setOneShotPreviewCallback 未在回调中点击 onPreviewFrame()

我正在为我正在制作的应用程序创建视频捕获活动，并且我想捕获预览帧以用作播放按钮。我已经设置了SurfaceView up in onCreate(...)并连接相机的预览显示SurfaceHolder.Callback:

private Callback mSurfaceViewCallback = new Callback() {

    @Override
    public void surfaceDestroyed(SurfaceHolder holder) {
        mSurfaceCreated = false;
    }

    @Override
    public void surfaceCreated(SurfaceHolder holder) {
        try {
            mSurfaceCreated = true;
            mCamera.setPreviewDisplay(mHolder);
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

    @Override
    public void surfaceChanged(SurfaceHolder holder, int format, int width,
            int height) {
        try {
            mSurfaceCreated = true;
            mCamera.setPreviewDisplay(mHolder);
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
};

这对于静态相机应用程序来说效果很好。相机预览打开和关闭onPause() and onResume():

@Override
protected void onResume() {
    super.onResume();
    if(mCamera!=null)
        mCamera.startPreview();
}

@Override
protected void onPause() {
    if(mCamera!=null)
        mCamera.stopPreview();
    super.onPause();
}

我已经设置了MediaRecorder像这样：

protected void startRecording() {
    mMediaRecorder = new MediaRecorder();
    mCamera.unlock();
    mMediaRecorder.setCamera(mCamera);
    mMediaRecorder.setAudioSource(MediaRecorder.AudioSource.CAMCORDER);
    mMediaRecorder.setVideoSource(MediaRecorder.VideoSource.CAMERA);
    mMediaRecorder.setProfile(CamcorderProfile.get(CamcorderProfile.QUALITY_LOW));

    mMediaRecorder.setMaxDuration(10000);
    mMediaRecorder.setOutputFile(new File(getFilesDir(), mChallengeUUID + ".mp4").getAbsolutePath());
    mMediaRecorder.setPreviewDisplay(mHolder.getSurface());
    try {
        mMediaRecorder.prepare();
    } catch (IllegalStateException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
    mCamera.setOneShotPreviewCallback(mPreviewCallback);
    mMediaRecorder.start();
}

然而，倒数第二行不起作用。我的Camera.PreviewCallback's onPreviewFrame(...)方法被遗漏了。

正如我所说，拥有它的目的是从录制视频的开头获取一帧以显示给用户。为什么回调没有被触发？

mCamera.unlock()将相机从进程中分离出来。有必要调用：

mCamera.reconnect();

致电之前：

mCamera.setOneShotPreviewCallback(mPreviewCallback);

（但在一切之后）。