Android Camera調用流程

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，對焦完成后，位于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();
   
   }</pre>
   
   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自己并沒有做太多事情，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. 位于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，用于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;
   };</pre>
   
   從上面的接口定義可以看到，這個類就是用于回調。
   
   
   
   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，并把自己的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. 然后自然是調用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);
   
   }
   }</pre>
   
   
   
   

6. Camera.cpp會繼續通知它的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);
   
    }
   }</pre>
   
   
   

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

   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;
   
   }
   }</pre>
   
   
   
   

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，收到消息后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。