Camera服务之--Service

在camera service这端的结构还是很容易让人迷惑的，我就是看了好久，才缕清楚关系。

service这部分包括以下几个头文件：ICamera.h, ICameraService.h, CameraService.h，对应的实现ICamera.cpp, ICameraService.cpp, CameraService.cpp。

CameraService中包含了一个内部类CameraService::Client，这个CameraService::Client是对ICamera的实现，camera client得到的ICamera就是这个CameraService::Client。

也就是说与camera client真正通信的，是这个CameraService::Client。下面我们来逐步分析service这块的实现。

1.ICameraService

（1）ICameraService.h

其中只定义了3个方法：

    virtual int32_t         getNumberOfCameras() = 0;
    virtual status_t        getCameraInfo(int cameraId, struct CameraInfo* cameraInfo) = 0;
    virtual sp<ICamera>     connect(const sp<ICameraClient>& cameraClient,  int cameraId) = 0;

（2） ICameraService.cpp

看代码：

status_t BnCameraService::onTransact( uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{
    switch(code) {
        case GET_NUMBER_OF_CAMERAS: {
            CHECK_INTERFACE(ICameraService, data, reply);
            reply->writeInt32(getNumberOfCameras());
            return NO_ERROR;
        } break;
        case GET_CAMERA_INFO: {
            CHECK_INTERFACE(ICameraService, data, reply);
            CameraInfo cameraInfo;
            memset(&cameraInfo, 0, sizeof(cameraInfo));
            status_t result = getCameraInfo(data.readInt32(), &cameraInfo);
            reply->writeInt32(cameraInfo.facing);
            reply->writeInt32(cameraInfo.orientation);
            reply->writeInt32(result);
            return NO_ERROR;
        } break;
        case CONNECT: {
            CHECK_INTERFACE(ICameraService, data, reply);
            sp<ICameraClient> cameraClient = interface_cast<ICameraClient>(data.readStrongBinder());
            sp<ICamera> camera = connect(cameraClient, data.readInt32());
            reply->writeStrongBinder(camera->asBinder());
            return NO_ERROR;
        } break;
        default:
            return BBinder::onTransact(code, data, reply, flags);
    }
}

这3个函数的真正实现在CameraService.cpp中。

sp<ICameraClient> cameraClient = interface_cast<ICameraClient>(data.readStrongBinder());中得到此次连接的client，并且传递到connect函数中。保存client信息的目的是保存在CameraService::Client中，在适当的时机回调client。

2.CameraService

（1）CameraService.h

class CameraService :  public BinderService<CameraService>, public BnCameraService

这个 BinderService<CameraService>是什么东西呢？看看它的定义，在frameworks/base/include/binder下BinderService.h。

template<typename SERVICE>
class BinderService
{
public:
    static status_t publish() {
        sp<IServiceManager> sm(defaultServiceManager());
        return sm->addService(String16(SERVICE::getServiceName()), new SERVICE());
    }

    static void publishAndJoinThreadPool() {
        sp<ProcessState> proc(ProcessState::self());
        sp<IServiceManager> sm(defaultServiceManager());
        sm->addService(String16(SERVICE::getServiceName()), new SERVICE());
        ProcessState::self()->startThreadPool();
        IPCThreadState::self()->joinThreadPool();
    }

    static void instantiate() { publish(); }

    static status_t shutdown() {
        return NO_ERROR;
    }
};

return sm->addService(String16(SERVICE::getServiceName()), new SERVICE());从这句代码，可以看出，这是native service在向SM注册。

其实这个模板类，就是为native service提供了一向service注册的统一方式。

publish和publishAndJoinThreadPool的区别就是在于，注册之后是否开启线程池来监听。

因为之前看过media server的代码，在media server的main函数里，调用的就是CameraService.instantiate来注册的，因为camera service是跑在media server进程里的，所以camera service不需要自己来开启线程来循环监听。

所以，我觉得调用publish和publishAndJoinThreadPool的不同场景是：如果这个service是跑在别的进程里的，就调用publish，如果是自己单独一个进程，就调用publishAndJoinThreadPool。

（2）CameraService.cpp

实现在ICameraService定义的3个函数。

getNumberOfCameras（）和getCameraInfo（）的实现很简单。

int32_t CameraService::getNumberOfCameras() {
    return mNumberOfCameras; //mNumberOfCameras是在构造函数中初始化的，mNumberOfCameras = HAL_getNumberOfCameras();
} 

status_t CameraService::getCameraInfo(int cameraId,
                                      struct CameraInfo* cameraInfo) {
    if (cameraId < 0 || cameraId >= mNumberOfCameras) {
        return BAD_VALUE;
    }

    HAL_getCameraInfo(cameraId, cameraInfo);
    return OK;
}

主要看connect函数：

sp<ICamera> CameraService::connect( const sp<ICameraClient>& cameraClient, int cameraId) {
    sp<Client> client;  //CameraService::Client
    …………
    if (mClient[cameraId] != 0) {
        client = mClient[cameraId].promote();
        if (client != 0) {
            if (cameraClient->asBinder() == client->getCameraClient()->asBinder()) {
                LOG1("CameraService::connect X (pid %d) (the same client)",
                    callingPid);
                return client;
            } 
        }
    }
    …………
    sp<CameraHardwareInterface> hardware = HAL_openCameraHardware(cameraId); //获取CameraHardwareInterface接口，在下边的代码构造Client时传递进入
    …………
    CameraInfo info;
    HAL_getCameraInfo(cameraId, &info);
    client = new Client(this, cameraClient, hardware, cameraId, info.facing, callingPid); //这里就是之前说过的，在onTransact保存的client
    mClient[cameraId] = client;
    return client;
}

3.CameraService::Client

class Client : public BnCamera， Client是对ICamera的实现。

构造函数：

CameraService::Client::Client(const sp<CameraService>& cameraService,
        const sp<ICameraClient>& cameraClient,
        const sp<CameraHardwareInterface>& hardware,
        int cameraId, int cameraFacing, int clientPid) {
    int callingPid = getCallingPid();
    LOG1("Client::Client E (pid %d)", callingPid);

    mCameraService = cameraService;
    mCameraClient = cameraClient;
    mHardware = hardware;
    mCameraId = cameraId;
    mCameraFacing = cameraFacing;
    mClientPid = clientPid;
    mUseOverlay = mHardware->useOverlay();
    mMsgEnabled = 0;

    mHardware->setCallbacks(notifyCallback,
                            dataCallback,
                            dataCallbackTimestamp,
                            (void *)cameraId);

    // Enable zoom, error, and focus messages by default
    enableMsgType(CAMERA_MSG_ERROR |
                  CAMERA_MSG_ZOOM |
                  CAMERA_MSG_FOCUS);
    mOverlayW = 0;
    mOverlayH = 0;

    // Callback is disabled by default
    mPreviewCallbackFlag = FRAME_CALLBACK_FLAG_NOOP;
    mOrientation = getOrientation(0, mCameraFacing == CAMERA_FACING_FRONT);
    mOrientationChanged = false;
    cameraService->setCameraBusy(cameraId);
    cameraService->loadSound();
    LOG1("Client::Client X (pid %d)", callingPid);
}

主要进行一些成员变量的初始化，主要有mCameraClient， mHardware等。都是在CameraService::connect时进行的。

Client的各个函数可以自己看一下，主要就是对mHardware的调用，这是一个CameraHardwareInterface接口，也就是对HAL层的一个封装。比如stopRecording函数：

// stop recording mode
void CameraService::Client::stopRecording() {
    ……
    mCameraService->playSound(SOUND_RECORDING);
    disableMsgType(CAMERA_MSG_VIDEO_FRAME);
    mHardware->stopRecording();
    …………
}

4.ICamera

最后再来说一下ICamera。

这个ICamera定义的其实就是camera client与camera service的接口。

当camera client连接camera service时，也就是调用CameraService::connect时，返回一个ICamera接口给camera client调用，这个ICamera接口的真正实现是CameraService::Client。