上一次讲解了一下startPreview过程，主要是为了画出一条大致的从上到下的线条，今天我们看一下Camera在Framework的sendCommand和dataCallback，这部分属于衔接过程，可以看到上下是如何交流沟通的。

首先，sendCommand

Camera.java中并没有sendCommand方法，在Camera.cpp中存在sendCommand函数，所以这个sendCommand是从android_hardware_interface.cpp中开始使用的

android_hardware_Camera.cpp (base\core\jni)

startSmoothZoom  
android_hardware_Camera_startSmoothZoom
     ------>CAMERA_CMD_START_SMOOTH_ZOOM

stopSmoothZoom
android_hardware_Camera_stopSmoothZoom
     ------>CAMERA_CMD_STOP_SMOOTH_ZOOM

setDisplayOrientation
android_hardware_Camera_setDisplayOrientation
     ------>CAMERA_CMD_SET_DISPLAY_ORIENTATION

_enableShutterSound
android_hardware_Camera_enableShutterSound
     ------>CAMERA_CMD_ENABLE_SHUTTER_SOUND

_startFaceDetection
android_hardware_Camera_startFaceDetection
     ------>CAMERA_CMD_START_FACE_DETECTION

_stopFaceDetection 
android_hardware_Camera_stopFaceDetection
     ------>CAMERA_CMD_STOP_FACE_DETECTION

enableFocusMoveCallback
android_hardware_Camera_enableFocusMoveCallback
     ------>CAMERA_CMD_ENABLE_FOCUS_MOVE_MSG

诸如此类的命令类型定义在Camera.h (system\core\include\system)

enum {
    CAMERA_CMD_START_SMOOTH_ZOOM = 1,
    CAMERA_CMD_STOP_SMOOTH_ZOOM = 2,
    CAMERA_CMD_SET_DISPLAY_ORIENTATION = 3,
    CAMERA_CMD_ENABLE_SHUTTER_SOUND = 4,
    CAMERA_CMD_PLAY_RECORDING_SOUND = 5,
    CAMERA_CMD_START_FACE_DETECTION = 6,
    CAMERA_CMD_STOP_FACE_DETECTION = 7,
    CAMERA_CMD_ENABLE_FOCUS_MOVE_MSG = 8,
    CAMERA_CMD_PING = 9,
    CAMERA_CMD_SET_VIDEO_BUFFER_COUNT = 10,
};

以上者几种操作都是采用sendCommand()的函数来实现的，对应的命令类型也列举出来了，HAL层会根据这个消息类型做出判断，然后做出对应的操作。
来看下sendCommand的实现：
Camera.cpp (frameworks\av\camera)

// send command to camera driver
status_t Camera::sendCommand(int32_t cmd, int32_t arg1, int32_t arg2)
{
    ALOGV("sendCommand");
    sp <ICamera> c = mCamera;
    if (c == 0) return NO_INIT;
    return c->sendCommand(cmd, arg1, arg2);  //三个参数，第一个是消息类型，后面两个参数有时候使用有时不使用，这里应该是具有扩展性的，如果需要添加更多的参数，上下接口同时修改就可以了。
}

然后通过Binder机制，

    virtual status_t sendCommand(int32_t cmd, int32_t arg1, int32_t arg2)
    {
        ALOGV("sendCommand");
        Parcel data, reply;
        data.writeInterfaceToken(ICamera::getInterfaceDescriptor());
        data.writeInt32(cmd);
        data.writeInt32(arg1);
        data.writeInt32(arg2);
        remote()->transact(SEND_COMMAND, data, &reply);
        return reply.readInt32();
    }

status_t BnCamera::onTransact(
    uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{
        ......
        case SEND_COMMAND: {
            ALOGV("SEND_COMMAND");
            CHECK_INTERFACE(ICamera, data, reply);
            int command = data.readInt32();
            int arg1 = data.readInt32();
            int arg2 = data.readInt32();
            reply->writeInt32(sendCommand(command, arg1, arg2));
            return NO_ERROR;
         } break;
}

然后调用到

status_t CameraClient::sendCommand(int32_t cmd, int32_t arg1, int32_t arg2) {
    ......
    if (cmd == CAMERA_CMD_SET_DISPLAY_ORIENTATION) {
        // Mirror the preview if the camera is front-facing.
        orientation = getOrientation(arg1, mCameraFacing == CAMERA_FACING_FRONT);
        if (orientation == -1) return BAD_VALUE;

        if (mOrientation != orientation) {
            mOrientation = orientation;
            if (mPreviewWindow != 0) {
                native_window_set_buffers_transform(mPreviewWindow.get(),
                        mOrientation);
            }
        }
        return OK;
    } else if (cmd == CAMERA_CMD_ENABLE_SHUTTER_SOUND) {
        switch (arg1) {    //这里就是对参数arg1的使用，通过参数来开关拍照声音
            case 0: 
                return enableShutterSound(false);
            case 1:
                return enableShutterSound(true);
            default:
                return BAD_VALUE;
        }
        return OK;
    } else if (cmd == CAMERA_CMD_PLAY_RECORDING_SOUND) { //录像声音
        mCameraService->playSound(CameraService::SOUND_RECORDING);
    } else if (cmd == CAMERA_CMD_SET_VIDEO_BUFFER_COUNT) {
        // Silently ignore this command
        return INVALID_OPERATION;
    } else if (cmd == CAMERA_CMD_PING) {
        // If mHardware is 0, checkPidAndHardware will return error.
        return OK;
    } //以上是framework可以处理的命令
    return mHardware->sendCommand(cmd, arg1, arg2);  //HAL层处理
}

在HAL层中处理的消息类型主要是打开和停止人脸检测过程，
QCamera2HWI.cpp (\device\asus\flo\camera\qcamera2\hal)

int QCamera2HardwareInterface::sendCommand(int32_t command, int32_t /*arg1*/, int32_t /*arg2*/)
{
    int rc = NO_ERROR;
    switch (command) {
    case CAMERA_CMD_START_FACE_DETECTION:
    case CAMERA_CMD_STOP_FACE_DETECTION:
        //开关人脸检测
        rc = setFaceDetection(command == CAMERA_CMD_START_FACE_DETECTION? true : false);
        break;
    default:
        rc = NO_ERROR;
        break;
    }
    return rc;
}

综上，如上就是sendCommand的过程

然后，回调Callback

从之前的文章可以看到callback主要有三种类型
notifyCallback
dataCallback
dataTimestampCallback

这个过程我们就不能按照之前的从上至下的跟过程了，这次需要反着来，从HAL层回调到
CameraHardwareInterface.h (frameworks\av\services\camera\libcameraservice\device1)

    static void __notify_cb(int32_t msg_type, int32_t ext1,
                            int32_t ext2, void *user)
    {
        ALOGV("%s", __FUNCTION__);
        CameraHardwareInterface *__this =
                static_cast<CameraHardwareInterface *>(user);
        __this->mNotifyCb(msg_type, ext1, ext2, __this->mCbUser);
    }

    static void __data_cb(int32_t msg_type,
                          const camera_memory_t *data, unsigned int index,
                          camera_frame_metadata_t *metadata,
                          void *user)
    {
        ALOGV("%s", __FUNCTION__);
        CameraHardwareInterface *__this =
                static_cast<CameraHardwareInterface *>(user);
        ......
        __this->mDataCb(msg_type, mem->mBuffers[index], metadata, __this->mCbUser);
    }

    static void __data_cb_timestamp(nsecs_t timestamp, int32_t msg_type,
                             const camera_memory_t *data, unsigned index,
                             void *user)
    {
        ALOGV("%s", __FUNCTION__);
        CameraHardwareInterface *__this =
                static_cast<CameraHardwareInterface *>(user);
        ......
        __this->mDataCbTimestamp(timestamp, msg_type, mem->mBuffers[index], __this->mCbUser);
    }

其中的mNotifyCb，mDataCb，mDataCbTimestamp是在CameraClient::initialize函数中设置的

    void setCallbacks(notify_callback notify_cb,
                      data_callback data_cb,
                      data_callback_timestamp data_cb_timestamp,
                      void* user)
    {
        mNotifyCb = notify_cb;
        mDataCb = data_cb;
        mDataCbTimestamp = data_cb_timestamp;
        mCbUser = user;

        ALOGV("%s(%s)", __FUNCTION__, mName.string());

        if (mDevice->ops->set_callbacks) {
            mDevice->ops->set_callbacks(mDevice,
                                   __notify_cb,
                                   __data_cb,
                                   __data_cb_timestamp,
                                   __get_memory,
                                   this);
        }
    }

回调自然是到CameraClient中去找了

void CameraClient::notifyCallback(int32_t msgType, int32_t ext1,
        int32_t ext2, void* user) {
    ......
}

void CameraClient::dataCallback(int32_t msgType,
        const sp<IMemory>& dataPtr, camera_frame_metadata_t *metadata, void* user) {
    LOG2("dataCallback(%d)", msgType);

    Mutex* lock = getClientLockFromCookie(user);
    if (lock == NULL) return;
    Mutex::Autolock alock(*lock);

    CameraClient* client =
            static_cast<CameraClient*>(getClientFromCookie(user));
    if (client == NULL) return;

    if (!client->lockIfMessageWanted(msgType)) return;
    if (dataPtr == 0 && metadata == NULL) {
        ALOGE("Null data returned in data callback");
        client->handleGenericNotify(CAMERA_MSG_ERROR, UNKNOWN_ERROR, 0);
        return;
    }

    switch (msgType & ~CAMERA_MSG_PREVIEW_METADATA) {
        case CAMERA_MSG_PREVIEW_FRAME:
            client->handlePreviewData(msgType, dataPtr, metadata);
            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, metadata);
            break;
    }
}

void CameraClient::dataCallbackTimestamp(nsecs_t timestamp,
        int32_t msgType, const sp<IMemory>& dataPtr, void* user) {
    ......
}

这里主要看下dataCallback的过程吧，

    switch (msgType & ~CAMERA_MSG_PREVIEW_METADATA) {
        case CAMERA_MSG_PREVIEW_FRAME:  //预览帧数据
            client->handlePreviewData(msgType, dataPtr, metadata);
            break;
        case CAMERA_MSG_POSTVIEW_FRAME: //postview image
            client->handlePostview(dataPtr);
            break;
        case CAMERA_MSG_RAW_IMAGE:   //原始数据
            client->handleRawPicture(dataPtr);
            break;
        case CAMERA_MSG_COMPRESSED_IMAGE: //真实图片
            client->handleCompressedPicture(dataPtr);
            break;
        default:
            client->handleGenericData(msgType, dataPtr, metadata);
            break;
    }

这里最后会调用到
c->dataCallback，然后根据消息类型来做处理，然后在通过binder机制

    // generic data callback from camera service to app with image data
    void dataCallback(int32_t msgType, const sp<IMemory>& imageData,
                      camera_frame_metadata_t *metadata)
    {
        ALOGV("dataCallback");
        Parcel data, reply;
        data.writeInterfaceToken(ICameraClient::getInterfaceDescriptor());
        data.writeInt32(msgType);
        data.writeStrongBinder(imageData->asBinder());
        if (metadata) {
            data.writeInt32(metadata->number_of_faces);
            data.write(metadata->faces, sizeof(camera_face_t) * metadata->number_of_faces);
        }
        remote()->transact(DATA_CALLBACK, data, &reply, IBinder::FLAG_ONEWAY);
    }

status_t BnCameraClient::onTransact(
    uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{
    switch(code) {
        ......
        case DATA_CALLBACK: {
            ALOGV("DATA_CALLBACK");
            CHECK_INTERFACE(ICameraClient, data, reply);
            int32_t msgType = data.readInt32();
            sp<IMemory> imageData = interface_cast<IMemory>(data.readStrongBinder());
            camera_frame_metadata_t *metadata = NULL;
            if (data.dataAvail() > 0) {
                metadata = new camera_frame_metadata_t;
                metadata->number_of_faces = data.readInt32();
                metadata->faces = (camera_face_t *) data.readInplace(
                        sizeof(camera_face_t) * metadata->number_of_faces);
            }
            dataCallback(msgType, imageData, metadata);
            if (metadata) delete metadata;
            return NO_ERROR;
        } break;
        ......
}

这里转到Camera.cpp

// callback from camera service when frame or image is ready
void Camera::dataCallback(int32_t msgType, const sp<IMemory>& dataPtr,
                          camera_frame_metadata_t *metadata)
{
    sp<CameraListener> listener;
    {
        Mutex::Autolock _l(mLock);
        listener = mListener;
    }
    if (listener != NULL) {
        listener->postData(msgType, dataPtr, metadata);
    }
}

通过listener的方式来往上层甩数据，那么问题来了，这个listener是什么时候设置的？
回想一下第一篇FWK分析博客中的native_setup过程中有这么一段

    sp<JNICameraContext> context = new JNICameraContext(env, weak_this, clazz, camera);
    context->incStrong((void*)android_hardware_Camera_native_setup);
    camera->setListener(context);

就是在这里设置的listener,JNICameraContext继承CameraListener，复写父类的方法

class JNICameraContext: public CameraListener{
    ......   
}

class CameraListener: virtual public RefBase
{
public:
    virtual void notify(int32_t msgType, int32_t ext1, int32_t ext2) = 0;
    virtual void postData(int32_t msgType, const sp<IMemory>& dataPtr,
                          camera_frame_metadata_t *metadata) = 0;
    virtual void postDataTimestamp(nsecs_t timestamp, int32_t msgType, const sp<IMemory>& dataPtr) = 0;
};

承接上面那一段，这里我们只看postData()
android_hardware_Camera.cpp (frameworks\base\core\jni)

void JNICameraContext::postData(int32_t msgType, const sp<IMemory>& dataPtr,
                                camera_frame_metadata_t *metadata)
{
    ......
    int32_t dataMsgType = msgType & ~CAMERA_MSG_PREVIEW_METADATA;

    // return data based on callback type
    switch (dataMsgType) {
        case CAMERA_MSG_VIDEO_FRAME:
            // should never happen
            break;

        // For backward-compatibility purpose, if there is no callback
        // buffer for raw image, the callback returns null.
        case CAMERA_MSG_RAW_IMAGE:
            ALOGV("rawCallback");
            if (mRawImageCallbackBuffers.isEmpty()) {
                env->CallStaticVoidMethod(mCameraJClass, fields.post_event,
                        mCameraJObjectWeak, dataMsgType, 0, 0, NULL);
            } else {
                copyAndPost(env, dataPtr, dataMsgType);
            }
            break;

        // There is no data.
        case 0:
            break;

        default:
            ALOGV("dataCallback(%d, %p)", dataMsgType, dataPtr.get());
            copyAndPost(env, dataPtr, dataMsgType);
            break;
    }
    // post frame metadata to Java
    if (metadata && (msgType & CAMERA_MSG_PREVIEW_METADATA)) {
        postMetadata(env, CAMERA_MSG_PREVIEW_METADATA, metadata);//这里有人脸数据
    }
}

这里涉及到的
env->CallStaticVoidMethod(mCameraJClass, fields.post_event,
mCameraJObjectWeak, dataMsgType, 0, 0, NULL);
copyAndPost(env, dataPtr, dataMsgType);
postMetadata(env, CAMERA_MSG_PREVIEW_METADATA, metadata);
直接或间接的使用到fileds.post_event函数，这里是JNI中的方法注册，
fields.post_event = env->GetStaticMethodID(clazz, "postEventFromNative",
"(Ljava/lang/Object;IIILjava/lang/Object;)V");
这个是在register_android_hardware_Camera()函数中调用的，这里不做过多停留，这个实际会调用到
Camera.java (frameworks\base\core\java\android\hardware)

    private static void postEventFromNative(Object camera_ref,
                                            int what, int arg1, int arg2, Object obj)
    {
        Camera c = (Camera)((WeakReference)camera_ref).get();
        if (c == null)
            return;

        if (c.mEventHandler != null) {
            Message m = c.mEventHandler.obtainMessage(what, arg1, arg2, obj);
            c.mEventHandler.sendMessage(m);
        }
    }

这里也是采用了JAVA中很常用的handler message处理

    private class EventHandler extends Handler
    {
        private final Camera mCamera;

        public EventHandler(Camera c, Looper looper) {
            super(looper);
            mCamera = c;
        }

        @Override
        public void handleMessage(Message msg) {
            switch(msg.what) {
            case CAMERA_MSG_SHUTTER:
                if (mShutterCallback != null) {
                    mShutterCallback.onShutter();
                }
                return;

            case CAMERA_MSG_RAW_IMAGE:
                if (mRawImageCallback != null) {
                    mRawImageCallback.onPictureTaken((byte[])msg.obj, mCamera);
                }
                return;

            case CAMERA_MSG_COMPRESSED_IMAGE:
                if (mJpegCallback != null) {
                    mJpegCallback.onPictureTaken((byte[])msg.obj, mCamera);
                }
                return;

            case CAMERA_MSG_PREVIEW_FRAME:
                PreviewCallback pCb = mPreviewCallback;
                if (pCb != null) {
                    if (mOneShot) {
                        // Clear the callback variable before the callback
                        // in case the app calls setPreviewCallback from
                        // the callback function
                        mPreviewCallback = null;
                    } else if (!mWithBuffer) {
                        // We're faking the camera preview mode to prevent
                        // the app from being flooded with preview frames.
                        // Set to oneshot mode again.
                        setHasPreviewCallback(true, false);
                    }
                    pCb.onPreviewFrame((byte[])msg.obj, mCamera);
                }
                return;

            case CAMERA_MSG_POSTVIEW_FRAME:
                if (mPostviewCallback != null) {
                    mPostviewCallback.onPictureTaken((byte[])msg.obj, mCamera);
                }
                return;

            case CAMERA_MSG_FOCUS:
                AutoFocusCallback cb = null;
                synchronized (mAutoFocusCallbackLock) {
                    cb = mAutoFocusCallback;
                }
                if (cb != null) {
                    boolean success = msg.arg1 == 0 ? false : true;
                    cb.onAutoFocus(success, mCamera);
                }
                return;

            case CAMERA_MSG_ZOOM:
                if (mZoomListener != null) {
                    mZoomListener.onZoomChange(msg.arg1, msg.arg2 != 0, mCamera);
                }
                return;

            case CAMERA_MSG_PREVIEW_METADATA:
                if (mFaceListener != null) {
                    mFaceListener.onFaceDetection((Face[])msg.obj, mCamera);
                }
                return;

            case CAMERA_MSG_ERROR :
                Log.e(TAG, "Error " + msg.arg1);
                if (mErrorCallback != null) {
                    mErrorCallback.onError(msg.arg1, mCamera);
                }
                return;

            case CAMERA_MSG_FOCUS_MOVE:
                if (mAutoFocusMoveCallback != null) {
                    mAutoFocusMoveCallback.onAutoFocusMoving(msg.arg1 == 0 ? false : true, mCamera);
                }
                return;

            default:
                Log.e(TAG, "Unknown message type " + msg.what);
                return;
            }
        }
    }

到这里基本上就是上层的处理了，callback都是在相机应用中设置的，然后各种数据就在相机应用中得到对应的处理。

具体的每一个数据怎么处理，这里我们不做分析，后续有需要，可以在细讲一下，旨在弄清楚代码是怎么走的。

备注

本文中代码使用的是Android5.0原始代码，最新的Android N版本除了把CameraService单独拎出来，其他的内容基本上大同小异。

版权声明：本文为博主原创文章，未经博主允许不得转载。

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