Android(安卓)Camera调用流程

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 readyvoid 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 readyvoid 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。

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