null, applicationContext, surfaceTextureHelper, mediaRecorder, Camera1Enumerator.getCameraIndex(cameraName), width, height, framerate); }

可以看到創建了一個Camera1Session，這個類就是實際操作相機的，在這個類里就看到了熟悉的Camera，在listenForBytebufferFrames函數中

private void listenForBytebufferFrames() {this.camera.setPreviewCallbackWithBuffer(new PreviewCallback() {public void onPreviewFrame(byte[] data, Camera callbackCamera) {Camera1Session.this.checkIsOnCameraThread();if (callbackCamera != Camera1Session.this.camera) {Logging.e("Camera1Session", "Callback from a different camera. This should never happen.");} else if (Camera1Session.this.state != Camera1Session.SessionState.RUNNING) {Logging.d("Camera1Session", "Bytebuffer frame captured but camera is no longer running.");} else {long captureTimeNs = TimeUnit.MILLISECONDS.toNanos(SystemClock.elapsedRealtime());if (!Camera1Session.this.firstFrameReported) {int startTimeMs = (int)TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - Camera1Session.this.constructionTimeNs);Camera1Session.camera1StartTimeMsHistogram.addSample(startTimeMs);Camera1Session.this.firstFrameReported = true;}Camera1Session.this.events.onByteBufferFrameCaptured(Camera1Session.this, data, Camera1Session.this.captureFormat.width, Camera1Session.this.captureFormat.height, Camera1Session.this.getFrameOrientation(), captureTimeNs);Camera1Session.this.camera.addCallbackBuffer(data);}}}); }

可以看到在通過預覽回調onPreviewFrame拿到視頻數據后，調用了events.onByteBufferFrameCaptured，這個events就是create時傳入的，回溯上面的流程可以發現這個events就是CameraCapturer中的cameraSessionEventsHandler，它的onByteBufferFrameCaptured函數如下：

public void onByteBufferFrameCaptured(CameraSession session, byte[] data, int width, int height, int rotation, long timestamp) {CameraCapturer.this.checkIsOnCameraThread();synchronized(CameraCapturer.this.stateLock) {if (session != CameraCapturer.this.currentSession) {Logging.w("CameraCapturer", "onByteBufferFrameCaptured from another session.");} else {if (!CameraCapturer.this.firstFrameObserved) {CameraCapturer.this.eventsHandler.onFirstFrameAvailable();CameraCapturer.this.firstFrameObserved = true;}CameraCapturer.this.cameraStatistics.addFrame();CameraCapturer.this.capturerObserver.onByteBufferFrameCaptured(data, width, height, rotation, timestamp);}} }

這里調用了capturerObserver.onByteBufferFrameCaptured，這個capturerObserver就是前面initialize時傳入的AndroidVideoTrackSourceObserver對象，它的onByteBufferFrameCaptured函數

public void onByteBufferFrameCaptured(byte[] data, int width, int height, int rotation, long timeStamp) {this.nativeOnByteBufferFrameCaptured(this.nativeSource, data, data.length, width, height, rotation, timeStamp); }

調用了native函數。這樣整個流程就結束了，應該在native中對數據進行處理并發送。

其實這里關鍵就是VideoCapturer，除了CameraCapturer及子類，還有FileVideoCapturer等。
如果我們需要直接發送byte[]原生數據，可以自定義實現一個VideoCapturer，獲取他的capturerObserver變量，主動調用它的onByteBufferFrameCaptured函數即可。

總結

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