Android Camera调用流程
jopen
12年前
Android中Camera的调用流程可分为以下几个层次：
Package->Framework->JNI->Camera(cpp)--(binder)-->CameraService->Camera HAL->Camera Driver
以拍照流程为例：
1. 各个参数设置完成，对焦完成后，位于Package的Camera.java会调用Framework中Camera.java的takePicture函数，如下：
public final void takePicture(ShutterCallback shutter, PictureCallback raw, PictureCallback postview, PictureCallback jpeg) { mShutterCallback = shutter; mRawImageCallback = raw; mPostviewCallback = postview; mJpegCallback = jpeg; native_takePicture(); }此函数保存Package层传下的callback函数，同时调用JNI层的native_takePicture
2. JNI层的native_takePicture自己并没有做太多事情，只是简单地调用cpp的Camera中的takePicture函数。此前已经把JNI中的一个对象注册成了Camera.cpp的listener
3. 位于frameworks/base/libs/camera是向CameraService请求服务的客户端，但它本身也继承了一个BnCameraClient类，用于CameraService回调自己。
class ICameraClient: public IInterface { public: DECLARE_META_INTERFACE(CameraClient); virtual void notifyCallback(int32_t msgType, int32_t ext1, int32_t ext2) = 0; virtual void dataCallback(int32_t msgType, const sp<IMemory>& data) = 0; virtual void dataCallbackTimestamp(nsecs_t timestamp, int32_t msgType, const sp<IMemory>& data) = 0; };从上面的接口定义可以看到，这个类就是用于回调。
Camera.cpp的takePicture函数是利用open Camera时得到的ICamera对象来继续调用takePicture
4. 接下来通过binder转到另一个进程CameraService中的处理。CameraService中之前已经实例化了一个HAL层的 CameraHardware，并把自己的data callback传递给了CameraHardware，这些工作都是由CameraService的内部类Client来完成的，这个Client类继承自BnCamera，是真正提供Camera操作API的类
5. 然后自然是调用HAL层CameraHardware的takePicture函数。从HAL层向下就不是Android的标准代码了，各个厂商有自己不同的实现。但思路应该都是相同的：Camera遵循V4L2架构，利用ioctl发送VIDIOC_DQBUF命令得到有效的图像数据，接着回调HAL层的data callback接口以通知CameraService，CameraService会通过binder通知Camera.cpp，如下：
void CameraService::Client::dataCallback(int32_t msgType, const sp<IMemory>& dataPtr, void* user) { LOG2("dataCallback(%d)", msgType); sp<Client> client = getClientFromCookie(user); if (client == 0) return; if (!client->lockIfMessageWanted(msgType)) return; if (dataPtr == 0) { LOGE("Null data returned in data callback"); client->handleGenericNotify(CAMERA_MSG_ERROR, UNKNOWN_ERROR, 0); return; } switch (msgType) { case CAMERA_MSG_PREVIEW_FRAME: client->handlePreviewData(dataPtr); break; case CAMERA_MSG_POSTVIEW_FRAME: client->handlePostview(dataPtr); break; case CAMERA_MSG_RAW_IMAGE: client->handleRawPicture(dataPtr); break; case CAMERA_MSG_COMPRESSED_IMAGE: client->handleCompressedPicture(dataPtr); break; default: client->handleGenericData(msgType, dataPtr); break; } } // picture callback - compressed picture ready void CameraService::Client::handleCompressedPicture(const sp<IMemory>& mem) { int restPictures = mHardware->getPictureRestCount(); if (!restPictures) { disableMsgType(CAMERA_MSG_COMPRESSED_IMAGE); } sp<ICameraClient> c = mCameraClient; mLock.unlock(); if (c != 0) { c->dataCallback(CAMERA_MSG_COMPRESSED_IMAGE, mem); } }6. Camera.cpp会继续通知它的listener：
// callback from camera service when frame or image is ready void Camera::dataCallback(int32_t msgType, const sp<IMemory>& dataPtr) { sp<CameraListener> listener; { Mutex::Autolock _l(mLock); listener = mListener; } if (listener != NULL) { listener->postData(msgType, dataPtr); } }7. 而这个listener就是我们的JNI层的JNICameraContext对象了：
void JNICameraContext::postData(int32_t msgType, const sp<IMemory>& dataPtr) { // VM pointer will be NULL if object is released Mutex::Autolock _l(mLock); JNIEnv *env = AndroidRuntime::getJNIEnv(); if (mCameraJObjectWeak == NULL) { LOGW("callback on dead camera object"); return; } // return data based on callback type switch(msgType) { case CAMERA_MSG_VIDEO_FRAME: // should never happen break; // don't return raw data to Java case CAMERA_MSG_RAW_IMAGE: LOGV("rawCallback"); env->CallStaticVoidMethod(mCameraJClass, fields.post_event, mCameraJObjectWeak, msgType, 0, 0, NULL); break; default: // TODO: Change to LOGV LOGV("dataCallback(%d, %p)", msgType, dataPtr.get()); copyAndPost(env, dataPtr, msgType); break; } }8. 可以看到JNI层最终都会调用来自java层的函数postEventFromNative，这个函数会发送对应的消息给自己的 eventhandler，收到消息后就会根据消息的类型回调Package层Camera.java最初传下来的callback函数。至此，我们就在最上层拿到了图像数据。