引言 {#引言}

UVCCamera工程主要驱动USB摄像头，进行打开，关闭，录像，拍照等一些操作，调用平台为Android，通过外接USBCamera进行调用。

libuvccamera调用过程简介 {#libuvccamera调用过程简介}

工程主要依赖库有libuvccamera和usbCameraCommon两个库，其中libuvccamera库是底层库，包括C++实现部分，外加几个基本控制类，如USBMonitor和UVCCamera类等。

USBMonitor类负责管理USB设备类，从该类可以选出自己需要的USBCamera设备。

UVCCamera该类就是一个USB的摄像头，基本方法均调用native，通过jni方式直接调用C++实现。该类是一个桥接类，连接上层控制到底层实现的基本事件转发。

比如聚焦，亮度，缩放，白平衡，预览，打开，关闭等，均实际通过该类来实现。

CameraDialog则是以对话框的形式提供对USBMonitor的调用，如发现并找寻自己需要操作的设备，里面用到了DeviceFilter类，主要用来过滤设备，如果不过滤设备，则认为是所有USB设备，主要还是通过USBManager来获取设备，然后进行筛选，如：

|-----------|------------------------------------------------------------------------------| | 1 | HashMap<String, UsbDevice> deviceList = mUsbManager.getDeviceList(); |

DeviceFilter类，这个是实际的筛选类，主要依据XML中的filter进行筛选，也就是说可以在XML中定义自己想要的设备的class，或者说不想要的设备的class，然后用XMLParser进行解析该XML文件，之后用此filter就可以过滤出实际想要的deviceList。

usbCameraCommon调用过程简介 {#usbCameraCommon调用过程简介}

调用过程分析 {#调用过程分析}

usbCameraCommon库是上层封装库，主要用来操作调用摄像头，也就是USBMonitor和UVCCamera等。

上层调用比较主要的是AbstractUVCCameraHandler类，该类基本提供了对于UVCCamera的调用，然后同时开放API给Activity使用。

因为AbstractUVCCameraHandler类是一个Handler类，类定义是：

|-----------|-------------------------------------------------------------------------| | 1 | abstract class AbstractUVCCameraHandler extends Handler { ... } |

也就是说它主要负责消息发送接收，任何调用它的方法它都会转发给内部类，也就是其实内部主要维护的是一个Thread类，叫CameraThread，定义为：

|-----------|----------------------------------------------------------------| | 1 | static final class CameraThread extends Thread { ... } |

该CameraThread类为static类。任何发送给AbstractUVCCameraHandler的方法都会以消息的方式发送给CameraThread进行处理，当没有消息的时候，就什么也不做。以下可以看到转发过程：

|---------------------------------------------------------------------------------------------------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 | public void handleMessage(final Message msg) { final CameraThread thread = mWeakThread.get(); if (thread == null) return; switch (msg.what) { case MSG_OPEN: thread.handleOpen((USBMonitor.UsbControlBlock)msg.obj); break; case MSG_CLOSE: thread.handleClose(); break; case MSG_PREVIEW_START: thread.handleStartPreview(msg.obj); break; case MSG_PREVIEW_STOP: thread.handleStopPreview(); break; case MSG_CAPTURE_STILL: thread.handleCaptureStill((String)msg.obj); break; case MSG_CAPTURE_START: thread.handleStartRecording(); break; case MSG_CAPTURE_STOP: thread.handleStopRecording(); break; case MSG_MEDIA_UPDATE: thread.handleUpdateMedia((String)msg.obj); break; case MSG_RELEASE: thread.handleRelease(); break; default: throw new RuntimeException("unsupported message:what=" + msg.what); } } |

然后CameraThread类内部持有UVCCamera类实例，所有调用CameraThread的方法又会发送给UVCCamera实例进行处理，举例如下：

开始预览的事件传递：

|------------------------------------------------------------------------------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 | public void handleStartPreview(final Object surface) { if (DEBUG) Log.v(TAG_THREAD, "handleStartPreview:"); if ((mUVCCamera == null) || mIsPreviewing) return; try { mUVCCamera.setPreviewSize(mWidth, mHeight, 1, 31, mPreviewMode, mBandwidthFactor); } catch (final IllegalArgumentException e) { try { // fallback to YUV mode mUVCCamera.setPreviewSize(mWidth, mHeight, 1, 31, UVCCamera.DEFAULT_PREVIEW_MODE, mBandwidthFactor); } catch (final IllegalArgumentException e1) { callOnError(e1); return; } } if (surface instanceof SurfaceHolder) { mUVCCamera.setPreviewDisplay((SurfaceHolder)surface); } if (surface instanceof Surface) { mUVCCamera.setPreviewDisplay((Surface)surface); } else { mUVCCamera.setPreviewTexture((SurfaceTexture)surface); } mUVCCamera.startPreview(); mUVCCamera.updateCameraParams(); synchronized (mSync) { mIsPreviewing = true; } callOnStartPreview(); } |

开始录像：

|---------------------------------------------------------------------------------------------------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 | public void handleStartRecording() { if (DEBUG) Log.v(TAG_THREAD, "handleStartRecording:"); try { if ((mUVCCamera == null) || (mMuxer != null)) return; final MediaMuxerWrapper muxer = new MediaMuxerWrapper(".mp4"); // if you record audio only, ".m4a" is also OK. MediaVideoBufferEncoder videoEncoder = null; switch (mEncoderType) { case 1: // for video capturing using MediaVideoEncoder new MediaVideoEncoder(muxer, getWidth(), getHeight(), mMediaEncoderListener); break; case 2: // for video capturing using MediaVideoBufferEncoder videoEncoder = new MediaVideoBufferEncoder(muxer, getWidth(), getHeight(), mMediaEncoderListener); break; // case 0: // for video capturing using MediaSurfaceEncoder default: new MediaSurfaceEncoder(muxer, getWidth(), getHeight(), mMediaEncoderListener); break; } if (true) { // for audio capturing new MediaAudioEncoder(muxer, mMediaEncoderListener); } muxer.prepare(); muxer.startRecording(); if (videoEncoder != null) { mUVCCamera.setFrameCallback(mIFrameCallback, UVCCamera.PIXEL_FORMAT_NV21); } synchronized (mSync) { mMuxer = muxer; mVideoEncoder = videoEncoder; } callOnStartRecording(); } catch (final IOException e) { callOnError(e); Log.e(TAG, "startCapture:", e); } } |

可以看到，实际都是通过mUVCCamera进行调用的。
而UVCCamera则直接调用native层进行处理。

所以事件的处理实际上是这个流程：

|-----------|--------------------------------------------------------------------------------------------------------| | 1 | Activity-->UVCCameraHandler--->AbstractUVCCameraHandler--->CameraThread--->UVCCamera--->native |

为什么说Activity先发送给了UVCCameraHandler，需要看构造过程。

在Activity调用示例中引用了UVCCameraHandler，也就是说UVCCameraHandler是实际操作摄像头的类，AbstractUVCCameraHandler因为是abstract，无法实例化，同时UVCCameraHandler继承了AbstractUVCCameraHandler，类关系为：

|-----------|--------------------------------------------------------------------------------| | 1 | public class UVCCameraHandler extends AbstractUVCCameraHandler { ... } |

所以Activity中实例化了UVCCameraHandler后，发送给UVCCameraHandler的事件都经过消息发送给了CameraThread，然后CameraThread又发送给了UVCCamera类。

如下为实例化过程：

在Activity中实例化UVCCameraHandler代码为：

|-------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 | final View view = findViewById(R.id.camera_view); mUVCCameraView = (CameraViewInterface)view; mUSBMonitor = new USBMonitor(this, mOnDeviceConnectListener); mCameraHandler = UVCCameraHandler.createHandler(this, mUVCCameraView, USE_SURFACE_ENCODER ? 0 : 1, PREVIEW_WIDTH, PREVIEW_HEIGHT, PREVIEW_MODE); |

这里多贴了几行，方便后面介绍。
先看这句：

|-------------|----------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 | mCameraHandler = UVCCameraHandler.createHandler(this, mUVCCameraView, USE_SURFACE_ENCODER ? 0 : 1, PREVIEW_WIDTH, PREVIEW_HEIGHT, PREVIEW_MODE); |

等于：

|---------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 | public static final UVCCameraHandler createHandler( final Activity parent, final CameraViewInterface cameraView, final int encoderType, final int width, final int height, final int format) { return createHandler(parent, cameraView, encoderType, width, height, format, UVCCamera.DEFAULT_BANDWIDTH); } |

等于：

|-------------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 7 8 | public static final UVCCameraHandler createHandler( final Activity parent, final CameraViewInterface cameraView, final int encoderType, final int width, final int height, final int format, final float bandwidthFactor) { final CameraThread thread = new CameraThread(UVCCameraHandler.class, parent, cameraView, encoderType, width, height, format, bandwidthFactor); thread.start(); return (UVCCameraHandler)thread.getHandler(); } |

也就是说，在Activity中实例化UVCCameraHandler其实就是构造了一个CameraThread，并将CameraThread中引用的UVCCameraHandler返回了回去。也就是说UVCCameraHandler一旦实例化，则CameraThread线程也创建了。
可以看到CameraThread中确实引用了UVCCameraHandler：

|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 | static final class CameraThread extends Thread { private static final String TAG_THREAD = "CameraThread"; private final Object mSync = new Object(); private final Class<? extends AbstractUVCCameraHandler> mHandlerClass; private final WeakReference<Activity> mWeakParent; private final WeakReference<CameraViewInterface> mWeakCameraView; private final int mEncoderType; private final Set<CameraCallback> mCallbacks = new CopyOnWriteArraySet<CameraCallback>(); private int mWidth, mHeight, mPreviewMode; private float mBandwidthFactor; private boolean mIsPreviewing; private boolean mIsRecording; /** * shutter sound */ private SoundPool mSoundPool; private int mSoundId; private AbstractUVCCameraHandler mHandler; /** * for accessing UVC camera */ private UVCCamera mUVCCamera; /** * muxer for audio/video recording */ private MediaMuxerWrapper mMuxer; private MediaVideoBufferEncoder mVideoEncoder; /** * * @param clazz Class extends AbstractUVCCameraHandler * @param parent parent Activity * @param cameraView for still capturing * @param encoderType 0: use MediaSurfaceEncoder, 1: use MediaVideoEncoder, 2: use MediaVideoBufferEncoder * @param width * @param height * @param format either FRAME_FORMAT_YUYV(0) or FRAME_FORMAT_MJPEG(1) * @param bandwidthFactor */ CameraThread(final Class<? extends AbstractUVCCameraHandler> clazz, final Activity parent, final CameraViewInterface cameraView, final int encoderType, final int width, final int height, final int format, final float bandwidthFactor) { super("CameraThread"); mHandlerClass = clazz; mEncoderType = encoderType; mWidth = width; mHeight = height; mPreviewMode = format; mBandwidthFactor = bandwidthFactor; mWeakParent = new WeakReference<Activity>(parent); mWeakCameraView = new WeakReference<CameraViewInterface>(cameraView); loadShutterSound(parent); } // ... } |

其中mHandlerClass就是UVCCameraHandler。

在上面Activity中的调用中，初始化代码为：

其中mUSBMonitor类负责扫描设备并发现目标USB Camera设备，然后mUVCCameraView负责预览页面，并将该预览页面传给UVCCameraHandler并传给CameraThread，见弱引用变量：

|-----------|---------------------------------------------------------------------------| | 1 | private final WeakReference<CameraViewInterface> mWeakCameraView; |

也就是CameraThread完成了承上启下的作用，负责对上提供API调用，对下进行实际操作。

既有页面预览，声音播放，操作控制UVCCamera，进行录像等各功能。

native层 {#native层}

native层由Java类UVCCamera发起调用，调用传递为jni-->UVCCamera-->UVCCamear.cpp--->UVCPreview.cpp

举例，如startPreview，stopPreview两个方法则又调用了UVCPreview 的方法，因为UVCCamear.cpp有引用UVCPreview.cpp指针：UVCPreview *mPreview;，代码如下所示：

|---------------------------------------------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 | int UVCCamera::startPreview() { ENTER(); int result = EXIT_FAILURE; if (mDeviceHandle) { return mPreview->startPreview(); } RETURN(result, int); } int UVCCamera::stopPreview() { ENTER(); if (LIKELY(mPreview)) { mPreview->stopPreview(); } RETURN(0, int); } |

UVCPreview.cpp的startPreview，stopPreview方法内容为：

|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 | int UVCPreview::startPreview() { ENTER(); int result = EXIT_FAILURE; if (!isRunning()) { mIsRunning = true; pthread_mutex_lock(&preview_mutex); { if (LIKELY(mPreviewWindow)) { result = pthread_create(&preview_thread, NULL, preview_thread_func, (void *)this); } } pthread_mutex_unlock(&preview_mutex); if (UNLIKELY(result != EXIT_SUCCESS)) { LOGW("UVCCamera::window does not exist/already running/could not create thread etc."); mIsRunning = false; pthread_mutex_lock(&preview_mutex); { pthread_cond_signal(&preview_sync); } pthread_mutex_unlock(&preview_mutex); } } RETURN(result, int); } int UVCPreview::stopPreview() { ENTER(); bool b = isRunning(); if (LIKELY(b)) { mIsRunning = false; pthread_cond_signal(&preview_sync); pthread_cond_signal(&capture_sync); if (pthread_join(capture_thread, NULL) != EXIT_SUCCESS) { LOGW("UVCPreview::terminate capture thread: pthread_join failed"); } if (pthread_join(preview_thread, NULL) != EXIT_SUCCESS) { LOGW("UVCPreview::terminate preview thread: pthread_join failed"); } clearDisplay(); } clearPreviewFrame(); clearCaptureFrame(); pthread_mutex_lock(&preview_mutex); if (mPreviewWindow) { ANativeWindow_release(mPreviewWindow); mPreviewWindow = NULL; } pthread_mutex_unlock(&preview_mutex); pthread_mutex_lock(&capture_mutex); if (mCaptureWindow) { ANativeWindow_release(mCaptureWindow); mCaptureWindow = NULL; } pthread_mutex_unlock(&capture_mutex); RETURN(0, int); } |

UVCCameraTextureView类，该类是显示摄像头预览页面的类，非常重要

|-----------------|-------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 | public class UVCCameraTextureView extends AspectRatioTextureView implements TextureView.SurfaceTextureListener, CameraViewInterface { //... } |