移动端的UVC协议分析是基于github上开源项目UVCCamera的。
谷歌官方推荐使用该项目作为开发UVC相关APP的参考实现。

这个项目整合了libjpeg-turbo, libusb, libuvc和rapidjson等基础c开源库，封装了供Java层调用的native接口函数（jni），增加了logcat相关日志打印，然后封装成为libuvccamera的Java包。
同时，该项目也提供了8个例子供大家调试使用，每个例子都可以单独编译成独立APP。

UVCCamera工程的源文件结构

以usbCameraTest8为例，其源文件结构如下：

可以看到，就一个MainActivity.java文件。
打开这个文件，可以看到，这是一个标准的Android APP的activity类，里面实现了onCreate()，onDestroy()，onStart()，onStop()等基本接口函数。

onCreate()

在onCreate函数中，定义了两个实例：

注意一下mOnDeviceConnectListener这个listener，它的实现如下：

private final OnDeviceConnectListener mOnDeviceConnectListener = new OnDeviceConnectListener() {
    @Override
    public void onAttach(final UsbDevice device) {
        Toast.makeText(MainActivity.this, "USB_DEVICE_ATTACHED", Toast.LENGTH_SHORT).show();
    }

    @Override
    public void onConnect(final UsbDevice device, final UsbControlBlock ctrlBlock, final boolean createNew) {
        if (DEBUG) Log.v(TAG, "onConnect:");
        mCameraHandler.open(ctrlBlock);
        startPreview();
        updateItems();
    }

    @Override
    public void onDisconnect(final UsbDevice device, final UsbControlBlock ctrlBlock) {
        if (DEBUG) Log.v(TAG, "onDisconnect:");
        if (mCameraHandler != null) {
            queueEvent(new Runnable() {
                @Override
                public void run() {
                    mCameraHandler.close();
                }
            }, 0);
            setCameraButton(false);
            updateItems();
        }
    }
    @Override
    public void onDettach(final UsbDevice device) {
        Toast.makeText(MainActivity.this, "USB_DEVICE_DETACHED", Toast.LENGTH_SHORT).show();
    }

    @Override
    public void onCancel(final UsbDevice device) {
        setCameraButton(false);
    }
};

在这个listener中，定义了USB摄像头与手机连接/断开等情况下，程序将如何执行，比如，当设备attach上后，会有一行提示语：“USB_DEVICE_ATTACHED”，当点击APP上的按钮去打开设备时，会调用onConnect()函数，调用UVCCameraHandler.open，同时启动预览。

onStart()

在onStart()中，注册了USBMonitor，从此刻起，系统开始监听USB口上的消息。

onStop()

关闭UVCCameraHandler，关闭相机控制相关按钮动作；

onDestroy()

释放UVCCameraHandler，关闭USBMonitor；

再回头看一下刚才在onCreate()中定义的那两个实例：
mUSBMonitor，mCameraHandler
它们的类USBMonitor和UVCCameraHandler分别位于：

USBMonitor用于监听手机USB口上的各种消息，并作出相应的动作，比如启动预览什么的。但是如果用户希望从APP上操作相机，比如拍照、录像或者设置相关相机参数，就需要调用UVCCameraHandler了。

UVCCameraHandler相关代码位于usbCameraCommon文件夹下，它继承了AbstractUVCCameraHandler这个类。在AbstractUVCCameraHandler这个类中，定义了一个名为CameraThread的类，在这个线程中，具体执行了拍照、录像、预览等相关函数；

到现在为止，可以看到，APP的MainActivity主要是调用USBCameraCommon中的UVCCameraHandler和AbstractUVCCameraHandler，而后者又进一步去调用libuvccamera文件夹下的UVCCamera.java。

可以看到，在libuvccamera目录里的文件如下：

UVCCamera.java是很薄的一层接口层，类似一个glue layer，向上对接UVCCameraHandler和APP，向下对接native层的c语言实现，显然，重头戏都在下面的c代码里，但是UVCCamera.java可以直观的展现手机APP可以操作相机的哪些功能，下面是native函数列表:

Get Method

Reset Method

Set Method

Camera Operation

USBMonitor.java的作用主要是用来监控USB口的事件（OnDeviceConnectListener : interface），获取相关usb设备信息（UsbControlBlock : class & UsbDeviceInfo : class）等。

上面基本就是java层代码的一个大概层次结构，接下来我们就进入底层的native函数，看看uvc到底是怎么运作的。

《============================================》

在UVCCamera.java中，声明了大量的native接口函数：

    // #nativeCreate and #nativeDestroy are not static methods.
    private final native long nativeCreate();
    private final native void nativeDestroy(final long id_camera);

    private final native int nativeConnect(long id_camera, int venderId, int productId, int fileDescriptor, int busNum, int devAddr, String usbfs);
    private static final native int nativeRelease(final long id_camera);

    private static final native int nativeSetStatusCallback(final long mNativePtr, final IStatusCallback callback);
    private static final native int nativeSetButtonCallback(final long mNativePtr, final IButtonCallback callback);

    private static final native int nativeSetPreviewSize(final long id_camera, final int width, final int height, final int min_fps, final int max_fps, final int mode, final float bandwidth);
    private static final native String nativeGetSupportedSize(final long id_camera);
    private static final native int nativeStartPreview(final long id_camera);
    private static final native int nativeStopPreview(final long id_camera);
    private static final native int nativeSetPreviewDisplay(final long id_camera, final Surface surface);
    private static final native int nativeSetFrameCallback(final long mNativePtr, final IFrameCallback callback, final int pixelFormat);

    private static final native int nativeSetCaptureDisplay(final long id_camera, final Surface surface);

    private static final native long nativeGetCtrlSupports(final long id_camera);
    private static final native long nativeGetProcSupports(final long id_camera);

    private final native int nativeUpdateScanningModeLimit(final long id_camera);
    private static final native int nativeSetScanningMode(final long id_camera, final int scanning_mode);
    private static final native int nativeGetScanningMode(final long id_camera);

    private final native int nativeUpdateExposureModeLimit(final long id_camera);
    private static final native int nativeSetExposureMode(final long id_camera, final int exposureMode);
    private static final native int nativeGetExposureMode(final long id_camera);

    private final native int nativeUpdateExposurePriorityLimit(final long id_camera);
    private static final native int nativeSetExposurePriority(final long id_camera, final int priority);
    private static final native int nativeGetExposurePriority(final long id_camera);

    private final native int nativeUpdateExposureLimit(final long id_camera);
    private static final native int nativeSetExposure(final long id_camera, final int exposure);
    private static final native int nativeGetExposure(final long id_camera);

    private final native int nativeUpdateExposureRelLimit(final long id_camera);
    private static final native int nativeSetExposureRel(final long id_camera, final int exposure_rel);
    private static final native int nativeGetExposureRel(final long id_camera);

    private final native int nativeUpdateAutoFocusLimit(final long id_camera);
    private static final native int nativeSetAutoFocus(final long id_camera, final boolean autofocus);
    private static final native int nativeGetAutoFocus(final long id_camera);

    private final native int nativeUpdateFocusLimit(final long id_camera);
    private static final native int nativeSetFocus(final long id_camera, final int focus);
    private static final native int nativeGetFocus(final long id_camera);

    private final native int nativeUpdateFocusRelLimit(final long id_camera);
    private static final native int nativeSetFocusRel(final long id_camera, final int focus_rel);
    private static final native int nativeGetFocusRel(final long id_camera);

    private final native int nativeUpdateIrisLimit(final long id_camera);
    private static final native int nativeSetIris(final long id_camera, final int iris);
    private static final native int nativeGetIris(final long id_camera);

    private final native int nativeUpdateIrisRelLimit(final long id_camera);
    private static final native int nativeSetIrisRel(final long id_camera, final int iris_rel);
    private static final native int nativeGetIrisRel(final long id_camera);

    private final native int nativeUpdatePanLimit(final long id_camera);
    private static final native int nativeSetPan(final long id_camera, final int pan);
    private static final native int nativeGetPan(final long id_camera);

    private final native int nativeUpdatePanRelLimit(final long id_camera);
    private static final native int nativeSetPanRel(final long id_camera, final int pan_rel);
    private static final native int nativeGetPanRel(final long id_camera);

    private final native int nativeUpdateTiltLimit(final long id_camera);
    private static final native int nativeSetTilt(final long id_camera, final int tilt);
    private static final native int nativeGetTilt(final long id_camera);

    private final native int nativeUpdateTiltRelLimit(final long id_camera);
    private static final native int nativeSetTiltRel(final long id_camera, final int tilt_rel);
    private static final native int nativeGetTiltRel(final long id_camera);

    private final native int nativeUpdateRollLimit(final long id_camera);
    private static final native int nativeSetRoll(final long id_camera, final int roll);
    private static final native int nativeGetRoll(final long id_camera);

    private final native int nativeUpdateRollRelLimit(final long id_camera);
    private static final native int nativeSetRollRel(final long id_camera, final int roll_rel);
    private static final native int nativeGetRollRel(final long id_camera);

    private final native int nativeUpdateAutoWhiteBlanceLimit(final long id_camera);
    private static final native int nativeSetAutoWhiteBlance(final long id_camera, final boolean autoWhiteBlance);
    private static final native int nativeGetAutoWhiteBlance(final long id_camera);

    private final native int nativeUpdateAutoWhiteBlanceCompoLimit(final long id_camera);
    private static final native int nativeSetAutoWhiteBlanceCompo(final long id_camera, final boolean autoWhiteBlanceCompo);
    private static final native int nativeGetAutoWhiteBlanceCompo(final long id_camera);

    private final native int nativeUpdateWhiteBlanceLimit(final long id_camera);
    private static final native int nativeSetWhiteBlance(final long id_camera, final int whiteBlance);
    private static final native int nativeGetWhiteBlance(final long id_camera);

    private final native int nativeUpdateWhiteBlanceCompoLimit(final long id_camera);
    private static final native int nativeSetWhiteBlanceCompo(final long id_camera, final int whiteBlance_compo);
    private static final native int nativeGetWhiteBlanceCompo(final long id_camera);

    private final native int nativeUpdateBacklightCompLimit(final long id_camera);
    private static final native int nativeSetBacklightComp(final long id_camera, final int backlight_comp);
    private static final native int nativeGetBacklightComp(final long id_camera);

    private final native int nativeUpdateBrightnessLimit(final long id_camera);
    private static final native int nativeSetBrightness(final long id_camera, final int brightness);
    private static final native int nativeGetBrightness(final long id_camera);

    private final native int nativeUpdateContrastLimit(final long id_camera);
    private static final native int nativeSetContrast(final long id_camera, final int contrast);
    private static final native int nativeGetContrast(final long id_camera);

    private final native int nativeUpdateAutoContrastLimit(final long id_camera);
    private static final native int nativeSetAutoContrast(final long id_camera, final boolean autocontrast);
    private static final native int nativeGetAutoContrast(final long id_camera);

    private final native int nativeUpdateSharpnessLimit(final long id_camera);
    private static final native int nativeSetSharpness(final long id_camera, final int sharpness);
    private static final native int nativeGetSharpness(final long id_camera);

    private final native int nativeUpdateGainLimit(final long id_camera);
    private static final native int nativeSetGain(final long id_camera, final int gain);
    private static final native int nativeGetGain(final long id_camera);

    private final native int nativeUpdateGammaLimit(final long id_camera);
    private static final native int nativeSetGamma(final long id_camera, final int gamma);
    private static final native int nativeGetGamma(final long id_camera);

    private final native int nativeUpdateSaturationLimit(final long id_camera);
    private static final native int nativeSetSaturation(final long id_camera, final int saturation);
    private static final native int nativeGetSaturation(final long id_camera);

    private final native int nativeUpdateHueLimit(final long id_camera);
    private static final native int nativeSetHue(final long id_camera, final int hue);
    private static final native int nativeGetHue(final long id_camera);

    private final native int nativeUpdateAutoHueLimit(final long id_camera);
    private static final native int nativeSetAutoHue(final long id_camera, final boolean autohue);
    private static final native int nativeGetAutoHue(final long id_camera);

    private final native int nativeUpdatePowerlineFrequencyLimit(final long id_camera);
    private static final native int nativeSetPowerlineFrequency(final long id_camera, final int frequency);
    private static final native int nativeGetPowerlineFrequency(final long id_camera);

    private final native int nativeUpdateZoomLimit(final long id_camera);
    private static final native int nativeSetZoom(final long id_camera, final int zoom);
    private static final native int nativeGetZoom(final long id_camera);

    private final native int nativeUpdateZoomRelLimit(final long id_camera);
    private static final native int nativeSetZoomRel(final long id_camera, final int zoom_rel);
    private static final native int nativeGetZoomRel(final long id_camera);

    private final native int nativeUpdateDigitalMultiplierLimit(final long id_camera);
    private static final native int nativeSetDigitalMultiplier(final long id_camera, final int multiplier);
    private static final native int nativeGetDigitalMultiplier(final long id_camera);

    private final native int nativeUpdateDigitalMultiplierLimitLimit(final long id_camera);
    private static final native int nativeSetDigitalMultiplierLimit(final long id_camera, final int multiplier_limit);
    private static final native int nativeGetDigitalMultiplierLimit(final long id_camera);

    private final native int nativeUpdateAnalogVideoStandardLimit(final long id_camera);
    private static final native int nativeSetAnalogVideoStandard(final long id_camera, final int standard);
    private static final native int nativeGetAnalogVideoStandard(final long id_camera);

    private final native int nativeUpdateAnalogVideoLockStateLimit(final long id_camera);
    private static final native int nativeSetAnalogVideoLoackState(final long id_camera, final int state);
    private static final native int nativeGetAnalogVideoLoackState(final long id_camera);

    private final native int nativeUpdatePrivacyLimit(final long id_camera);
    private static final native int nativeSetPrivacy(final long id_camera, final boolean privacy);
    private static final native int nativeGetPrivacy(final long id_camera);

这些native函数与libuvc库函数的对接位于UVCCamera/libuvccamera/src/main/jni/UVCCamera/serenegiant_usb_UVCCamera.cpp这个文件中。

我们以简单的nativeConnect函数为例，看看它的实现：

这个camera->connect定义在类UVCCamera中，进入UVCCamera.cpp文件，我们可以看到，其实现如下：

忍不住吐槽一句，这么知名的项目居然用日文注释，实在是不专业！

不过骂归骂，从这层函数开始，我们就开始真正调用libuvc库函数了。比如uvc_open()。接下来的故事，就可以参考本系列第二篇libuvc一文。