打开Camera第一键事情就是预览取景preview的动作，我们先从Camera app分析起 。所有拥有拍照功能的应用，它在预览时候都要实现SurfaceHolder.Callback接口，并实现其surfaceCreated、surfaceChanged、surfaceDestroyed三个函数，同时声明一个用于预览的窗口SurfaceView ，以下是系统自带ap的源代码
SurfaceView preview = (SurfaceView) findViewById(R.id.camera_preview);
SurfaceHolder holder = preview.getHolder();holder.addCallback(this);
还要设置camera预览的surface缓存区 ，系统自带app实在surfaceChange()方法里面设置Camera的预览区，以供底层获取的preview数据不断投递到这个surface缓存区内。
public void surfaceChanged(SurfaceHolder holder, int format, int w, int h) {
mSurfaceHolder = holder;// The mCameraDevice will be null if it fails to connect to the camera// hardware. In this case we will show a dialog and then finish the// activity, so it's OK to ignore it.if (mCameraDevice == null) return;// Sometimes surfaceChanged is called after onPause or before onResume.// Ignore it.if (mPausing || isFinishing()) return;// Set preview display if the surface is being created. Preview was// already started. Also restart the preview if display rotation has// changed. Sometimes this happens when the device is held in portrait// and camera app is opened. Rotation animation takes some time and// display rotation in onCreate may not be what we want.if (mCameraState == PREVIEW_STOPPED) {startPreview();startFaceDetection();} else {if (Util.getDisplayRotation(this) != mDisplayRotation) {setDisplayOrientation();}if (holder.isCreating()) {// Set preview display if the surface is being created and preview// was already started. That means preview display was set to null// and we need to set it now.setPreviewDisplay(holder);}}
设置好以上参数后，就可以调用startPreview（）进行取景预览
startPreview（）也是一层层往下调用，最后到Camera的服务端CameraService,我们看下它的过程
Camera.java(应用）-------------> Camera.java(框架）-------------->android_hardware_camera.cpp(JNI)-------------------->Camera.cpp(客户端）------------------->CameraService.cpp(服务端）--------------------->CameraHarwareInterface(HAL接口)
在CameraService端将处理preview的请求并进入HAL层
status_t CameraService::Client::startPreview() {
enableMsgType(CAMERA_MSG_PREVIEW_METADATA);
return startCameraMode(CAMERA_PREVIEW_MODE);}
先是传递preview的消息到HAL层，然后执行preview
status_t CameraService::Client::startCameraMode(camera_mode mode) {switch(mode) {case CAMERA_PREVIEW_MODE:if (mSurface == 0 && mPreviewWindow == 0) {LOG1("mSurface is not set yet.");// still able to start preview in this case.}return startPreviewMode();}}
status_t CameraService::Client::startPreviewMode() {LOG1("startPreviewMode");status_t result = NO_ERROR;// if preview has been enabled, nothing needs to be doneif (mHardware->previewEnabled()) {return NO_ERROR;}if (mPreviewWindow != 0) {native_window_set_scaling_mode(mPreviewWindow.get(),NATIVE_WINDOW_SCALING_MODE_SCALE_TO_WINDOW);native_window_set_buffers_transform(mPreviewWindow.get(),mOrientation);}mHardware->setPreviewWindow(mPreviewWindow);result = mHardware->startPreview();return result;}
然后就近去HAL层调用，并通过回调函数源源不断的将数据投递到surfaceview的缓存去，因为preview的数据是比较大的，所以数据不会携带着传上上层，而是直接在两个缓存区之间copy，一个是底层采集数据的缓存区，另一个是用于显示的surfaceview缓存区
我们看看preview的回调函数是怎么处理的
首先在Camera客户端与服务端连接成功的时候就会设置一个回调函数dataCallBack
CameraService::Client::Client(const sp<CameraService>& cameraService,const sp<ICameraClient>& cameraClient,const sp<CameraHardwareInterface>& hardware,int cameraId, int cameraFacing, int clientPid) {......mHardware->setCallbacks(notifyCallback,dataCallback,dataCallbackTimestamp,(void *)cameraId);}
在上篇有介绍到，client与server连接成功后就会new 一个client返回,在client的构造函数中，就对camera设置了notifyCallback、dataCallback、dataCallbackTimestamp三个回调函数，用于返回底层数据用于处理，看下它的处理方法
void CameraService::Client::dataCallback(int32_t msgType,const sp<IMemory>& dataPtr, camera_frame_metadata_t *metadata, void* user) {switch (msgType & ~CAMERA_MSG_PREVIEW_METADATA) {case CAMERA_MSG_PREVIEW_FRAME:client->handlePreviewData(msgType, dataPtr, metadata);break;.......
}
void CameraService::Client::handlePreviewData(int32_t msgType,const sp<IMemory>& mem,camera_frame_metadata_t *metadata) {sp<ICameraClient> c = mCameraClient;.......if (c != 0) {// Is the received frame copied out or not?if (flags & CAMERA_FRAME_CALLBACK_FLAG_COPY_OUT_MASK) {LOG2("frame is copied");copyFrameAndPostCopiedFrame(msgType, c, heap, offset, size, metadata);} else {LOG2("frame is forwarded");mLock.unlock();c->dataCallback(msgType, mem, metadata);}} else {mLock.unlock();}}
copyFrameAndPostCopiedFrame就是这个函数执行两个buff区preview数据的投递
void CameraService::Client::copyFrameAndPostCopiedFrame(int32_t msgType, const sp<ICameraClient>& client,const sp<IMemoryHeap>& heap, size_t offset, size_t size,camera_frame_metadata_t *metadata) {......previewBuffer = mPreviewBuffer;memcpy(previewBuffer->base(), (uint8_t *)heap->base() + offset, size);sp<MemoryBase> frame = new MemoryBase(previewBuffer, 0, size);if (frame == 0) {LOGE("failed to allocate space for frame callback");mLock.unlock();return;}mLock.unlock();client->dataCallback(msgType, frame, metadata);}
将数据处理成frame，继续调用客户端client的回调函数 client->dataCallback(msgType, frame, metadata);
// callback from camera service when frame or image is readyvoid Camera::dataCallback(int32_t msgType, const sp<IMemory>& dataPtr,camera_frame_metadata_t *metadata){sp<CameraListener> listener;{Mutex::Autolock _l(mLock);listener = mListener;}if (listener != NULL) {listener->postData(msgType, dataPtr, metadata);}}
还记得初始化的时候，在jni里面有设置listener吗？
static void android_hardware_Camera_native_setup(JNIEnv *env, jobject thiz,jobject weak_this, jint cameraId){
sp<JNICameraContext> context = new JNICameraContext(env, weak_this, clazz, camera);context->incStrong(thiz);camera->setListener(context);
}
继续 listener->postData(msgType, dataPtr, metadata);
void JNICameraContext::postData(int32_t msgType, const sp<IMemory>& dataPtr,camera_frame_metadata_t *metadata){......switch (dataMsgType) {case CAMERA_MSG_VIDEO_FRAME:// should never happenbreak;default:LOGV("dataCallback(%d, %p)", dataMsgType, dataPtr.get());copyAndPost(env, dataPtr, dataMsgType);break;}}
继续copyAndPost(env, dataPtr, dataMsgType);
void JNICameraContext::copyAndPost(JNIEnv* env, const sp<IMemory>& dataPtr, int msgType){jbyteArray obj = NULL;// allocate Java byte array and copy dataif (dataPtr != NULL) {....... } else {LOGV("Allocating callback buffer");obj = env->NewByteArray(size);.......env->SetByteArrayRegion(obj, 0, size, data);}} else {LOGE("image heap is NULL");}}
void JNICameraContext::copyAndPost(JNIEnv* env, const sp<IMemory>& dataPtr, int msgType){jbyteArray obj = NULL;// allocate Java byte array and copy dataif (dataPtr != NULL) {....... } else {LOGV("Allocating callback buffer");obj = env->NewByteArray(size);.......env->SetByteArrayRegion(obj, 0, size, data);}} else {LOGE("image heap is NULL");}}// post image data to Javaenv->CallStaticVoidMethod(mCameraJClass, fields.post_event,mCameraJObjectWeak, msgType, 0, 0, obj);if (obj) {env->DeleteLocalRef(obj);}}
解释一下标红的部分，先建立一个byte数组obj，将data缓存数据存储进obj数组，CallStaticVoidMethod是C调用java函数，最后执行实在Camera.java(框架）的postEventFromNative（）
private static void postEventFromNative(Object camera_ref,int what, int arg1, int arg2, Object obj){Camera c = (Camera)((WeakReference)camera_ref).get();if (c == null)return;if (c.mEventHandler != null) {Message m = c.mEventHandler.obtainMessage(what, arg1, arg2, obj);c.mEventHandler.sendMessage(m);}}
public void handleMessage(Message msg) {switch(msg.what) {case CAMERA_MSG_SHUTTER:if (mShutterCallback != null) {mShutterCallback.onShutter();}return;case CAMERA_MSG_RAW_IMAGE:if (mRawImageCallback != null) {mRawImageCallback.onPictureTaken((byte[])msg.obj, mCamera);}return;case CAMERA_MSG_COMPRESSED_IMAGE:if (mJpegCallback != null) {mJpegCallback.onPictureTaken((byte[])msg.obj, mCamera);}return;case CAMERA_MSG_PREVIEW_FRAME:if (mPreviewCallback != null) {PreviewCallback cb = mPreviewCallback;if (mOneShot) {// Clear the callback variable before the callback// in case the app calls setPreviewCallback from// the callback functionmPreviewCallback = null;} else if (!mWithBuffer) {// We're faking the camera preview mode to prevent// the app from being flooded with preview frames.// Set to oneshot mode again.setHasPreviewCallback(true, false);}cb.onPreviewFrame((byte[])msg.obj, mCamera);}return;}}}
