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Android—Camera Framework C/S架构服务请求

由 时光毁灭记忆、已成空白 提交于 2020-03-15 11:20:03

  在前一篇文章Camera Client/Server的binder IPC机制中阐述了Android进程间通信binder IPC机制基础，Android Camera基于这个基础设计了Camera Framework Client/Server架构，文中给出了其类图设计。本文从Android 4.4 版本源码入手，分析Camera App向Camera Service请求服务与返回的过程。服务请求的过程从上到下依次要经过三个大的过程：1. Camera App调用Framework Camera类Java API；2. Camera类Java API通过JNI调用JNI层的C++接口函数；3. 基于binder通信设计的Camera IPC通信过程。下面结合代码分析整个服务请求过程。

 

Camera应用调用Framework Camera类API

        在Android Kitkat原生Camera2应用（packages/apps/Camera2/）的PhotoModule, VideoModule, WideAnglePanoramaModule类中用CameraUtil.open()方法来打开Camera。然后依次调用：CameraHolder的open()方法，AndroidCameraManagerImpl的cameraOpen()方法，CameraHandler的handleMessage()【message为OPEN_CAMERA】，直到调用Framework Camera类（frameworks/base/core/java/android/hardware/Camera.java）的open()方法。在这里，Camera2应用程序暂不做分析，我们着重看程序向下调用的服务请求过程。

mCameraDevice = CameraUtil.openCamera( mActivity, mCameraId, mHandler, mActivity.getCameraOpenErrorCallback()); // (1)
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public class CameraUtil {
    public static CameraManager.CameraProxy openCamera(
            Activity activity, final int cameraId,
            Handler handler, final CameraManager.CameraOpenErrorCallback cb) {
        try {
            throwIfCameraDisabled(activity);
            return CameraHolder.instance().open(handler, cameraId, cb); // (2)
        } catch (CameraDisabledException ex) {
            handler.post(new Runnable() {
                @Override
                public void run() {
                    cb.onCameraDisabled(cameraId);
                }
            });
        }
        return null;
    }
}
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public class CameraHolder {
    public synchronized CameraProxy open(
            Handler handler, int cameraId,
            CameraManager.CameraOpenErrorCallback cb) {
        …………
        if (mCameraDevice == null) {
            Log.v(TAG, "open camera " + cameraId);
            if (mMockCameraInfo == null) {
                mCameraDevice = CameraManagerFactory
                        .getAndroidCameraManager().cameraOpen(handler, cameraId, cb); // (3)
            …………
        } else {
            …………
        }
        mCameraOpened = true;
        mHandler.removeMessages(RELEASE_CAMERA);
        …………
        return mCameraDevice;
    }
}
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class AndroidCameraManagerImpl implements CameraManager {
    public CameraManager.CameraProxy cameraOpen(
        Handler handler, int cameraId, CameraOpenErrorCallback callback) {
        mCameraHandler.obtainMessage(OPEN_CAMERA, cameraId, 0,
                CameraOpenErrorCallbackForward.getNewInstance(
                        handler, callback)).sendToTarget(); // (4)
        …………
    }
}
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private class CameraHandler extends Handler {
        @Override
        public void handleMessage(final Message msg) {
            try {
                switch (msg.what) {
                    case OPEN_CAMERA:
                        mCamera = android.hardware.Camera.open(msg.arg1); // (5)
                        …………
                        return;
                }
            }
        }
    }
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JNI层调用
          Framework Camera类API调用本地方法，而本地方法被注册到JNI，因此通过JNI调用android_hardware_Camera.cpp（/framworks/base/core/jni/）中对应的方法。在打开相机的过程中，Framework Camera类的open()方法调用本地方法native_setup()。native_setup()被注册到JNI，通过JNI调用android_hardware_Camera_native_setup()方法。再通过android_hardware_Camera_native_setup()调用Camera::connect()函数（frameworks/av/camera/Camera.cpp）请求连接CameraService服务。
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public class Camera {
    public static Camera open(int cameraId) {
        return new Camera(cameraId);
    }
    Camera(int cameraId) {
        …………
        String packageName = ActivityThread.currentPackageName();
        native_setup(new WeakReference<Camera>(this), cameraId, packageName);
    }
}
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JNI层注册native_setup方法

static JNINativeMethod camMethods[] = {
  { "native_setup",
    "(Ljava/lang/Object;ILjava/lang/String;)V",
    (void*)android_hardware_Camera_native_setup }
    ……
};
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// connect to camera service
static void android_hardware_Camera_native_setup(JNIEnv *env, jobject thiz,
    jobject weak_this, jint cameraId, jstring clientPackageName)
{
    …………
    sp<Camera> camera = Camera::connect(cameraId, clientName,
            Camera::USE_CALLING_UID);
    …………
}
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CameraService服务连接（IPC通信）

        JNI调用Camera::connect()请求CameraService服务。Camera类继承模板类CameraBase<Camera>和BnCameraClient。首先调用模板类的connect()函数，在函数中向ServiceManager获取Camera服务信息，并生成CameraService服务代理BpCameraService（/frameworks/av/camera/ICameraService.cpp），然后通过Binder通信发送CONNECT命令，当BnCameraService收到CONNECT命令后调用CameraService的connect()成员函数来做相应的处理。

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template <typename TCam, typename TCamTraits>
sp<TCam> CameraBase<TCam, TCamTraits>::connect(int cameraId,
                                               const String16& clientPackageName,
                                               int clientUid)
{
    sp<TCam> c = new TCam(cameraId);    // BnCameraClient
    sp<TCamCallbacks> cl = c;
    status_t status = NO_ERROR;
    const sp<ICameraService>& cs = getCameraService();    // return BpCameraService
    if (cs != 0) {
        TCamConnectService fnConnectService = TCamTraits::fnConnectService;
        status = (cs.get()->*fnConnectService)(cl, cameraId, clientPackageName, clientUid,
                                             /*out*/ c->mCamera);
    }
    if (status == OK && c->mCamera != 0) {
        c->mCamera->asBinder()->linkToDeath(c);
        c->mStatus = NO_ERROR;
    } else {
        ALOGW("An error occurred while connecting to camera: %d", cameraId);
        c.clear();
    }
    return c;
}
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       下面我们来分析BpCameraService的connect()成员函数。首先将传递过来的Camera对象转换成IBinder类型，将调用的参数写到Parcel中，通过BpBinder的transact()函数发送消息，然后由BnCameraService去响应该连接，最后就是等待服务端返回，如果成功这里为我们生成一个BpCamera实例。

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class BpCameraService: public BpInterface<ICameraService>
{
    // connect to camera service (android.hardware.Camera)
    virtual status_t connect(const sp<ICameraClient>& cameraClient, int cameraId,
                             const String16 &clientPackageName, int clientUid,
                             /*out*/
                             sp<ICamera>& device)
    {
        Parcel data, reply;
        data.writeInterfaceToken(ICameraService::getInterfaceDescriptor());
        data.writeStrongBinder(cameraClient->asBinder());
        data.writeInt32(cameraId);
        data.writeString16(clientPackageName);
        data.writeInt32(clientUid);
        remote()->transact(BnCameraService::CONNECT, data, &reply); // BpBinder的transact()函数向IPCThreadState实例发送消息，通知其有消息要发送给binder driver
        if (readExceptionCode(reply)) return -EPROTO;
        status_t status = reply.readInt32();
        if (reply.readInt32() != 0) {
            device = interface_cast<ICamera>(reply.readStrongBinder()); // client端读出server返回的binder
        }
        return status;
    }
}
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        BnCameraService的onTransact()函数负责解包收到的Parcel并执行client端的请求的方法。服务端收到CONNECT命令之后，
(1) 使用Camera的Binder对象生成Camera客户代理BpCameraClient实例；
(2) 将生成的BpCameraClient对象作为参数传递到CameraService（/frameworks/av/services/camera/libcameraservice/CameraService.cpp）的connect()函数中，该函数会返回一个BpCamera实例；
(3) 将在(2)中返回的实例对象以IBinder的形式打包到Parcel中返回。
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status_t BnCameraService::onTransact(
    uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{
    switch(code) {
        case CONNECT: {
            CHECK_INTERFACE(ICameraService, data, reply);
            sp<ICameraClient> cameraClient =
                    interface_cast<ICameraClient>(data.readStrongBinder()); // （1）
            int32_t cameraId = data.readInt32();
            const String16 clientName = data.readString16();
            int32_t clientUid = data.readInt32();
            sp<ICamera> camera;
            status_t status = connect(cameraClient, cameraId,
                    clientName, clientUid, /*out*/ camera); // （2）
            reply->writeNoException();
            reply->writeInt32(status);
            if (camera != NULL) {
                reply->writeInt32(1);
                reply->writeStrongBinder(camera->asBinder()); // （3）
            } else {
                reply->writeInt32(0);
            }
            return NO_ERROR;
        } break;
    }
}
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        接下来看CameraService::connect()函数，该函数返回一个BpCamera实例。

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status_t CameraService::connect(
        const sp<ICameraClient>& cameraClient,
        int cameraId,
        const String16& clientPackageName,
        int clientUid,
        /*out*/
        sp<ICamera>& device) {
    String8 clientName8(clientPackageName);
    int callingPid = getCallingPid();
    LOG1("CameraService::connect E (pid %d \"%s\", id %d)", callingPid,
            clientName8.string(), cameraId);
    status_t status = validateConnect(cameraId, /*inout*/clientUid);
    if (status != OK) {
        return status;
    }
    sp<Client> client;
    {
        Mutex::Autolock lock(mServiceLock);
        sp<BasicClient> clientTmp;
        if (!canConnectUnsafe(cameraId, clientPackageName,
                              cameraClient->asBinder(),
                              /*out*/clientTmp)) {
            return -EBUSY;
        } else if (client.get() != NULL) {
            device = static_cast<Client*>(clientTmp.get());
            return OK;
        }
        int facing = -1;
        int deviceVersion = getDeviceVersion(cameraId, &facing);
        // If there are other non-exclusive users of the camera,
        // this will tear them down before we can reuse the camera
        if (isValidCameraId(cameraId)) {
            // transition from PRESENT -> NOT_AVAILABLE
            updateStatus(ICameraServiceListener::STATUS_NOT_AVAILABLE,
                         cameraId);
        }
        /* 根据HAL不同API的版本创建不同的client实例 */
        switch(deviceVersion) {
          case CAMERA_DEVICE_API_VERSION_1_0:
            client = new CameraClient(this, cameraClient,
                    clientPackageName, cameraId,
                    facing, callingPid, clientUid, getpid());
            break;
          case CAMERA_DEVICE_API_VERSION_2_0:
          case CAMERA_DEVICE_API_VERSION_2_1:
          case CAMERA_DEVICE_API_VERSION_3_0:
            client = new Camera2Client(this, cameraClient,
                    clientPackageName, cameraId,
                    facing, callingPid, clientUid, getpid(),
                    deviceVersion);
            break;
          case -1:
            ALOGE("Invalid camera id %d", cameraId);
            return BAD_VALUE;
          default:
            ALOGE("Unknown camera device HAL version: %d", deviceVersion);
            return INVALID_OPERATION;
        }
        status_t status = connectFinishUnsafe(client, client->getRemote());
        if (status != OK) {
            // this is probably not recoverable.. maybe the client can try again
            // OK: we can only get here if we were originally in PRESENT state
            updateStatus(ICameraServiceListener::STATUS_PRESENT, cameraId);
            return status;
        }
        mClient[cameraId] = client; // every camera is a Client class
        LOG1("CameraService::connect X (id %d, this pid is %d)", cameraId,
             getpid());
    }
    // important: release the mutex here so the client can call back
    // into the service from its destructor (can be at the end of the call)
    device = client; // 返回的camera device实例
    return OK;
}
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         至此，一次Framework层的Camera服务请求过程完成。

标签
handler机制
函数调用
android架构
JNI
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