WebRTC中Android Demo中的攝像頭從採集到預覽流程

 

APPRTC-Demo調用流程

　　1.CallActivity#onCreate 執行startCall開始鏈接或建立房間

 

　　2.WebSocketClient#connectToRoom 請求一次服務器

 

　　3.回調到CallActivity#onConnectToRoom 開始建立對等鏈接，同時將視頻採集對象，本地和遠程的VideoSink，相關參數傳入

 

 

　　　　localProxyVideoSink代理本地視頻渲染器

　　　　remoteSinks是代理遠程視頻的渲染器，這裏是一個集合

　　　　videoCapture是本地視頻採集器

 

      4.PeerConnectionClient#createPeerConnectionInternal 建立PeerConnection對象和建立視頻軌道

 　　factory是在CallActivity#onCreate中建立的

　 　pcObserver是一個對等鏈接觀察者，用於底層消息的回調

 

　　若是開啓了視頻功能，則將本地採集的數據添加到軌道（經過C++底層完成）

　　若是是遠程的數據，經過(getRemoteVideoTrack調用C++底層方法)獲取到遠程視頻軌道，添加傳入進來的remoteSinks

 

　　這裏繼續添加音頻軌道。

　　APPRTC的demo調用基本結束，到這裏手機上就能預覽出視頻了。

　　下面具體分析這個VideoCapture怎麼來的。

 

 

VideoCapture怎麼來的(或者說如何採集數據的)

　　首先來一個參考文獻==>webrtc源碼分析之視頻採集之一

　　1.回到最初的起點CallActivity#onConnectedToRoomInternal

　　

 

　　2.CallActivity#createVideoCapturer

　　　　webrtc針對視頻採集對外主要提供了3個接口。

　　　　ScreenCapturerAndroid==>屏幕的視頻來源採集

　　　　FileVideoCapturer==>文件的視頻來源採集

　　　　CameraCapturer==>攝像頭的視頻來源採集 這裏有分爲Camera1Capturer和Camera2Capturer

　　　　反正返回一個VideoCapturer對象就ok啦。

　　　　下面進入如何建立攝像頭採集器的方法體中。

 

　　3.CallActivity#createCameraCapturer　　　　

　　這裏經過一個CameraEnumerator對象來建立視頻採集器。

　　

　　詳細看一下這個CameraEnumerator接口。　

　

　　這個對象能夠獲取到設備名，判斷是不是前置或後置攝像頭，還能夠建立一個攝像頭的視頻採集器。

 

　　拿攝像頭1的CamearEnumerator實現類爲例。

　　#Camera1Enumerator#createCapturer

　　因此最終仍是調用了Camera1Capturer來建立攝像頭採集器。

　　下面分析Camera1Capturer實現過程。

 

　　4.Camera1Capturer實現過程。

　　相機採集的實現是CameraCapturer

　　針對不一樣相機API又分爲Camera1Capturer和Camera2Capturer。

　　相機採集的邏輯都封裝在CameraCapturer中

　　只有建立CameraSession的代碼在兩個子類中有不一樣的實現。

 

　　5.無奈，再進入CameraCapturer類中再次尋覓。

abstract class CameraCapturer implements CameraVideoCapturer {
  enum SwitchState {
    IDLE, // No switch requested.
    PENDING, // Waiting for previous capture session to open.
    IN_PROGRESS, // Waiting for new switched capture session to start.
  }

  private static final String TAG = "CameraCapturer";
  private final static int MAX_OPEN_CAMERA_ATTEMPTS = 3;
  private final static int OPEN_CAMERA_DELAY_MS = 500;
  private final static int OPEN_CAMERA_TIMEOUT = 10000;

  private final CameraEnumerator cameraEnumerator;
  @Nullable private final CameraEventsHandler eventsHandler;
  private final Handler uiThreadHandler;

  @Nullable
  private final CameraSession.CreateSessionCallback createSessionCallback =
      new CameraSession.CreateSessionCallback() {
        @Override
        public void onDone(CameraSession session) {
          checkIsOnCameraThread();
          Logging.d(TAG, "Create session done. Switch state: " + switchState);
          uiThreadHandler.removeCallbacks(openCameraTimeoutRunnable);
          synchronized (stateLock) {
            capturerObserver.onCapturerStarted(true /* success */);
            sessionOpening = false;
            currentSession = session;
            cameraStatistics = new CameraStatistics(surfaceHelper, eventsHandler);
            firstFrameObserved = false;
            stateLock.notifyAll();

            if (switchState == SwitchState.IN_PROGRESS) {
              if (switchEventsHandler != null) {
                switchEventsHandler.onCameraSwitchDone(cameraEnumerator.isFrontFacing(cameraName));
                switchEventsHandler = null;
              }
              switchState = SwitchState.IDLE;
            } else if (switchState == SwitchState.PENDING) {
              switchState = SwitchState.IDLE;
              switchCameraInternal(switchEventsHandler);
            }
          }
        }

        @Override
        public void onFailure(CameraSession.FailureType failureType, String error) {
          checkIsOnCameraThread();
          uiThreadHandler.removeCallbacks(openCameraTimeoutRunnable);
          synchronized (stateLock) {
            capturerObserver.onCapturerStarted(false /* success */);
            openAttemptsRemaining--;

            if (openAttemptsRemaining <= 0) {
              Logging.w(TAG, "Opening camera failed, passing: " + error);
              sessionOpening = false;
              stateLock.notifyAll();

              if (switchState != SwitchState.IDLE) {
                if (switchEventsHandler != null) {
                  switchEventsHandler.onCameraSwitchError(error);
                  switchEventsHandler = null;
                }
                switchState = SwitchState.IDLE;
              }

              if (failureType == CameraSession.FailureType.DISCONNECTED) {
                eventsHandler.onCameraDisconnected();
              } else {
                eventsHandler.onCameraError(error);
              }
            } else {
              Logging.w(TAG, "Opening camera failed, retry: " + error);
              createSessionInternal(OPEN_CAMERA_DELAY_MS);
            }
          }
        }
      };

  @Nullable
  private final CameraSession.Events cameraSessionEventsHandler = new CameraSession.Events() {
    @Override
    public void onCameraOpening() {
      checkIsOnCameraThread();
      synchronized (stateLock) {
        if (currentSession != null) {
          Logging.w(TAG, "onCameraOpening while session was open.");
          return;
        }
        eventsHandler.onCameraOpening(cameraName);
      }
    }

    @Override
    public void onCameraError(CameraSession session, String error) {
      checkIsOnCameraThread();
      synchronized (stateLock) {
        if (session != currentSession) {
          Logging.w(TAG, "onCameraError from another session: " + error);
          return;
        }
        eventsHandler.onCameraError(error);
        stopCapture();
      }
    }

    @Override
    public void onCameraDisconnected(CameraSession session) {
      checkIsOnCameraThread();
      synchronized (stateLock) {
        if (session != currentSession) {
          Logging.w(TAG, "onCameraDisconnected from another session.");
          return;
        }
        eventsHandler.onCameraDisconnected();
        stopCapture();
      }
    }

    @Override
    public void onCameraClosed(CameraSession session) {
      checkIsOnCameraThread();
      synchronized (stateLock) {
        if (session != currentSession && currentSession != null) {
          Logging.d(TAG, "onCameraClosed from another session.");
          return;
        }
        eventsHandler.onCameraClosed();
      }
    }

    @Override
    public void onFrameCaptured(CameraSession session, VideoFrame frame) {
      checkIsOnCameraThread();
      synchronized (stateLock) {
        if (session != currentSession) {
          Logging.w(TAG, "onFrameCaptured from another session.");
          return;
        }
        if (!firstFrameObserved) {
          eventsHandler.onFirstFrameAvailable();
          firstFrameObserved = true;
        }
        cameraStatistics.addFrame();
        capturerObserver.onFrameCaptured(frame);
      }
    }
  };

  private final Runnable openCameraTimeoutRunnable = new Runnable() {
    @Override
    public void run() {
      eventsHandler.onCameraError("Camera failed to start within timeout.");
    }
  };

  // Initialized on initialize
  // -------------------------
  @Nullable private Handler cameraThreadHandler;
  private Context applicationContext;
  private CapturerObserver capturerObserver;
  @Nullable private SurfaceTextureHelper surfaceHelper;

  private final Object stateLock = new Object();
  private boolean sessionOpening; /* guarded by stateLock */
  @Nullable private CameraSession currentSession; /* guarded by stateLock */
  private String cameraName; /* guarded by stateLock */
  private int width; /* guarded by stateLock */
  private int height; /* guarded by stateLock */
  private int framerate; /* guarded by stateLock */
  private int openAttemptsRemaining; /* guarded by stateLock */
  private SwitchState switchState = SwitchState.IDLE; /* guarded by stateLock */
  @Nullable private CameraSwitchHandler switchEventsHandler; /* guarded by stateLock */
  // Valid from onDone call until stopCapture, otherwise null.
  @Nullable private CameraStatistics cameraStatistics; /* guarded by stateLock */
  private boolean firstFrameObserved; /* guarded by stateLock */

  public CameraCapturer(String cameraName, @Nullable CameraEventsHandler eventsHandler,
      CameraEnumerator cameraEnumerator) {
    if (eventsHandler == null) {
      eventsHandler = new CameraEventsHandler() {
        @Override
        public void onCameraError(String errorDescription) {}
        @Override
        public void onCameraDisconnected() {}
        @Override
        public void onCameraFreezed(String errorDescription) {}
        @Override
        public void onCameraOpening(String cameraName) {}
        @Override
        public void onFirstFrameAvailable() {}
        @Override
        public void onCameraClosed() {}
      };
    }

    this.eventsHandler = eventsHandler;
    this.cameraEnumerator = cameraEnumerator;
    this.cameraName = cameraName;
    uiThreadHandler = new Handler(Looper.getMainLooper());

    final String[] deviceNames = cameraEnumerator.getDeviceNames();

    if (deviceNames.length == 0) {
      throw new RuntimeException("No cameras attached.");
    }
    if (!Arrays.asList(deviceNames).contains(this.cameraName)) {
      throw new IllegalArgumentException(
          "Camera name " + this.cameraName + " does not match any known camera device.");
    }
  }

  @Override
  public void initialize(@Nullable SurfaceTextureHelper surfaceTextureHelper,
      Context applicationContext, CapturerObserver capturerObserver) {
    this.applicationContext = applicationContext;
    this.capturerObserver = capturerObserver;
    this.surfaceHelper = surfaceTextureHelper;
    this.cameraThreadHandler =
        surfaceTextureHelper == null ? null : surfaceTextureHelper.getHandler();
  }

  @Override
  public void startCapture(int width, int height, int framerate) {
    Logging.d(TAG, "startCapture: " + width + "x" + height + "@" + framerate);
    if (applicationContext == null) {
      throw new RuntimeException("CameraCapturer must be initialized before calling startCapture.");
    }

    synchronized (stateLock) {
      if (sessionOpening || currentSession != null) {
        Logging.w(TAG, "Session already open");
        return;
      }

      this.width = width;
      this.height = height;
      this.framerate = framerate;

      sessionOpening = true;
      openAttemptsRemaining = MAX_OPEN_CAMERA_ATTEMPTS;
      createSessionInternal(0);
    }
  }

  private void createSessionInternal(int delayMs) {
    uiThreadHandler.postDelayed(openCameraTimeoutRunnable, delayMs + OPEN_CAMERA_TIMEOUT);
    cameraThreadHandler.postDelayed(new Runnable() {
      @Override
      public void run() {
        createCameraSession(createSessionCallback, cameraSessionEventsHandler, applicationContext,
            surfaceHelper, cameraName, width, height, framerate);
      }
    }, delayMs);
  }

  @Override
  public void stopCapture() {
    Logging.d(TAG, "Stop capture");

    synchronized (stateLock) {
      while (sessionOpening) {
        Logging.d(TAG, "Stop capture: Waiting for session to open");
        try {
          stateLock.wait();
        } catch (InterruptedException e) {
          Logging.w(TAG, "Stop capture interrupted while waiting for the session to open.");
          Thread.currentThread().interrupt();
          return;
        }
      }

      if (currentSession != null) {
        Logging.d(TAG, "Stop capture: Nulling session");
        cameraStatistics.release();
        cameraStatistics = null;
        final CameraSession oldSession = currentSession;
        cameraThreadHandler.post(new Runnable() {
          @Override
          public void run() {
            oldSession.stop();
          }
        });
        currentSession = null;
        capturerObserver.onCapturerStopped();
      } else {
        Logging.d(TAG, "Stop capture: No session open");
      }
    }

    Logging.d(TAG, "Stop capture done");
  }

  @Override
  public void changeCaptureFormat(int width, int height, int framerate) {
    Logging.d(TAG, "changeCaptureFormat: " + width + "x" + height + "@" + framerate);
    synchronized (stateLock) {
      stopCapture();
      startCapture(width, height, framerate);
    }
  }

  @Override
  public void dispose() {
    Logging.d(TAG, "dispose");
    stopCapture();
  }

  @Override
  public void switchCamera(final CameraSwitchHandler switchEventsHandler) {
    Logging.d(TAG, "switchCamera");
    cameraThreadHandler.post(new Runnable() {
      @Override
      public void run() {
        switchCameraInternal(switchEventsHandler);
      }
    });
  }

  @Override
  public boolean isScreencast() {
    return false;
  }

  public void printStackTrace() {
    Thread cameraThread = null;
    if (cameraThreadHandler != null) {
      cameraThread = cameraThreadHandler.getLooper().getThread();
    }
    if (cameraThread != null) {
      StackTraceElement[] cameraStackTrace = cameraThread.getStackTrace();
      if (cameraStackTrace.length > 0) {
        Logging.d(TAG, "CameraCapturer stack trace:");
        for (StackTraceElement traceElem : cameraStackTrace) {
          Logging.d(TAG, traceElem.toString());
        }
      }
    }
  }

  private void reportCameraSwitchError(
      String error, @Nullable CameraSwitchHandler switchEventsHandler) {
    Logging.e(TAG, error);
    if (switchEventsHandler != null) {
      switchEventsHandler.onCameraSwitchError(error);
    }
  }

  private void switchCameraInternal(@Nullable final CameraSwitchHandler switchEventsHandler) {
    Logging.d(TAG, "switchCamera internal");

    final String[] deviceNames = cameraEnumerator.getDeviceNames();

    if (deviceNames.length < 2) {
      if (switchEventsHandler != null) {
        switchEventsHandler.onCameraSwitchError("No camera to switch to.");
      }
      return;
    }

    synchronized (stateLock) {
      if (switchState != SwitchState.IDLE) {
        reportCameraSwitchError("Camera switch already in progress.", switchEventsHandler);
        return;
      }
      if (!sessionOpening && currentSession == null) {
        reportCameraSwitchError("switchCamera: camera is not running.", switchEventsHandler);
        return;
      }

      this.switchEventsHandler = switchEventsHandler;
      if (sessionOpening) {
        switchState = SwitchState.PENDING;
        return;
      } else {
        switchState = SwitchState.IN_PROGRESS;
      }

      Logging.d(TAG, "switchCamera: Stopping session");
      cameraStatistics.release();
      cameraStatistics = null;
      final CameraSession oldSession = currentSession;
      cameraThreadHandler.post(new Runnable() {
        @Override
        public void run() {
          oldSession.stop();
        }
      });
      currentSession = null;

      int cameraNameIndex = Arrays.asList(deviceNames).indexOf(cameraName);
      cameraName = deviceNames[(cameraNameIndex + 1) % deviceNames.length];

      sessionOpening = true;
      openAttemptsRemaining = 1;
      createSessionInternal(0);
    }
    Logging.d(TAG, "switchCamera done");
  }

  private void checkIsOnCameraThread() {
    if (Thread.currentThread() != cameraThreadHandler.getLooper().getThread()) {
      Logging.e(TAG, "Check is on camera thread failed.");
      throw new RuntimeException("Not on camera thread.");
    }
  }

  protected String getCameraName() {
    synchronized (stateLock) {
      return cameraName;
    }
  }

  abstract protected void createCameraSession(
      CameraSession.CreateSessionCallback createSessionCallback, CameraSession.Events events,
      Context applicationContext, SurfaceTextureHelper surfaceTextureHelper, String cameraName,
      int width, int height, int framerate);
}

View Code

　　哎呀，這是一個抽象類，把最關鍵的createCameraSession交給子類去實現了。　　

　　因此又要回到CameraCapturer的子類中看看具體實現方案。

 

　　6.如上圖的第4點中的Camera1Capturer#createCameraSession　　

　　原來源頭是CameraSession。

　　由它來採集視頻流的。

 

　　7.首先看看CameraSession接口。

　　

　　主要有兩個回調類。

　　一個建立Session的回調，正在建立onDone和建立失敗onFailure。

　　一個是事件回調，攝像頭打開，失敗，斷開鏈接，關閉，採集。

　　中止採集stop()。

 

　　8.再來看看CameraSession的實現類，如Camera1Session。

class Camera1Session implements CameraSession {
  private static final String TAG = "Camera1Session";
  private static final int NUMBER_OF_CAPTURE_BUFFERS = 3;

  private static final Histogram camera1StartTimeMsHistogram =
      Histogram.createCounts("WebRTC.Android.Camera1.StartTimeMs", 1, 10000, 50);
  private static final Histogram camera1StopTimeMsHistogram =
      Histogram.createCounts("WebRTC.Android.Camera1.StopTimeMs", 1, 10000, 50);
  private static final Histogram camera1ResolutionHistogram = Histogram.createEnumeration(
      "WebRTC.Android.Camera1.Resolution", CameraEnumerationAndroid.COMMON_RESOLUTIONS.size());

  private static enum SessionState { RUNNING, STOPPED }

  private final Handler cameraThreadHandler;
  private final Events events;
  private final boolean captureToTexture;
  private final Context applicationContext;
  private final SurfaceTextureHelper surfaceTextureHelper;
  private final int cameraId;
  private final android.hardware.Camera camera;
  private final android.hardware.Camera.CameraInfo info;
  private final CaptureFormat captureFormat;
  // Used only for stats. Only used on the camera thread.
  private final long constructionTimeNs; // Construction time of this class.

  private SessionState state;
  private boolean firstFrameReported = false;

  // TODO(titovartem) make correct fix during webrtc:9175
  @SuppressWarnings("ByteBufferBackingArray")
  public static void create(final CreateSessionCallback callback, final Events events,
      final boolean captureToTexture, final Context applicationContext,
      final SurfaceTextureHelper surfaceTextureHelper, final int cameraId, final int width,
      final int height, final int framerate) {
    final long constructionTimeNs = System.nanoTime();
    Logging.d(TAG, "Open camera " + cameraId);
    events.onCameraOpening();

    final android.hardware.Camera camera;
    try {
      camera = android.hardware.Camera.open(cameraId);
    } catch (RuntimeException e) {
      callback.onFailure(FailureType.ERROR, e.getMessage());
      return;
    }

    if (camera == null) {
      callback.onFailure(FailureType.ERROR,
          "android.hardware.Camera.open returned null for camera id = " + cameraId);
      return;
    }

    try {
      camera.setPreviewTexture(surfaceTextureHelper.getSurfaceTexture());
    } catch (IOException | RuntimeException e) {
      camera.release();
      callback.onFailure(FailureType.ERROR, e.getMessage());
      return;
    }

    final android.hardware.Camera.CameraInfo info = new android.hardware.Camera.CameraInfo();
    android.hardware.Camera.getCameraInfo(cameraId, info);

    final CaptureFormat captureFormat;
    try {
      final android.hardware.Camera.Parameters parameters = camera.getParameters();
      captureFormat = findClosestCaptureFormat(parameters, width, height, framerate);
      final Size pictureSize = findClosestPictureSize(parameters, width, height);
      updateCameraParameters(camera, parameters, captureFormat, pictureSize, captureToTexture);
    } catch (RuntimeException e) {
      camera.release();
      callback.onFailure(FailureType.ERROR, e.getMessage());
      return;
    }

    if (!captureToTexture) {
      final int frameSize = captureFormat.frameSize();
      for (int i = 0; i < NUMBER_OF_CAPTURE_BUFFERS; ++i) {
        final ByteBuffer buffer = ByteBuffer.allocateDirect(frameSize);
        camera.addCallbackBuffer(buffer.array());
      }
    }

    // Calculate orientation manually and send it as CVO insted.
    camera.setDisplayOrientation(0 /* degrees */);

    callback.onDone(new Camera1Session(events, captureToTexture, applicationContext,
        surfaceTextureHelper, cameraId, camera, info, captureFormat, constructionTimeNs));
  }

  private static void updateCameraParameters(android.hardware.Camera camera,
      android.hardware.Camera.Parameters parameters, CaptureFormat captureFormat, Size pictureSize,
      boolean captureToTexture) {
    final List<String> focusModes = parameters.getSupportedFocusModes();

    parameters.setPreviewFpsRange(captureFormat.framerate.min, captureFormat.framerate.max);
    parameters.setPreviewSize(captureFormat.width, captureFormat.height);
    parameters.setPictureSize(pictureSize.width, pictureSize.height);
    if (!captureToTexture) {
      parameters.setPreviewFormat(captureFormat.imageFormat);
    }

    if (parameters.isVideoStabilizationSupported()) {
      parameters.setVideoStabilization(true);
    }
    if (focusModes.contains(android.hardware.Camera.Parameters.FOCUS_MODE_CONTINUOUS_VIDEO)) {
      parameters.setFocusMode(android.hardware.Camera.Parameters.FOCUS_MODE_CONTINUOUS_VIDEO);
    }
    camera.setParameters(parameters);
  }

  private static CaptureFormat findClosestCaptureFormat(
      android.hardware.Camera.Parameters parameters, int width, int height, int framerate) {
    // Find closest supported format for |width| x |height| @ |framerate|.
    final List<CaptureFormat.FramerateRange> supportedFramerates =
        Camera1Enumerator.convertFramerates(parameters.getSupportedPreviewFpsRange());
    Logging.d(TAG, "Available fps ranges: " + supportedFramerates);

    final CaptureFormat.FramerateRange fpsRange =
        CameraEnumerationAndroid.getClosestSupportedFramerateRange(supportedFramerates, framerate);

    final Size previewSize = CameraEnumerationAndroid.getClosestSupportedSize(
        Camera1Enumerator.convertSizes(parameters.getSupportedPreviewSizes()), width, height);
    CameraEnumerationAndroid.reportCameraResolution(camera1ResolutionHistogram, previewSize);

    return new CaptureFormat(previewSize.width, previewSize.height, fpsRange);
  }

  private static Size findClosestPictureSize(
      android.hardware.Camera.Parameters parameters, int width, int height) {
    return CameraEnumerationAndroid.getClosestSupportedSize(
        Camera1Enumerator.convertSizes(parameters.getSupportedPictureSizes()), width, height);
  }

  private Camera1Session(Events events, boolean captureToTexture, Context applicationContext,
      SurfaceTextureHelper surfaceTextureHelper, int cameraId, android.hardware.Camera camera,
      android.hardware.Camera.CameraInfo info, CaptureFormat captureFormat,
      long constructionTimeNs) {
    Logging.d(TAG, "Create new camera1 session on camera " + cameraId);

    this.cameraThreadHandler = new Handler();
    this.events = events;
    this.captureToTexture = captureToTexture;
    this.applicationContext = applicationContext;
    this.surfaceTextureHelper = surfaceTextureHelper;
    this.cameraId = cameraId;
    this.camera = camera;
    this.info = info;
    this.captureFormat = captureFormat;
    this.constructionTimeNs = constructionTimeNs;

    startCapturing();
  }

  @Override
  public void stop() {
    Logging.d(TAG, "Stop camera1 session on camera " + cameraId);
    checkIsOnCameraThread();
    if (state != SessionState.STOPPED) {
      final long stopStartTime = System.nanoTime();
      stopInternal();
      final int stopTimeMs = (int) TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - stopStartTime);
      camera1StopTimeMsHistogram.addSample(stopTimeMs);
    }
  }

  private void startCapturing() {
    Logging.d(TAG, "Start capturing");
    checkIsOnCameraThread();

    state = SessionState.RUNNING;

    camera.setErrorCallback(new android.hardware.Camera.ErrorCallback() {
      @Override
      public void onError(int error, android.hardware.Camera camera) {
        String errorMessage;
        if (error == android.hardware.Camera.CAMERA_ERROR_SERVER_DIED) {
          errorMessage = "Camera server died!";
        } else {
          errorMessage = "Camera error: " + error;
        }
        Logging.e(TAG, errorMessage);
        stopInternal();
        if (error == android.hardware.Camera.CAMERA_ERROR_EVICTED) {
          events.onCameraDisconnected(Camera1Session.this);
        } else {
          events.onCameraError(Camera1Session.this, errorMessage);
        }
      }
    });

    if (captureToTexture) {
      listenForTextureFrames();
    } else {
      listenForBytebufferFrames();
    }
    try {
      camera.startPreview();
    } catch (RuntimeException e) {
      stopInternal();
      events.onCameraError(this, e.getMessage());
    }
  }

  private void stopInternal() {
    Logging.d(TAG, "Stop internal");
    checkIsOnCameraThread();
    if (state == SessionState.STOPPED) {
      Logging.d(TAG, "Camera is already stopped");
      return;
    }

    state = SessionState.STOPPED;
    surfaceTextureHelper.stopListening();
    // Note: stopPreview or other driver code might deadlock. Deadlock in
    // android.hardware.Camera._stopPreview(Native Method) has been observed on
    // Nexus 5 (hammerhead), OS version LMY48I.
    camera.stopPreview();
    camera.release();
    events.onCameraClosed(this);
    Logging.d(TAG, "Stop done");
  }

  private void listenForTextureFrames() {
    surfaceTextureHelper.startListening(new SurfaceTextureHelper.OnTextureFrameAvailableListener() {
      @Override
      public void onTextureFrameAvailable(
          int oesTextureId, float[] transformMatrix, long timestampNs) {
        checkIsOnCameraThread();

        if (state != SessionState.RUNNING) {
          Logging.d(TAG, "Texture frame captured but camera is no longer running.");
          surfaceTextureHelper.returnTextureFrame();
          return;
        }

        if (!firstFrameReported) {
          final int startTimeMs =
              (int) TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - constructionTimeNs);
          camera1StartTimeMsHistogram.addSample(startTimeMs);
          firstFrameReported = true;
        }

        int rotation = getFrameOrientation();
        if (info.facing == android.hardware.Camera.CameraInfo.CAMERA_FACING_FRONT) {
          // Undo the mirror that the OS "helps" us with.
          // http://developer.android.com/reference/android/hardware/Camera.html#setDisplayOrientation(int)
          transformMatrix = RendererCommon.multiplyMatrices(
              transformMatrix, RendererCommon.horizontalFlipMatrix());
        }
        final VideoFrame.Buffer buffer =
            surfaceTextureHelper.createTextureBuffer(captureFormat.width, captureFormat.height,
                RendererCommon.convertMatrixToAndroidGraphicsMatrix(transformMatrix));
        final VideoFrame frame = new VideoFrame(buffer, rotation, timestampNs);
        events.onFrameCaptured(Camera1Session.this, frame);
        frame.release();
      }
    });
  }

  private void listenForBytebufferFrames() {
    camera.setPreviewCallbackWithBuffer(new android.hardware.Camera.PreviewCallback() {
      @Override
      public void onPreviewFrame(final byte[] data, android.hardware.Camera callbackCamera) {
        checkIsOnCameraThread();

        if (callbackCamera != camera) {
          Logging.e(TAG, "Callback from a different camera. This should never happen.");
          return;
        }

        if (state != SessionState.RUNNING) {
          Logging.d(TAG, "Bytebuffer frame captured but camera is no longer running.");
          return;
        }

        final long captureTimeNs = TimeUnit.MILLISECONDS.toNanos(SystemClock.elapsedRealtime());

        if (!firstFrameReported) {
          final int startTimeMs =
              (int) TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - constructionTimeNs);
          camera1StartTimeMsHistogram.addSample(startTimeMs);
          firstFrameReported = true;
        }

        VideoFrame.Buffer frameBuffer = new NV21Buffer(
            data, captureFormat.width, captureFormat.height, () -> cameraThreadHandler.post(() -> {
              if (state == SessionState.RUNNING) {
                camera.addCallbackBuffer(data);
              }
            }));
        final VideoFrame frame = new VideoFrame(frameBuffer, getFrameOrientation(), captureTimeNs);
        events.onFrameCaptured(Camera1Session.this, frame);
        frame.release();
      }
    });
  }

  private int getDeviceOrientation() {
    int orientation = 0;

    WindowManager wm = (WindowManager) applicationContext.getSystemService(Context.WINDOW_SERVICE);
    switch (wm.getDefaultDisplay().getRotation()) {
      case Surface.ROTATION_90:
        orientation = 90;
        break;
      case Surface.ROTATION_180:
        orientation = 180;
        break;
      case Surface.ROTATION_270:
        orientation = 270;
        break;
      case Surface.ROTATION_0:
      default:
        orientation = 0;
        break;
    }
    return orientation;
  }

  private int getFrameOrientation() {
    int rotation = getDeviceOrientation();
    if (info.facing == android.hardware.Camera.CameraInfo.CAMERA_FACING_BACK) {
      rotation = 360 - rotation;
    }
    return (info.orientation + rotation) % 360;
  }

  private void checkIsOnCameraThread() {
    if (Thread.currentThread() != cameraThreadHandler.getLooper().getThread()) {
      throw new IllegalStateException("Wrong thread");
    }
  }
}

View Code

　　　　再Camera1Session#create中會建立一個Camera1Session對象。　　 

　　　　直接在構造函數中開始採集。

　　　　Camera1Session#startCapturing具體怎麼作的呢？

　　

　　　　再進入Camera1Session#listenForBytebufferFrames看看是什麼狀況。

　　　　原來這裏纔是真正進行採集的地方。

　　　　Camera#setPreviewCallbackWithBuffer，將相機預覽的數據填充到一個緩衝區。

　　　　而後將預覽的二進制數據添加到一個VideoFrame.Buffer中。這裏採用的是NV21Buffer來處理該過程的。

　　　　最後經過event一路回調到Camera1Capture中去了，隨便把frame也帶過去了。