Android Camera原理之openCamera模块(一)

我们平时开发，知道怎么调度api，怎么调起camera，怎么调用camera的实例来操作camera就可以了，但是这些调度的背后都做了什么事情，我们可能不太清楚，本文打算从openCamera这个调用谈起，展开说下camera调起之后底层是怎么工作的？

Camera操作过程中最重要的四个步骤：

CameraManager-->openCamera ---> 打开相机

CameraDeviceImpl-->createCaptureSession ---> 创建捕获会话

CameraCaptureSession-->setRepeatingRequest ---> 设置预览界面

CameraDeviceImpl-->capture ---> 开始捕获图片

1.CameraManager

CameraManager是本地的SystemService集合中一个service，在SystemServiceRegistry中注册：

        registerService(Context.CAMERA_SERVICE, CameraManager.class,
                new CachedServiceFetcher<CameraManager>() { @Override public CameraManager createService(ContextImpl ctx) { return new CameraManager(ctx); }});

SystemServiceRegistry中有两个HashMap集合来存储本地的SystemService数据，有一点要注意点一些，这和Binder的service不同，他们不是binder service，只是普通的调用模块，集成到一个本地service中，便于管理。

    private static final HashMap<Class<?>, String> SYSTEM_SERVICE_NAMES = new HashMap<Class<?>, String>(); private static final HashMap<String, ServiceFetcher<?>> SYSTEM_SERVICE_FETCHERS = new HashMap<String, ServiceFetcher<?>>();

2.openCamera函数

CameraManager中两个openCamera(...)，只是一个传入Handler，一个传入Executor，是想用线程池来执行Camera中耗时操作。

public void openCamera(@NonNull String cameraId,
            @NonNull final CameraDevice.StateCallback callback, @Nullable Handler handler)

public void openCamera(@NonNull String cameraId,
            @NonNull @CallbackExecutor Executor executor,
            @NonNull final CameraDevice.StateCallback callback)

cameraId 是一个标识，标识当前要打开的camera

callback 是一个状态回调，当前camera被打开的时候，这个状态回调会被触发的。

handler 是传入的一个执行耗时操作的handler

executor 操作线程池

了解一下openCamera的调用流程：

openCamera流程.jpg

2.1 openCameraDeviceUserAsync函数

private CameraDevice openCameraDeviceUserAsync(String cameraId, CameraDevice.StateCallback callback, Executor executor, final int uid) throws CameraAccessException { //...... }

返回值是CameraDevice，从《Android Camera模块解析之拍照》中讲解了Camera framework模块中主要类之间的关系，CameraDevice是抽象类，CameraDeviceImpl是其实现类，就是要获取CameraDeviceImpl的实例。
这个函数的主要作用就是到底层获取相机设备的信息，并获取当前指定cameraId的设备实例。
本函数的主要工作可以分为下面五点：

获取当前cameraId指定相机的设备信息

利用获取相机的设备信息创建CameraDeviceImpl实例

调用远程CameraService获取当前相机的远程服务

将获取的远程服务设置到CameraDeviceImpl实例中

返回CameraDeviceImpl实例

2.2 获取当前cameraId指定相机的设备信息

CameraCharacteristics characteristics = getCameraCharacteristics(cameraId);

一句简单的调用，返回值是CameraCharacteristics，CameraCharacteristics提供了CameraDevice的各种属性，可以通过getCameraCharacteristics函数来查询。

    public CameraCharacteristics getCameraCharacteristics(@NonNull String cameraId) throws CameraAccessException { CameraCharacteristics characteristics = null; if (CameraManagerGlobal.sCameraServiceDisabled) { throw new IllegalArgumentException("No cameras available on device"); } synchronized (mLock) { ICameraService cameraService = CameraManagerGlobal.get().getCameraService(); if (cameraService == null) { throw new CameraAccessException(CameraAccessException.CAMERA_DISCONNECTED, "Camera service is currently unavailable"); } try { if (!supportsCamera2ApiLocked(cameraId)) { int id = Integer.parseInt(cameraId); String parameters = cameraService.getLegacyParameters(id); CameraInfo info = cameraService.getCameraInfo(id); characteristics = LegacyMetadataMapper.createCharacteristics(parameters, info); } else { CameraMetadataNative info = cameraService.getCameraCharacteristics(cameraId); characteristics = new CameraCharacteristics(info); } } catch (ServiceSpecificException e) { throwAsPublicException(e); } catch (RemoteException e) { throw new CameraAccessException(CameraAccessException.CAMERA_DISCONNECTED, "Camera service is currently unavailable", e); } } return characteristics; }

一个关键的函数----> supportsCamera2ApiLocked(cameraId)，这个函数的意思是当前camera服务是否支持camera2 api，如果支持，返回true，如果不支持，返回false。

    private boolean supportsCameraApiLocked(String cameraId, int apiVersion) { /* * Possible return values: * - NO_ERROR => CameraX API is supported * - CAMERA_DEPRECATED_HAL => CameraX API is *not* supported (thrown as an exception) * - Remote exception => If the camera service died * * Anything else is an unexpected error we don't want to recover from. */ try { ICameraService cameraService = CameraManagerGlobal.get().getCameraService(); // If no camera service, no support if (cameraService == null) return false; return cameraService.supportsCameraApi(cameraId, apiVersion); } catch (RemoteException e) { // Camera service is now down, no support for any API level } return false; }

调用的CameraService对应的是ICameraService.aidl，对应的实现类在frameworks/av/services/camera/libcameraservice/CameraService.h
下面是CameraManager与CameraService之间的连接关系图示：

CameraService生成.jpg

CameraManagerGlobal是CameraManager中的内部类，服务端在native层，《Android Camera模块解析之拍照》中camera2介绍的时候已经说明了当前cameraservice是放在frameworks/av/services/camera/libcameraservice/中的，编译好了之后会生成一个libcameraservices.so的共享库。熟悉camera代码，首先应该熟悉camera架构的代码。

这儿监测的是当前camera架构是基于HAL什么版本的，看下面的switch判断：

当前device是基于HAL1.0 HAL3.0 HAL3.1，并且apiversion不是API_VERSION_2，此时支持，这里要搞清楚了，这里的API_VERSION_2不是api level 2，而是camera1还是camera2.

当前device是基于HAL3.2 HAL3.3 HAL3.4，此时支持

目前android版本，正常情况下都是支持camera2的

Status CameraService::supportsCameraApi(const String16& cameraId, int apiVersion, /*out*/ bool *isSupported) { ATRACE_CALL(); const String8 id = String8(cameraId); ALOGV("%s: for camera ID = %s", __FUNCTION__, id.string()); switch (apiVersion) { case API_VERSION_1: case API_VERSION_2: break; default: String8 msg = String8::format("Unknown API version %d", apiVersion); ALOGE("%s: %s", __FUNCTION__, msg.string()); return STATUS_ERROR(ERROR_ILLEGAL_ARGUMENT, msg.string()); } int deviceVersion = getDeviceVersion(id); switch(deviceVersion) { case CAMERA_DEVICE_API_VERSION_1_0: case CAMERA_DEVICE_API_VERSION_3_0: case CAMERA_DEVICE_API_VERSION_3_1: if (apiVersion == API_VERSION_2) { ALOGV("%s: Camera id %s uses HAL version %d <3.2, doesn't support api2 without shim", __FUNCTION__, id.string(), deviceVersion); *isSupported = false; } else { // if (apiVersion == API_VERSION_1) { ALOGV("%s: Camera id %s uses older HAL before 3.2, but api1 is always supported", __FUNCTION__, id.string()); *isSupported = true; } break; case CAMERA_DEVICE_API_VERSION_3_2: case CAMERA_DEVICE_API_VERSION_3_3: case CAMERA_DEVICE_API_VERSION_3_4: ALOGV("%s: Camera id %s uses HAL3.2 or newer, supports api1/api2 directly", __FUNCTION__, id.string()); *isSupported = true; break; case -1: { String8 msg = String8::format("Unknown camera ID %s", id.string()); ALOGE("%s: %s", __FUNCTION__, msg.string()); return STATUS_ERROR(ERROR_ILLEGAL_ARGUMENT, msg.string()); } default: { String8 msg = String8::format("Unknown device version %x for device %s", deviceVersion, id.string()); ALOGE("%s: %s", __FUNCTION__, msg.string()); return STATUS_ERROR(ERROR_INVALID_OPERATION, msg.string()); } } return Status::ok(); }

采用camera2 api来获取相机设备的信息。

CameraMetadataNative info = cameraService.getCameraCharacteristics(cameraId);
characteristics = new CameraCharacteristics(info);

getCameraCharacteristics调用流程.jpg

其中DeviceInfo3是CameraProviderManager::ProviderInfo::DeviceInfo3，CameraProviderManager中的结构体，最终返回的是CameraMetadata类型，它是一个Parcelable类型，native中对应的代码是frameworks/av/camera/include/camera/CameraMetadata.h，java中对应的是frameworks/base/core/java/android/hardware/camera2/impl/CameraMetadataNative.java，Parcelable类型是可以跨进程传输的。下面是在native中定义CameraMetadata为CameraMetadataNative

namespace hardware { namespace camera2 { namespace impl { using ::android::CameraMetadata; typedef CameraMetadata CameraMetadataNative; } } }

我们关注其中的一个调用函数：

status_t CameraProviderManager::getCameraCharacteristicsLocked(const std::string &id, CameraMetadata* characteristics) const { auto deviceInfo = findDeviceInfoLocked(id, /*minVersion*/ {3,0}, /*maxVersion*/ {4,0}); if (deviceInfo == nullptr) return NAME_NOT_FOUND; return deviceInfo->getCameraCharacteristics(characteristics); }

发现了调用了一个findDeviceInfoLocked(...)函数，返回类型是一个DeviceInfo结构体，CameraProviderManager.h中定义了三个DeviceInfo结构体，除了DeviceInfo之外，还有DeviceInfo1与DeviceInfo3，他们都继承DeviceInfo，其中DeviceInfo1为HALv1服务，DeviceInfo3为HALv3-specific服务，都是提供camera device一些基本信息。这里主要看下findDeviceInfoLocked(...)函数：

CameraProviderManager::ProviderInfo::DeviceInfo* CameraProviderManager::findDeviceInfoLocked(
        const std::string& id, hardware::hidl_version minVersion, hardware::hidl_version maxVersion) const { for (auto& provider : mProviders) { for (auto& deviceInfo : provider->mDevices) { if (deviceInfo->mId == id && minVersion <= deviceInfo->mVersion && maxVersion >= deviceInfo->mVersion) { return deviceInfo.get(); } } } return nullptr; }

这儿的是mProviders是ProviderInfo类型的列表，这个ProviderInfo也是CameraProviderManager.h中定义的结构体，并且上面3种DeviceInfo都是定义在ProviderInfo里面的。下面给出了ProviderInfo的代码大纲，裁剪了很多代码，但是我们还是能看到核心的代码：ProviderInfo是管理当前手机的camera device设备的，通过addDevice保存在mDevices中，接下来我们看下这个addDevice是如何工作的。

    struct ProviderInfo : virtual public hardware::camera::provider::V2_4::ICameraProviderCallback, virtual public hardware::hidl_death_recipient { //...... ProviderInfo(const std::string &providerName, sp<hardware::camera::provider::V2_4::ICameraProvider>& interface, CameraProviderManager *manager); ~ProviderInfo(); status_t initialize(); const std::string& getType() const; status_t addDevice(const std::string& name, hardware::camera::common::V1_0::CameraDeviceStatus initialStatus = hardware::camera::common::V1_0::CameraDeviceStatus::PRESENT, /*out*/ std::string *parsedId = nullptr); // ICameraProviderCallbacks interface - these lock the parent mInterfaceMutex virtual hardware::Return<void> cameraDeviceStatusChange( const hardware::hidl_string& cameraDeviceName, hardware::camera::common::V1_0::CameraDeviceStatus newStatus) override; virtual hardware::Return<void> torchModeStatusChange( const hardware::hidl_string& cameraDeviceName, hardware::camera::common::V1_0::TorchModeStatus newStatus) override; // hidl_death_recipient interface - this locks the parent mInterfaceMutex virtual void serviceDied(uint64_t cookie, const wp<hidl::base::V1_0::IBase>& who) override; // Basic device information, common to all camera devices struct DeviceInfo { //...... }; std::vector<std::unique_ptr<DeviceInfo>> mDevices; std::unordered_set<std::string> mUniqueCameraIds; int mUniqueDeviceCount; // HALv1-specific camera fields, including the actual device interface struct DeviceInfo1 : public DeviceInfo { //...... }; // HALv3-specific camera fields, including the actual device interface struct DeviceInfo3 : public DeviceInfo { //...... }; private: void removeDevice(std::string id); };

mProviders是如何添加的？

addDevice是如何工作的？

mProviders添加的流程：
1.CameraService --> onFirstRef()
2.CameraService --> enumerateProviders()
3.CameraProviderManager --> initialize(this)
initialize(...)函数原型是：

status_t initialize(wp<StatusListener> listener, ServiceInteractionProxy *proxy = &sHardwareServiceInteractionProxy);

第2个参数默认是sHardwareServiceInteractionProxy类型，

    struct ServiceInteractionProxy { virtual bool registerForNotifications( const std::string &serviceName, const sp<hidl::manager::V1_0::IServiceNotification> &notification) = 0; virtual sp<hardware::camera::provider::V2_4::ICameraProvider> getService( const std::string &serviceName) = 0; virtual ~ServiceInteractionProxy() {} }; // Standard use case - call into the normal generated static methods which invoke // the real hardware service manager struct HardwareServiceInteractionProxy : public ServiceInteractionProxy { virtual bool registerForNotifications( const std::string &serviceName, const sp<hidl::manager::V1_0::IServiceNotification> &notification) override { return hardware::camera::provider::V2_4::ICameraProvider::registerForNotifications( serviceName, notification); } virtual sp<hardware::camera::provider::V2_4::ICameraProvider> getService( const std::string &serviceName) override { return hardware::camera::provider::V2_4::ICameraProvider::getService(serviceName); } };

hardware::camera::provider::V2_4::ICameraProvider::getService(serviceName)出处在./hardware/interfaces/camera/provider/2.4/default/CameraProvider.cpp，传入的参数可能是下面两种的一个：
const std::string kLegacyProviderName("legacy/0"); 代表 HALv1
const std::string kExternalProviderName("external/0"); 代码HALv3-specific

ICameraProvider* HIDL_FETCH_ICameraProvider(const char* name) { if (strcmp(name, kLegacyProviderName) == 0) { CameraProvider* provider = new CameraProvider(); if (provider == nullptr) { ALOGE("%s: cannot allocate camera provider!", __FUNCTION__); return nullptr; } if (provider->isInitFailed()) { ALOGE("%s: camera provider init failed!", __FUNCTION__); delete provider; return nullptr; } return provider; } else if (strcmp(name, kExternalProviderName) == 0) { ExternalCameraProvider* provider = new ExternalCameraProvider(); return provider; } ALOGE("%s: unknown instance name: %s", __FUNCTION__, name); return nullptr; }

addDevice是如何工作的？
1.CameraProviderManager::ProviderInfo::initialize()初始化的时候是检查当前的camera device，检查的执行函数是：

    std::vector<std::string> devices; hardware::Return<void> ret = mInterface->getCameraIdList([&status, &devices]( Status idStatus, const hardware::hidl_vec<hardware::hidl_string>& cameraDeviceNames) { status = idStatus; if (status == Status::OK) { for (size_t i = 0; i < cameraDeviceNames.size(); i++) { devices.push_back(cameraDeviceNames[i]); } } });

最终调用到./hardware/interfaces/camera/provider/2.4/default/CameraProvider.cpp中的getCameraIdList函数：CAMERA_DEVICE_STATUS_PRESENT表明当前的camera是可用的，mCameraStatusMap存储了所有的camera 设备列表。

Return<void> CameraProvider::getCameraIdList(getCameraIdList_cb _hidl_cb) { std::vector<hidl_string> deviceNameList; for (auto const& deviceNamePair : mCameraDeviceNames) { if (mCameraStatusMap[deviceNamePair.first] == CAMERA_DEVICE_STATUS_PRESENT) { deviceNameList.push_back(deviceNamePair.second); } } hidl_vec<hidl_string> hidlDeviceNameList(deviceNameList); _hidl_cb(Status::OK, hidlDeviceNameList); return Void(); }

我们理一下整体的调用结构：

Camera分层体系.jpg

1.上面谈的camera2 api就是在framework层的，在应用程序进程中。
2.CameraService，是camera2 api binder IPC通信方式调用到服务端的，camera相关的操作都在在服务端进行。所在的位置就是./frameworks/av/services/camera/下面
3.服务端也只是一个桥梁，service也会调用到HAL，硬件抽象层，具体位置在./hardware/interfaces/camera/provider/2.4
4.camera driver，底层的驱动层了，这是真正操作硬件的地方。

2.3 利用获取相机的设备信息创建CameraDeviceImpl实例

android.hardware.camera2.impl.CameraDeviceImpl deviceImpl =
                    new android.hardware.camera2.impl.CameraDeviceImpl( cameraId, callback, executor, characteristics, mContext.getApplicationInfo().targetSdkVersion);

创建CameraDevice实例，传入了刚刚获取的characteristics参数（camera设备信息赋值为CameraDevice实例）。这个实例接下来还是使用，使用的时候再谈一下。

2.4 调用远程CameraService获取当前相机的远程服务

                    // Use cameraservice's cameradeviceclient implementation for HAL3.2+ devices ICameraService cameraService = CameraManagerGlobal.get().getCameraService(); if (cameraService == null) { throw new ServiceSpecificException( ICameraService.ERROR_DISCONNECTED, "Camera service is currently unavailable"); } cameraUser = cameraService.connectDevice(callbacks, cameraId, mContext.getOpPackageName(), uid);

这个函数的主要目的就是连接当前的cameraDevice设备。调用到CameraService::connectDevice中。

connectDevice调用流程.jpg

Status CameraService::connectDevice(
        const sp<hardware::camera2::ICameraDeviceCallbacks>& cameraCb, const String16& cameraId, const String16& clientPackageName, int clientUid, /*out*/ sp<hardware::camera2::ICameraDeviceUser>* device) { ATRACE_CALL(); Status ret = Status::ok(); String8 id = String8(cameraId); sp<CameraDeviceClient> client = nullptr; ret = connectHelper<hardware::camera2::ICameraDeviceCallbacks,CameraDeviceClient>(cameraCb, id, /*api1CameraId*/-1, CAMERA_HAL_API_VERSION_UNSPECIFIED, clientPackageName, clientUid, USE_CALLING_PID, API_2, /*legacyMode*/ false, /*shimUpdateOnly*/ false, /*out*/client); if(!ret.isOk()) { logRejected(id, getCallingPid(), String8(clientPackageName), ret.toString8()); return ret; } *device = client; return ret; }

connectDevice函数的第5个参数就是当前binder ipc的返回值，我们connectDevice之后，会得到一个cameraDeviceClient对象，这个对象会返回到应用程序进程中。我们接下来主要看看这个对象是如何生成的。

validateConnectLocked：检查当前的camera device是否可用，这儿的判断比较简单，只是简单判断当前设备是否存在。

handleEvictionsLocked：处理camera独占情况，主要的工作是当前的cameradevice如果已经被其他的设备使用了，或者是否有比当前调用优先级更高的调用等等，在执行完这个函数之后，才能完全判断当前的camera device是可用的，并且开始获取camera device的一些信息开始工作了。

CameraFlashlight-->prepareDeviceOpen：此时准备连接camera device 了，需要判断一下如果当前的camera device有可用的flashlight，那就要开始准备好了，但是flashlight被占用的那就没有办法了。只是一个通知作用。

getDeviceVersion：判断一下当前的camera device的version 版本，主要判断在CameraProviderManager::getHighestSupportedVersion函数中，这个函数中将camera device支持的最高和最低版本查清楚，然后我们判断当前的camera facing，只有两种情况CAMERA_FACING_BACK = 0与CAMERA_FACING_FRONT = 1，这些都是先置判断条件，只有这些检查都通过，说明当前camera device是确实可用的。

makeClient：是根据当前的CAMERA_DEVICE_API_VERSION来判断的，当前最新的HAL架构都是基于HALv3的，所以我们采用的client都是CameraDeviceClient

Status CameraService::makeClient(const sp<CameraService>& cameraService, const sp<IInterface>& cameraCb, const String16& packageName, const String8& cameraId, int api1CameraId, int facing, int clientPid, uid_t clientUid, int servicePid, bool legacyMode, int halVersion, int deviceVersion, apiLevel effectiveApiLevel, /*out*/sp<BasicClient>* client) { if (halVersion < 0 || halVersion == deviceVersion) { // Default path: HAL version is unspecified by caller, create CameraClient // based on device version reported by the HAL. switch(deviceVersion) { case CAMERA_DEVICE_API_VERSION_1_0: if (effectiveApiLevel == API_1) { // Camera1 API route sp<ICameraClient> tmp = static_cast<ICameraClient*>(cameraCb.get()); *client = new CameraClient(cameraService, tmp, packageName, api1CameraId, facing, clientPid, clientUid, getpid(), legacyMode); } else { // Camera2 API route ALOGW("Camera using old HAL version: %d", deviceVersion); return STATUS_ERROR_FMT(ERROR_DEPRECATED_HAL, "Camera device \"%s\" HAL version %d does not support camera2 API", cameraId.string(), deviceVersion); } break; case CAMERA_DEVICE_API_VERSION_3_0: case CAMERA_DEVICE_API_VERSION_3_1: case CAMERA_DEVICE_API_VERSION_3_2: case CAMERA_DEVICE_API_VERSION_3_3: case CAMERA_DEVICE_API_VERSION_3_4: if (effectiveApiLevel == API_1) { // Camera1 API route sp<ICameraClient> tmp = static_cast<ICameraClient*>(cameraCb.get()); *client = new Camera2Client(cameraService, tmp, packageName, cameraId, api1CameraId, facing, clientPid, clientUid, servicePid, legacyMode); } else { // Camera2 API route sp<hardware::camera2::ICameraDeviceCallbacks> tmp = static_cast<hardware::camera2::ICameraDeviceCallbacks*>(cameraCb.get()); *client = new CameraDeviceClient(cameraService, tmp, packageName, cameraId, facing, clientPid, clientUid, servicePid); } break; default: // Should not be reachable ALOGE("Unknown camera device HAL version: %d", deviceVersion); return STATUS_ERROR_FMT(ERROR_INVALID_OPERATION, "Camera device \"%s\" has unknown HAL version %d", cameraId.string(), deviceVersion); } } else { // A particular HAL version is requested by caller. Create CameraClient // based on the requested HAL version. if (deviceVersion > CAMERA_DEVICE_API_VERSION_1_0 && halVersion == CAMERA_DEVICE_API_VERSION_1_0) { // Only support higher HAL version device opened as HAL1.0 device. sp<ICameraClient> tmp = static_cast<ICameraClient*>(cameraCb.get()); *client = new CameraClient(cameraService, tmp, packageName, api1CameraId, facing, clientPid, clientUid, servicePid, legacyMode); } else { // Other combinations (e.g. HAL3.x open as HAL2.x) are not supported yet. ALOGE("Invalid camera HAL version %x: HAL %x device can only be" " opened as HAL %x device", halVersion, deviceVersion, CAMERA_DEVICE_API_VERSION_1_0); return STATUS_ERROR_FMT(ERROR_ILLEGAL_ARGUMENT, "Camera device \"%s\" (HAL version %d) cannot be opened as HAL version %d", cameraId.string(), deviceVersion, halVersion); } } return Status::ok(); }

CameraClient.jpg

CameraClient与Camera2Client是之前系统版本使用的camera client对象，现在都使用CameraDeviceClient了
BnCamera --> ./frameworks/av/camera/include/camera/android/hardware/ICamera.h
ICamera --> ./frameworks/av/camera/include/camera/android/hardware/ICamera.h
BnCameraDeviceUser --> android/hardware/camera2/BnCameraDeviceUser.h 这是ICameraDeviceUser.aidl自动生成的binder 对象。所以最终得到的client对象就是ICameraDeviceUser.Stub对象。

2.5 将获取的远程服务设置到CameraDeviceImpl实例中

deviceImpl.setRemoteDevice(cameraUser);
device = deviceImpl;

这个cameraUser就是cameraservice端设置的ICameraDeviceUser.Stub对象，

    public void setRemoteDevice(ICameraDeviceUser remoteDevice) throws CameraAccessException { synchronized(mInterfaceLock) { // TODO: Move from decorator to direct binder-mediated exceptions // If setRemoteFailure already called, do nothing if (mInError) return; mRemoteDevice = new ICameraDeviceUserWrapper(remoteDevice); IBinder remoteDeviceBinder = remoteDevice.asBinder(); // For legacy camera device, remoteDevice is in the same process, and // asBinder returns NULL. if (remoteDeviceBinder != null) { try { remoteDeviceBinder.linkToDeath(this, /*flag*/ 0); } catch (RemoteException e) { CameraDeviceImpl.this.mDeviceExecutor.execute(mCallOnDisconnected); throw new CameraAccessException(CameraAccessException.CAMERA_DISCONNECTED, "The camera device has encountered a serious error"); } } mDeviceExecutor.execute(mCallOnOpened); mDeviceExecutor.execute(mCallOnUnconfigured); } }

这个mRemoteDevice是应用程序进程和android camera service端之间链接的桥梁，上层操作camera的方法会通过调用mRemoteDevice来调用到camera service端来实现操作底层camera驱动的目的。

小结

本文通过我们熟知的openCamera函数讲起，openCamera串起应用程序和cameraService之间的联系，通过研究cameraservice代码，我们知道了底层是如何通过HAL调用camera驱动设备的。下面会逐渐深入讲解camera底层知识，不足之处，敬请谅解。