Android Camera調用流程

Android中Camera的調用流程可分爲以下幾個層次：
Package->Framework->JNI->Camera(cpp)--(binder)-->CameraService->Camera HAL->Camera Driver

以拍照流程爲例：
1. 各個參數設置完成，對焦完成後，位於Package的Camera.java會調用Framework中Camera.java的takePicture函數，如下：

1	public final void takePicture(ShutterCallback shutter, PictureCallback raw,

2	PictureCallback postview, PictureCallback jpeg) {

3	mShutterCallback = shutter;

4	mRawImageCallback = raw;

5	mPostviewCallback = postview;

6	mJpegCallback = jpeg;

7	native_takePicture();

8

}

此函數保存Package層傳下的callback函數，同時調用JNI層的native_takePicture

2. JNI層的native_takePicture自己並沒有做太多事情，僅僅是通過./base/core/jni/android_hardware_Camera.cpp中的android_hardware_Camera_takePicture，簡單地調用./av/camera/Camera.cpp中的takePicture函數。此前已經把JNI中的一個對象註冊成了Camera.cpp的listener

3. 位於frameworks/base/libs/camera是向CameraService請求服務的客戶端，但它本身也繼承了一個BnCameraClient類，用於CameraService回調自己。

1	class ICameraClient: public IInterface

2

{

3

public:

4	DECLARE_META_INTERFACE(CameraClient);

5

6	virtual void            notifyCallback(int32_t msgType, int32_t ext1, int32_t ext2) = 0;

7	virtual void            dataCallback(int32_t msgType, const sp<IMemory>& data) =0;

8	virtual void            dataCallbackTimestamp(nsecs_t timestamp, int32_t msgType,const sp<IMemory>& data) = 0;

9

};

從上面的接口定義可以看到，這個類就是用於回調。

Camera.cpp的takePicture函數是利用open Camera時得到的ICamera對象來繼續調用takePicture

4. 接下來通過binder轉到另一個進程CameraService中的處理。CameraService中之前已經實例化了一個HAL層的 CameraHardware，並把自己的data callback傳遞給了CameraHardware，這些工作都是由CameraService的內部類Client來完成的，這個Client類繼承自BnCamera，是真正提供Camera操作API的類

5. 然後自然是調用HAL層CameraHardware的takePicture函數。從HAL層向下就不是Android的標準代碼了，各個廠商有自己不同的實現。但思路應該都是相同的：Camera遵循V4L2架構，利用ioctl發送VIDIOC_DQBUF命令得到有效的圖像數據，接着回調HAL層的data callback接口以通知CameraService，CameraService會通過binder通知Camera.cpp，如下：

01	void CameraService::Client::dataCallback(int32_t msgType,

02	const sp<IMemory>& dataPtr, void* user) {

03	LOG2("dataCallback(%d)", msgType);

04

05	sp<Client> client = getClientFromCookie(user);

06	if (client == 0) return;

07	if (!client->lockIfMessageWanted(msgType)) return;

08

09	if (dataPtr == 0) {

10	LOGE("Null data returned in data callback");

11	client->handleGenericNotify(CAMERA_MSG_ERROR, UNKNOWN_ERROR, 0);

12

return;

13

}

14

15	switch (msgType) {

16	case CAMERA_MSG_PREVIEW_FRAME:

17	client->handlePreviewData(dataPtr);

18

break;

19	case CAMERA_MSG_POSTVIEW_FRAME:

20	client->handlePostview(dataPtr);

21

break;

22	case CAMERA_MSG_RAW_IMAGE:

23	client->handleRawPicture(dataPtr);

24

break;

25	case CAMERA_MSG_COMPRESSED_IMAGE:

26	client->handleCompressedPicture(dataPtr);

27

break;

28

default:

29	client->handleGenericData(msgType, dataPtr);

30

break;

31

}

32

}

33	// picture callback - compressed picture ready

34	void CameraService::Client::handleCompressedPicture(const sp<IMemory>& mem) {

35	int restPictures =  mHardware->getPictureRestCount();

36	if (!restPictures)

37

{

38	disableMsgType(CAMERA_MSG_COMPRESSED_IMAGE);

39

}

40

41	sp<ICameraClient> c = mCameraClient;

42	mLock.unlock();

43	if (c != 0) {

44	c->dataCallback(CAMERA_MSG_COMPRESSED_IMAGE, mem);

45

}

46

}

6. Camera.cpp會繼續通知它的listener：

01	// callback from camera service when frame or image is ready

02	void Camera::dataCallback(int32_t msgType, const sp<IMemory>& dataPtr)

03

{

04	sp<CameraListener> listener;

05

{

06	Mutex::Autolock _l(mLock);

07	listener = mListener;

08

}

09	if (listener != NULL) {

10	listener->postData(msgType, dataPtr);

11

}

12

}

7. 而這個listener就是我們的JNI層的JNICameraContext對象了：

01	void JNICameraContext::postData(int32_t msgType, const sp<IMemory>& dataPtr)

02

{

03	// VM pointer will be NULL if object is released

04	Mutex::Autolock _l(mLock);

05	JNIEnv *env = AndroidRuntime::getJNIEnv();

06	if (mCameraJObjectWeak == NULL) {

07	LOGW("callback on dead camera object");

08

return;

09

}

10

11	// return data based on callback type

12	switch(msgType) {

13	case CAMERA_MSG_VIDEO_FRAME:

14	// should never happen

15

break;

16	// don't return raw data to Java

17	case CAMERA_MSG_RAW_IMAGE:

18	LOGV("rawCallback");

19	env->CallStaticVoidMethod(mCameraJClass, fields.post_event,

20	mCameraJObjectWeak, msgType, 0, 0, NULL);

21

break;

22

default:

23	// TODO: Change to LOGV

24	LOGV("dataCallback(%d, %p)", msgType, dataPtr.get());

25	copyAndPost(env, dataPtr, msgType);

26

break;

27

}

28

}

8. 可以看到JNI層最終都會調用來自java層的函數postEventFromNative，這個函數會發送對應的消息給自己的 eventhandler，收到消息後就會根據消息的類型回調Package層Camera.java最初傳下來的callback函數。至此，我們就在最上層拿到了圖像數據。