【Android】 Camera Framework层分析

易寒

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)
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;
    }
}
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;
    }
}
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)
        …………
    }
}

    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;
                }
            }
        }
    }

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。

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);
 }
}

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 }
    ……
};
// connect to camera servicestatic 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);
    …………
}

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。

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;
}

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。

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;
    }
}

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。

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;
    }
}

接下来看CameraService::connect()函数,该函数返回一个BpCamera实例。

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;
}

至此,一次Framework层的Camera服务请求过程完成。
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