Android中Camera的調用流程可分為以下幾個層次：
Package->Framework->JNI->Camera(cpp)--(binder)-->CameraService->Camera HAL->Camera Driver
以拍照流程為例：
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：
public final void takePicture(ShutterCallback shutter, PictureCallback raw,
PictureCallback postview, PictureCallback jpeg) {
mShutterCallback = shutter;
mRawImageCallback = raw;
mPostviewCallback = postview;
mJpegCallback = jpeg;
native_takePicture();
}
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，只是簡單地調用cpp的Camera中的takePicture函數。此前已經把JNI中的一個對象注冊成了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。
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;
};
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的標准代碼了，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，接著回調HAL層的data callback接口以通知CameraService，CameraService會通過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);
}
}
