startPreview()调用细节分析一

在开发相机预览功能时，App会调用Android API函数startPreview()，在Camera.java中定义：

public native final void startPreview();

这是个native函数，会直接调用jni层的函数，在android_hardware_Camera.cpp中定义：

static void android_hardware_Camera_startPreview(JNIEnv *env, jobject thiz)
{
    ALOGV("startPreview");
    sp<Camera> camera = get_native_camera(env, thiz, NULL);
    if (camera == 0) return;

    if (camera->startPreview() != NO_ERROR) {
        jniThrowRuntimeException(env, "startPreview failed");
        return;
    }
}

sp<Camera> get_native_camera(JNIEnv *env, jobject thiz, JNICameraContext** pContext)
{
    sp<Camera> camera;
    Mutex::Autolock _l(sLock);
    JNICameraContext* context = reinterpret_cast<JNICameraContext*>(env->GetLongField(thiz, fields.context));
    if (context != NULL) {
        camera = context->getCamera();
    }
    ALOGV("get_native_camera: context=%p, camera=%p", context, camera.get());
    if (camera == 0) {
        jniThrowRuntimeException(env,
                "Camera is being used after Camera.release() was called");
    }

    if (pContext != NULL) *pContext = context;
    return camera;
}

sp<Camera> getCamera() { Mutex::Autolock _l(mLock); return mCamera; }

如上跟踪代码可以了解到，jni的startPreview()会调用C++Camera类的startPreview()，在Camera.cpp中定义：

/ start preview mode
status_t Camera::startPreview()
{
    ALOGV("startPreview");
    sp <ICamera> c = mCamera;
    if (c == 0) return NO_INIT;
    return c->startPreview();
}

下一步毫无疑问会调用ICamera类中startPreview()函数，只是ICamera对象mCamera是从哪来的呢？

首先看下声明mCamera的地方，在CameraBase.h中：

sp<TCamUser> mCamera;

跟踪代码可知mCamera是在CameraBase类中声明的sp<TCamUser>的对象，TCamUser在Camera.h中定义：

template <>
struct CameraTraits<Camera>
{
    typedef CameraListener        TCamListener;
    typedef ICamera               TCamUser;
    typedef ICameraClient         TCamCallbacks;
    typedef status_t (ICameraService::*TCamConnectService)(const sp<ICameraClient>&,
                                                           int, const String16&, int,
                                                           /*out*/
                                                           sp<ICamera>&);
    static TCamConnectService     fnConnectService;
};

从以上代码中可知，mCamera是ICamera类型的对象，它是在哪儿实例化的呢？下面我们逆向跟踪下代码，注意是逆向，

首先Camera::create()函数为mCamera赋值，如下：

// construct a camera client from an existing camera remote
sp<Camera> Camera::create(const sp<ICamera>& camera)
{
     ALOGV("create");
     if (camera == 0) {
         ALOGE("camera remote is a NULL pointer");
         return 0;
     }

    sp<Camera> c = new Camera(-1);
    if (camera->connect(c) == NO_ERROR) {
        c->mStatus = NO_ERROR;
        c->mCamera = camera;
        camera->asBinder()->linkToDeath(c);
        return c;
    }
    return 0;
}


status_t CameraSource::isCameraAvailable(
    省略...
    if (camera == 0) {
        mCamera = Camera::connect(cameraId, clientName, clientUid);
        if (mCamera == 0) return -EBUSY;
        mCameraFlags &= ~FLAGS_HOT_CAMERA;
    } else {
        // We get the proxy from Camera, not ICamera. We need to get the proxy
        // to the remote Camera owned by the application. Here mCamera is a
        // local Camera object created by us. We cannot use the proxy from
        // mCamera here.
        mCamera = Camera::create(camera);
        if (mCamera == 0) return -EBUSY;
        省略...
    }

    mCamera->lock();

    return OK;
}


status_t CameraSource::initWithCameraAccess(
        const sp<ICamera>& camera,
        const sp<ICameraRecordingProxy>& proxy,
        int32_t cameraId,
        const String16& clientName,
        uid_t clientUid,
        Size videoSize,
        int32_t frameRate,
        bool storeMetaDataInVideoBuffers) {

    省略...
    if ((err = isCameraAvailable(camera, proxy, cameraId,
            clientName, clientUid)) != OK) {
        ALOGE("Camera connection could not be established.");
        return err;
    }

    省略...
    return OK;
}

status_t CameraSource::init(
        const sp<ICamera>& camera,
        const sp<ICameraRecordingProxy>& proxy,
        int32_t cameraId,
        const String16& clientName,
        uid_t clientUid,
        Size videoSize,
        int32_t frameRate,
        bool storeMetaDataInVideoBuffers) {

    ALOGV("init");
    status_t err = OK;
    int64_t token = IPCThreadState::self()->clearCallingIdentity();
    err = initWithCameraAccess(camera, proxy, cameraId, clientName, clientUid,
                               videoSize, frameRate,
                               storeMetaDataInVideoBuffers);
    IPCThreadState::self()->restoreCallingIdentity(token);
    return err;
}

// static
CameraSource *CameraSource::CreateFromCamera(
    const sp<ICamera>& camera,
    const sp<ICameraRecordingProxy>& proxy,
    int32_t cameraId,
    const String16& clientName,
    uid_t clientUid,
    Size videoSize,
    int32_t frameRate,
    const sp<IGraphicBufferProducer>& surface,
    bool storeMetaDataInVideoBuffers) {

    CameraSource *source = new CameraSource(camera, proxy, cameraId,
            clientName, clientUid, videoSize, frameRate, surface,
            storeMetaDataInVideoBuffers);
    return source;
}

status_t StagefrightRecorder::setupCameraSource(
        sp<CameraSource> *cameraSource) {
    省略...
    if (mCaptureTimeLapse) {
        省略...
    } else {
        *cameraSource = CameraSource::CreateFromCamera(
                mCamera, mCameraProxy, mCameraId, mClientName, mClientUid,
                videoSize, mFrameRate,
                mPreviewSurface, encoderSupportsCameraSourceMetaDataMode);
    }
    mCamera.clear();
    mCameraProxy.clear();
    if (*cameraSource == NULL) {
        return UNKNOWN_ERROR;
    }

    if ((*cameraSource)->initCheck() != OK) {
        (*cameraSource).clear();
        *cameraSource = NULL;
        return NO_INIT;
    }

    省略...

    return OK;
}


status_t StagefrightRecorder::setCamera(const sp<ICamera> &camera,
                                        const sp<ICameraRecordingProxy> &proxy) {
    ALOGV("setCamera");
    if (camera == 0) {
        ALOGE("camera is NULL");
        return BAD_VALUE;
    }
    if (proxy == 0) {
        ALOGE("camera proxy is NULL");
        return BAD_VALUE;
    }

    mCamera = camera;
    mCameraProxy = proxy;
    return OK;
}


status_t MediaRecorderClient::setCamera(const sp<ICamera>& camera,
                                        const sp<ICameraRecordingProxy>& proxy)
{
    ALOGV("setCamera");
    Mutex::Autolock lock(mLock);
    if (mRecorder == NULL) {
        ALOGE("recorder is not initialized");
        return NO_INIT;
    }
    return mRecorder->setCamera(camera, proxy);
}

MediaRecorderClient是BnMediaRecorder的派生类，MediaRecorderClient的setCamera()函数会在BnMediaRecorder的

onTransact()函数中被调用，如下：

status_t BnMediaRecorder::onTransact(
                          uint32_t code, const Parcel& data,

                     Parcel* reply, uint32_t flags)
{
    switch (code) {
        省略...
        case SET_CAMERA: {
            ALOGV("SET_CAMERA");
            CHECK_INTERFACE(IMediaRecorder, data, reply);
            sp<ICamera> camera = interface_cast<ICamera>(data.readStrongBinder());
            sp<ICameraRecordingProxy> proxy =
                interface_cast<ICameraRecordingProxy>(data.readStrongBinder());
            reply->writeInt32(setCamera(camera, proxy));
            return NO_ERROR;
        } break;

       省略...
        default:
            return BBinder::onTransact(code, data, reply, flags);
    }
}

在前面章节，我们已经介绍了interface_cast()函数，这里的interface_cast<ICamera>函数会返回保存的BpCamera对象，如果没有保存的则会实例化BpCamera类并返回该对象．另外BpCamera在实例化时会注入一个IBinder对象．

好了，在Camera.cpp中的startPreview()函数中注入的mCamera对象已经找到源头了，实际上就是上述BpCamera的实例．

然而注入的IBinder又是在哪实例化的？这个问题我们留待后续章节分析．