前言

随着Android系统的不断升级，相机子系统框架也在不断进化，由最初的API1和HAL1到现在的API2和HAL3，由最初简单的拍照，录制到现在的连拍，AI人脸识别；可以说是架构上变动最大最频繁的子系统。

如果您希望通过相机 HAL1 支持性能略低的前置摄像头，并通过相机 HAL3 支持更为高级的后置摄像头,Android 相机服务还支持同时实现两种 HAL1 和 HAL3，
Android 的相机硬件抽象层 (HAL) 可将 Camera Api v2 中较高级别的相机框架 API 连接到底层的相机驱动程序和硬件,相机 HAL 则可提供用于实现您的这些组件版本的接口。

在前面的文章中描述 Android 系统 android.hardware.Camera (api1) 是如何通过JNI接口调用 libcamera_client.so库、管理系统摄像头; 本篇从 CameraManager 看如何使用 android.hardware.Camera2 接口框架来使用相机,Android 应用中涉及到相机类如下:

Android Camera-Camera2

CameraManager 相机系统服务，用于管理和连接相机设备

CameraDevice 相机设备类，和Camera1中的Camera同级

CameraCharacteristics 主要用于获取相机信息，内部携带大量的相机信息，包含摄像头的正反(LENS_FACING)、AE模式、AF模式等，
和Camera1中的Camera.Parameters类似

CaptureRequest 相机捕获图像的设置请求，包含传感器，镜头，闪光灯等

CaptureRequest.Builder CaptureRequest的构造器，使用Builder模式，设置更加方便

CameraCaptureSession 请求抓取相机图像帧的会话，会话的建立主要会建立起一个通道。一个CameraDevice一次只能开启一个CameraCaptureSession。
源端是相机，另一端是 Target，Target可以是Preview，也可以是ImageReader。

ImageReader 用于从相机打开的通道中读取需要的格式的原始图像数据，可以设置多个ImageReader。

用户App 使用 api V2 接口、打开摄像头

用户通过 CameraManager类 openCamera 方法打开摄像头、代码如下:

    public static final String CAMERA_SERVICE = "camera";

    private void openCamera(int width, int height) {
        if (ContextCompat.checkSelfPermission(getActivity(), Manifest.permission.CAMERA)
                != PackageManager.PERMISSION_GRANTED) {
            requestCameraPermission();
            return;
        }
        setUpCameraOutputs(width, height);
        configureTransform(width, height);
        Activity activity = getActivity();

        CameraManager manager = (CameraManager) activity.getSystemService(Context.CAMERA_SERVICE);
        try {
            if (!mCameraOpenCloseLock.tryAcquire(2500, TimeUnit.MILLISECONDS)) {
                throw new RuntimeException("Time out waiting to lock camera opening.");
            }
            manager.openCamera(mCameraId, mStateCallback, mBackgroundHandler);
        } catch (CameraAccessException e) {
            e.printStackTrace();
        } catch (InterruptedException e) {
            throw new RuntimeException("Interrupted while trying to lock camera opening.", e);
        }
    }
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源码 manager.openCamera(mCameraId, mStateCallback, mBackgroundHandler); 调用 CameraManager 类的 openCamera()方法内容如下:
@frameworks/base/core/java/android/hardware/camera2/CameraManager.java

@SystemService(Context.CAMERA_SERVICE)
public final class CameraManager {
    @SuppressWarnings("unused")
    private static final int API_VERSION_1 = 1;
    private static final int API_VERSION_2 = 2;

    private static final int CAMERA_TYPE_BACKWARD_COMPATIBLE = 0;
    private static final int CAMERA_TYPE_ALL = 1;

    private ArrayList<String> mDeviceIdList;

    private final Context mContext;
    private final Object mLock = new Object();

    public CameraManager(Context context) {
        synchronized(mLock) {
            mContext = context;
        }
    }

    //> 1. openCamera 方法
    @RequiresPermission(android.Manifest.permission.CAMERA)
    public void openCamera(@NonNull String cameraId,
            @NonNull final CameraDevice.StateCallback callback, @Nullable Handler handler)
            throws CameraAccessException {
        openCameraForUid(cameraId, callback, handler, USE_CALLING_UID);
    }

    //> 2 进一步调用 openCameraForUid 函数
    public void openCameraForUid(@NonNull String cameraId,
            @NonNull final CameraDevice.StateCallback callback, @Nullable Handler handler,
            int clientUid)
            throws CameraAccessException {

        if (cameraId == null) {
            throw new IllegalArgumentException("cameraId was null");
        } else if (callback == null) {
            throw new IllegalArgumentException("callback was null");
        } else if (handler == null) {
            if (Looper.myLooper() != null) {
                handler = new Handler();
            } else {
                throw new IllegalArgumentException(
                        "Handler argument is null, but no looper exists in the calling thread");
            }
        }
        if (CameraManagerGlobal.sCameraServiceDisabled) {
            throw new IllegalArgumentException("No cameras available on device");
        }

        openCameraDeviceUserAsync(cameraId, callback, handler, clientUid);
    }

    //> 3 最后调用到 openCameraDeviceUserAsync() 函数、返回 CameraDevice 抽象接口类对象
    private CameraDevice openCameraDeviceUserAsync(String cameraId,
            CameraDevice.StateCallback callback, Handler handler, final int uid)
            throws CameraAccessException {
        CameraCharacteristics characteristics = getCameraCharacteristics(cameraId);
        CameraDevice device = null;

        synchronized (mLock) {

            ICameraDeviceUser cameraUser = null;
            //> 通过 CameraDeviceImpl 获取 ICameraDeviceCallbacks 回调服务
            android.hardware.camera2.impl.CameraDeviceImpl deviceImpl =
                    new android.hardware.camera2.impl.CameraDeviceImpl(
                        cameraId,
                        callback,
                        handler,
                        characteristics,
                        mContext.getApplicationInfo().targetSdkVersion);

            ICameraDeviceCallbacks callbacks = deviceImpl.getCallbacks();
            
            try {
                //> 检查是否服务是否支持 Api V2 
                if (supportsCamera2ApiLocked(cameraId)) {
                    //> 获取 "media.camera" 的 ICameraService 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");
                    }
                    //> 4. 通过 cameraService服务 连接到摄像头设备
                    cameraUser = cameraService.connectDevice(callbacks, cameraId,
                            mContext.getOpPackageName(), uid);
                } else {
                    //> 使用 api v1 版本接口
                    // Use legacy camera implementation for HAL1 devices
                    int id;
                    try {
                        id = Integer.parseInt(cameraId);
                    } catch (NumberFormatException e) {
                        throw new IllegalArgumentException("Expected cameraId to be numeric, but it was: "
                                + cameraId);
                    }

                    //> 使用 V1 API 连接到摄像头设备
                    Log.i(TAG, "Using legacy camera HAL.");
                    cameraUser = CameraDeviceUserShim.connectBinderShim(callbacks, id);
                }

            } catch (ServiceSpecificException e) {
                if (e.errorCode == ICameraService.ERROR_DEPRECATED_HAL) {
                    throw new AssertionError("Should've gone down the shim path");
                } else if (e.errorCode == ICameraService.ERROR_CAMERA_IN_USE ||
                        e.errorCode == ICameraService.ERROR_MAX_CAMERAS_IN_USE ||
                        e.errorCode == ICameraService.ERROR_DISABLED ||
                        e.errorCode == ICameraService.ERROR_DISCONNECTED ||
                        e.errorCode == ICameraService.ERROR_INVALID_OPERATION) {
                    // Received one of the known connection errors
                    // The remote camera device cannot be connected to, so
                    // set the local camera to the startup error state
                    deviceImpl.setRemoteFailure(e);

                    if (e.errorCode == ICameraService.ERROR_DISABLED ||
                            e.errorCode == ICameraService.ERROR_DISCONNECTED ||
                            e.errorCode == ICameraService.ERROR_CAMERA_IN_USE) {
                        // Per API docs, these failures call onError and throw
                        throwAsPublicException(e);
                    }
                } else {
                    // Unexpected failure - rethrow
                    throwAsPublicException(e);
                }
            } catch (RemoteException e) {
                // Camera service died - act as if it's a CAMERA_DISCONNECTED case
                ServiceSpecificException sse = new ServiceSpecificException(
                    ICameraService.ERROR_DISCONNECTED,
                    "Camera service is currently unavailable");
                deviceImpl.setRemoteFailure(sse);
                throwAsPublicException(sse);
            }

            // TODO: factor out callback to be non-nested, then move setter to constructor
            // For now, calling setRemoteDevice will fire initial
            // onOpened/onUnconfigured callbacks.
            // This function call may post onDisconnected and throw CAMERA_DISCONNECTED if
            // cameraUser dies during setup.
            deviceImpl.setRemoteDevice(cameraUser);
            device = deviceImpl;
        }

        return device;
    }
}
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源码中关键部分笔者标注了注释,我们看到 supportsCamera2ApiLocked(cameraId) 检查该摄像头的api版本,此处是兼容性设计、我们只关注 API V2版本的过程,
ICameraService cameraService = CameraManagerGlobal.get().getCameraService(); 获取 ICameraService 服务和 ICameraDeviceUser cameraUser = cameraService.connectDevice(callbacks, cameraId, mContext.getOpPackageName(), uid); 过程,成功连接摄像头后，也即是 CameraManager::openCamera() 方法成功打开摄像头过程,用户 App 就可以实现拍照、连续拍照或录像的操作了。

先看如何获取 ICameraService 服务，CameraManagerGlobal 类是 CameraManager 的子类、内容如下:

/**
     * A per-process global camera manager instance, to retain a connection to the camera service,
     * and to distribute camera availability notices to API-registered callbacks
     */
    private static final class CameraManagerGlobal extends ICameraServiceListener.Stub
            implements IBinder.DeathRecipient {

        private static final String TAG = "CameraManagerGlobal";
        private final boolean DEBUG = false;

        private final int CAMERA_SERVICE_RECONNECT_DELAY_MS = 1000;

        // Singleton instance
        private static final CameraManagerGlobal gCameraManager =
            new CameraManagerGlobal();

        /**
         * This must match the ICameraService definition 服务名称 "media.camera"
         */
        private static final String CAMERA_SERVICE_BINDER_NAME = "media.camera";

        // Camera ID -> Status map
        private final ArrayMap<String, Integer> mDeviceStatus = new ArrayMap<String, Integer>();

        // Registered availablility callbacks and their handlers
        private final ArrayMap<AvailabilityCallback, Handler> mCallbackMap =
            new ArrayMap<AvailabilityCallback, Handler>();

        // torch client binder to set the torch mode with.
        private Binder mTorchClientBinder = new Binder();

        // Camera ID -> Torch status map
        private final ArrayMap<String, Integer> mTorchStatus = new ArrayMap<String, Integer>();

        // Registered torch callbacks and their handlers
        private final ArrayMap<TorchCallback, Handler> mTorchCallbackMap =
                new ArrayMap<TorchCallback, Handler>();

        private final Object mLock = new Object();

        // Access only through getCameraService to deal with binder death
        private ICameraService mCameraService;

        // Singleton, don't allow construction
        private CameraManagerGlobal() {
        }

        public static final boolean sCameraServiceDisabled =
                SystemProperties.getBoolean("config.disable_cameraservice", false);

        //> get 静态方法内容
        public static CameraManagerGlobal get() {
            return gCameraManager;
        }

        @Override
        public IBinder asBinder() {
            return this;
        }

        /**
         * Return a best-effort ICameraService.
         *
         * 
This will be null if the camera service is not currently available. If the camera
         * service has died since the last use of the camera service, will try to reconnect to the
         * service.
         */
        public ICameraService getCameraService() {
            synchronized(mLock) {
                connectCameraServiceLocked();
                if (mCameraService == null && !sCameraServiceDisabled) {
                    Log.e(TAG, "Camera service is unavailable");
                }
                return mCameraService;
            }
        }

        /**
         * Connect to the camera service if it's available, and set up listeners.
         * If the service is already connected, do nothing.
         *
         * 
Sets mCameraService to a valid pointer or null if the connection does not succeed.
         */
        private void connectCameraServiceLocked() {
            // Only reconnect if necessary
            if (mCameraService != null || sCameraServiceDisabled) return;

            Log.i(TAG, "Connecting to camera service");
            //> 获取 "media.camera" 名称的服务
            IBinder cameraServiceBinder = ServiceManager.getService(CAMERA_SERVICE_BINDER_NAME);
            if (cameraServiceBinder == null) {
                // Camera service is now down, leave mCameraService as null
                return;
            }
            try {
                cameraServiceBinder.linkToDeath(this, /*flags*/ 0);
            } catch (RemoteException e) {
                // Camera service is now down, leave mCameraService as null
                return;
            }

            //> 通过 ICameraService.Stub.asInterface() 把 cameraServiceBinder 转化为 ICameraService 代理
            ICameraService cameraService = ICameraService.Stub.asInterface(cameraServiceBinder);

            try {
                CameraMetadataNative.setupGlobalVendorTagDescriptor();
            } catch (ServiceSpecificException e) {
                handleRecoverableSetupErrors(e);
            }

            try {
                CameraStatus[] cameraStatuses = cameraService.addListener(this);
                for (CameraStatus c : cameraStatuses) {
                    onStatusChangedLocked(c.status, c.cameraId);
                }
                //> 把 cameraService 保存起来
                mCameraService = cameraService;
            } catch(ServiceSpecificException e) {
                // Unexpected failure
                throw new IllegalStateException("Failed to register a camera service listener", e);
            } catch (RemoteException e) {
                // Camera service is now down, leave mCameraService as null
            }
        }
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源码 ICameraService.Stub.asInterface() 做了什么转化,代码如下:
源码路径:@out/target/common/obj/JAVA_LIBRARIES/framework_intermediates/dotdot/av/camera/aidl/android/hardware/ICameraService.java

文件 ICameraService.java 是由 @frameworks/av/camera/aidl/android/hardware/ICameraService.aidl 文件内容编译系统自动生成的内容、如下:

public interface ICameraService extends android.os.IInterface
{
/** Local-side IPC implementation stub class. */
public static abstract class Stub extends android.os.Binder implements android.hardware.ICameraService
{
    private static final java.lang.String DESCRIPTOR = "android.hardware.ICameraService";
    /** Construct the stub at attach it to the interface. */
    public Stub()
    {
        this.attachInterface(this, DESCRIPTOR);
    }

    private static class Proxy implements android.hardware.ICameraService
    {
        private android.os.IBinder mRemote;
        Proxy(android.os.IBinder remote)
        {
            mRemote = remote;
        }
        @Override public android.os.IBinder asBinder()
        {
            return mRemote;
        }
    }
    /**
     * Cast an IBinder object into an android.hardware.ICameraService interface,
     * generating a proxy if needed.
     */
    public static android.hardware.ICameraService asInterface(android.os.IBinder obj)
    {
        if ((obj==null)) {
            return null;
        }
        android.os.IInterface iin = obj.queryLocalInterface(DESCRIPTOR);
        if (((iin!=null)&&(iin instanceof android.hardware.ICameraService))) {
            return ((android.hardware.ICameraService)iin);
        }
        return new android.hardware.ICameraService.Stub.Proxy(obj);
}
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通过 Stub.asInterface() 就生成 ICameraService 代理服务,
接下来 ICameraDeviceUser cameraUser = cameraService.connectDevice(callbacks, cameraId, mContext.getOpPackageName(), uid);把 android.os.Parcel 内容给了谁呢?
连接谁呢? 内容如下:

/**
     * Open a camera device through the new camera API
     * Only supported for device HAL versions >= 3.2
     */
@Override public android.hardware.camera2.ICameraDeviceUser connectDevice(android.hardware.camera2.ICameraDeviceCallbacks callbacks, java.lang.String cameraId, java.lang.String opPackageName, int clientUid) throws android.os.RemoteException
{
    android.os.Parcel _data = android.os.Parcel.obtain();
    android.os.Parcel _reply = android.os.Parcel.obtain();
    android.hardware.camera2.ICameraDeviceUser _result;
    try {
        _data.writeInterfaceToken(DESCRIPTOR);
        _data.writeStrongBinder((((callbacks!=null))?(callbacks.asBinder()):(null)));
        _data.writeString(cameraId);
        _data.writeString(opPackageName);
        _data.writeInt(clientUid);
        mRemote.transact(Stub.TRANSACTION_connectDevice, _data, _reply, 0);
        _reply.readException();
        _result = android.hardware.camera2.ICameraDeviceUser.Stub.asInterface(_reply.readStrongBinder());
    }
    finally {
        _reply.recycle();
        _data.recycle();
    }
    return _result;
}
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网上很多关于 Camera 框架的描述博客、都没有描述这个过程，就直接跳转到 CameraService 侧的 CameraService::connectDevice() 代码上,
分析 CameraService::connectDevice 是如何创建 CameraClient 的过程。

我们一起梳理这块内容:

分析如下:

 //> 获取 "media.camera" 名称的服务, cameraServiceBinder 对象持有 BnInterface
 IBinder cameraServiceBinder = ServiceManager.getService(CAMERA_SERVICE_BINDER_NAME);

 //> 通过 ICameraService.Stub.asInterface() 把 cameraServiceBinder 转化为 ICameraService 代理
 ICameraService cameraService = ICameraService.Stub.asInterface(cameraServiceBinder);
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代理就是 ServiceManager.getService(CAMERA_SERVICE_BINDER_NAME) 获取到的服务,就是 “media.camera” 服务;ICameraServer 所持有的 mRemote 是 ICameraService 对象，通过 Stub.TRANSACTION_connectDevice 命令、使用transact方法发送给
CameraService 的服务端、服务端解码后、调用 CameraService::connectDevice() 代码来处理;代码如下:
代码路径:@@out/soong/.intermediates/frameworks/av/camera/libcamera_client/android_arm64_armv8-a_cortex-a53_shared_core/
gen/aidl/android/hardware/ICameraService.cpp

::android::status_t BnCameraService::onTransact(uint32_t _aidl_code, const ::android::Parcel& _aidl_data, ::android::Parcel* _aidl_reply, uint32_t _aidl_flags) 
{
    ::android::status_t _aidl_ret_status = ::android::OK;
    switch (_aidl_code) {
        case Call::CONNECTDEVICE:
        {
            ::android::sp<::android::hardware::camera2::ICameraDeviceCallbacks> in_callbacks;
            ::android::String16 in_cameraId;
            ::android::String16 in_opPackageName;
            int32_t in_clientUid;
            ::android::sp<::android::hardware::camera2::ICameraDeviceUser> _aidl_return;
            if (!(_aidl_data.checkInterface(this))) {
            _aidl_ret_status = ::android::BAD_TYPE;
            break;
            }
            _aidl_ret_status = _aidl_data.readStrongBinder(&in_callbacks);
            if (((_aidl_ret_status) != (::android::OK))) {
            break;
            }
            _aidl_ret_status = _aidl_data.readString16(&in_cameraId);
            if (((_aidl_ret_status) != (::android::OK))) {
            break;
            }
            _aidl_ret_status = _aidl_data.readString16(&in_opPackageName);
            if (((_aidl_ret_status) != (::android::OK))) {
            break;
            }
            _aidl_ret_status = _aidl_data.readInt32(&in_clientUid);
            if (((_aidl_ret_status) != (::android::OK))) {
            break;
            }
            //> 此处调用 connectDevice(in_callbacks, in_cameraId, in_opPackageName, in_clientUid, &_aidl_return)
            ::android::binder::Status _aidl_status(connectDevice(in_callbacks, in_cameraId, in_opPackageName, in_clientUid, &_aidl_return));
            _aidl_ret_status = _aidl_status.writeToParcel(_aidl_reply);
            if (((_aidl_ret_status) != (::android::OK))) {
            break;
            }
            if (!_aidl_status.isOk()) {
            break;
            }
            _aidl_ret_status = _aidl_reply->writeStrongBinder(::android::hardware::camera2::ICameraDeviceUser::asBinder(_aidl_return));
            if (((_aidl_ret_status) != (::android::OK))) {
            break;
            }
        }
        break;
        case Call::CONNECTLEGACY:
        {
            ::android::sp<::android::hardware::ICameraClient> in_client;
            int32_t in_cameraId;
            int32_t in_halVersion;
            ::android::String16 in_opPackageName;
            int32_t in_clientUid;
            ::android::sp<::android::hardware::ICamera> _aidl_return;
            if (!(_aidl_data.checkInterface(this))) {
            _aidl_ret_status = ::android::BAD_TYPE;
            break;
            }
            _aidl_ret_status = _aidl_data.readStrongBinder(&in_client);
            if (((_aidl_ret_status) != (::android::OK))) {
            break;
            }
            _aidl_ret_status = _aidl_data.readInt32(&in_cameraId);
            if (((_aidl_ret_status) != (::android::OK))) {
            break;
            }
            _aidl_ret_status = _aidl_data.readInt32(&in_halVersion);
            if (((_aidl_ret_status) != (::android::OK))) {
            break;
            }
            _aidl_ret_status = _aidl_data.readString16(&in_opPackageName);
            if (((_aidl_ret_status) != (::android::OK))) {
            break;
            }
            _aidl_ret_status = _aidl_data.readInt32(&in_clientUid);
            if (((_aidl_ret_status) != (::android::OK))) {
            break;
            }
            ::android::binder::Status _aidl_status(connectLegacy(in_client, in_cameraId, in_halVersion, in_opPackageName, in_clientUid, &_aidl_return));
            _aidl_ret_status = _aidl_status.writeToParcel(_aidl_reply);
            if (((_aidl_ret_status) != (::android::OK))) {
            break;
            }
            if (!_aidl_status.isOk()) {
            break;
            }
            _aidl_ret_status = _aidl_reply->writeStrongBinder(::android::hardware::ICamera::asBinder(_aidl_return));
            if (((_aidl_ret_status) != (::android::OK))) {
            break;
            }
        }
        break;
        default:
            {
                _aidl_ret_status = ::android::BBinder::onTransact(_aidl_code, _aidl_data, _aidl_reply, _aidl_flags);
            }
        break;
    }
    if (_aidl_ret_status == ::android::UNEXPECTED_NULL) {
        _aidl_ret_status = ::android::binder::Status::fromExceptionCode(::android::binder::Status::EX_NULL_POINTER).writeToParcel(_aidl_reply);
    }
    return _aidl_ret_status;
}
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那接下来看看 CameraService 服务端的 connectDevice() 代码,内容如下：
源码路径:@frameworks/av/services/camera/libcameraservcie/CameraService.cpp 中 connectDevice 方法内容如下:

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,
            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;
}
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该方法中调用 connectHelper() 来连接摄像头,代码如下：

template<class CALLBACK, class CLIENT>
Status CameraService::connectHelper(const sp<CALLBACK>& cameraCb, const String8& cameraId,
        int halVersion, const String16& clientPackageName, int clientUid, int clientPid,
        apiLevel effectiveApiLevel, bool legacyMode, bool shimUpdateOnly,
        /*out*/sp<CLIENT>& device) {
    binder::Status ret = binder::Status::ok();

    String8 clientName8(clientPackageName);

    int originalClientPid = 0;

    ALOGI("CameraService::connect call (PID %d \"%s\", camera ID %s) for HAL version %s and "
            "Camera API version %d", clientPid, clientName8.string(), cameraId.string(),
            (halVersion == -1) ? "default" : std::to_string(halVersion).c_str(),
            static_cast<int>(effectiveApiLevel));

    sp<CLIENT> client = nullptr;
    {
        // Acquire mServiceLock and prevent other clients from connecting
        std::unique_ptr<AutoConditionLock> lock =
                AutoConditionLock::waitAndAcquire(mServiceLockWrapper, DEFAULT_CONNECT_TIMEOUT_NS);

        if (lock == nullptr) {
            ALOGE("CameraService::connect (PID %d) rejected (too many other clients connecting)."
                    , clientPid);
            return STATUS_ERROR_FMT(ERROR_MAX_CAMERAS_IN_USE,
                    "Cannot open camera %s for \"%s\" (PID %d): Too many other clients connecting",
                    cameraId.string(), clientName8.string(), clientPid);
        }

        // Enforce client permissions and do basic sanity checks
        if(!(ret = validateConnectLocked(cameraId, clientName8,
                /*inout*/clientUid, /*inout*/clientPid, /*out*/originalClientPid)).isOk()) {
            return ret;
        }

        // Check the shim parameters after acquiring lock, if they have already been updated and
        // we were doing a shim update, return immediately
        if (shimUpdateOnly) {
            auto cameraState = getCameraState(cameraId);
            if (cameraState != nullptr) {
                if (!cameraState->getShimParams().isEmpty()) return ret;
            }
        }

        status_t err;

        sp<BasicClient> clientTmp = nullptr;
        std::shared_ptr<resource_policy::ClientDescriptor<String8, sp<BasicClient>>> partial;

        if ((err = handleEvictionsLocked(cameraId, originalClientPid, effectiveApiLevel,
                IInterface::asBinder(cameraCb), clientName8, /*out*/&clientTmp,
                /*out*/&partial)) != NO_ERROR) {
            switch (err) {
                case -ENODEV:
                    return STATUS_ERROR_FMT(ERROR_DISCONNECTED,
                            "No camera device with ID \"%s\" currently available",
                            cameraId.string());
                case -EBUSY:
                    return STATUS_ERROR_FMT(ERROR_CAMERA_IN_USE,
                            "Higher-priority client using camera, ID \"%s\" currently unavailable",
                            cameraId.string());
                default:
                    return STATUS_ERROR_FMT(ERROR_INVALID_OPERATION,
                            "Unexpected error %s (%d) opening camera \"%s\"",
                            strerror(-err), err, cameraId.string());
            }
        }

        if (clientTmp.get() != nullptr) {
            // Handle special case for API1 MediaRecorder where the existing client is returned
            device = static_cast<CLIENT*>(clientTmp.get());
            return ret;
        }

        // give flashlight a chance to close devices if necessary.
        mFlashlight->prepareDeviceOpen(cameraId);

        int facing = -1;
        int deviceVersion = getDeviceVersion(cameraId, /*out*/&facing);
        if (facing == -1) {
            ALOGE("%s: Unable to get camera device \"%s\"  facing", __FUNCTION__, cameraId.string());
            return STATUS_ERROR_FMT(ERROR_INVALID_OPERATION,
                    "Unable to get camera device \"%s\" facing", cameraId.string());
        }

        //> 1. 创建 CameraClient
        sp<BasicClient> tmp = nullptr;
        if(!(ret = makeClient(this, cameraCb, clientPackageName, cameraId, facing, clientPid,
                clientUid, getpid(), legacyMode, halVersion, deviceVersion, effectiveApiLevel,
                /*out*/&tmp)).isOk()) {
            return ret;
        }
        client = static_cast<CLIENT*>(tmp.get());

        LOG_ALWAYS_FATAL_IF(client.get() == nullptr, "%s: CameraService in invalid state",
                __FUNCTION__);

        //> 2. 初始化 client
        err = client->initialize(mCameraProviderManager);

        if (err != OK) {
            ALOGE("%s: Could not initialize client from HAL.", __FUNCTION__);
            // Errors could be from the HAL module open call or from AppOpsManager
            switch(err) {
                case BAD_VALUE:
                    return STATUS_ERROR_FMT(ERROR_ILLEGAL_ARGUMENT,
                            "Illegal argument to HAL module for camera \"%s\"", cameraId.string());
                case -EBUSY:
                    return STATUS_ERROR_FMT(ERROR_CAMERA_IN_USE,
                            "Camera \"%s\" is already open", cameraId.string());
                case -EUSERS:
                    return STATUS_ERROR_FMT(ERROR_MAX_CAMERAS_IN_USE,
                            "Too many cameras already open, cannot open camera \"%s\"",
                            cameraId.string());
                case PERMISSION_DENIED:
                    return STATUS_ERROR_FMT(ERROR_PERMISSION_DENIED,
                            "No permission to open camera \"%s\"", cameraId.string());
                case -EACCES:
                    return STATUS_ERROR_FMT(ERROR_DISABLED,
                            "Camera \"%s\" disabled by policy", cameraId.string());
                case -ENODEV:
                default:
                    return STATUS_ERROR_FMT(ERROR_INVALID_OPERATION,
                            "Failed to initialize camera \"%s\": %s (%d)", cameraId.string(),
                            strerror(-err), err);
            }
        }

        // Update shim paremeters for legacy clients
        if (effectiveApiLevel == API_1) {
            // Assume we have always received a Client subclass for API1
            sp<Client> shimClient = reinterpret_cast<Client*>(client.get());
            String8 rawParams = shimClient->getParameters();
            CameraParameters params(rawParams);

            auto cameraState = getCameraState(cameraId);
            if (cameraState != nullptr) {
                cameraState->setShimParams(params);
            } else {
                ALOGE("%s: Cannot update shim parameters for camera %s, no such device exists.",
                        __FUNCTION__, cameraId.string());
            }
        }

        if (shimUpdateOnly) {
            // If only updating legacy shim parameters, immediately disconnect client
            mServiceLock.unlock();
            client->disconnect();
            mServiceLock.lock();
        } else {
            // Otherwise, add client to active clients list
            finishConnectLocked(client, partial);
        }
    } // lock is destroyed, allow further connect calls

    // 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;
    return ret;
}
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代码调用 makeClient() 和 client->initialize() 函数，打开相机就完成了。
我们先看 makeClient() 创建不同 hal 版本的 BasicClient 的相机。

Status CameraService::makeClient(const sp<CameraService>& cameraService,
        const sp<IInterface>& cameraCb, const String16& packageName, const String8& cameraId,
        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, cameraIdToInt(cameraId),
                        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, cameraIdToInt(cameraId),
                        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, cameraIdToInt(cameraId),
                    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();
}
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代码中创建 client 有 CameraClient()、Camera2Client() 和 CameraDeviceClient() 方法,根据相机 HAL 中的 deviceVersion 描述的内容，分别对应起来;
我关注的 CameraDeviceClient() 是 API_2 的方法, 因笔者虚拟摄像头就使用api V2 .
源码路径: @frameworks/av/services/camera/libcameraservcie/api2/CameraDeviceClient.h

class CameraDeviceClient :
        public Camera2ClientBase<CameraDeviceClientBase>,
        public camera2::FrameProcessorBase::FilteredListener
{
public:
    /**
     * ICameraDeviceUser interface (see ICameraDeviceUser for details)
     */

    // Note that the callee gets a copy of the metadata.
    virtual binder::Status submitRequest(
            const hardware::camera2::CaptureRequest& request,
            bool streaming = false,
            /*out*/
            hardware::camera2::utils::SubmitInfo *submitInfo = nullptr) override;

    virtual binder::Status createStream(
            const hardware::camera2::params::OutputConfiguration &outputConfiguration,
            /*out*/
            int32_t* newStreamId = NULL) override;
    
    // Create an input stream of width, height, and format.
    virtual binder::Status createInputStream(int width, int height, int format,
            /*out*/
            int32_t* newStreamId = NULL) override;
    /**
     * Interface used by CameraService
     */

    CameraDeviceClient(const sp<CameraService>& cameraService,
            const sp<hardware::camera2::ICameraDeviceCallbacks>& remoteCallback,
            const String16& clientPackageName,
            const String8& cameraId,
            int cameraFacing,
            int clientPid,
            uid_t clientUid,
            int servicePid);
}
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源码路径: @frameworks/av/services/camera/libcameraservcie/api2/CameraDeviceClient.cpp

// Interface used by CameraService
//> 构造函数
CameraDeviceClient::CameraDeviceClient(const sp<CameraService>& cameraService,
        const sp<hardware::camera2::ICameraDeviceCallbacks>& remoteCallback,
        const String16& clientPackageName,
        const String8& cameraId,
        int cameraFacing,
        int clientPid,
        uid_t clientUid,
        int servicePid) :
    Camera2ClientBase(cameraService, remoteCallback, clientPackageName,
                cameraId, cameraFacing, clientPid, clientUid, servicePid),
    mInputStream(),
    mStreamingRequestId(REQUEST_ID_NONE),
    mRequestIdCounter(0) {

    ATRACE_CALL();
    ALOGI("CameraDeviceClient %s: Opened", cameraId.string());
}
//> 初始化函数
status_t CameraDeviceClient::initialize(sp<CameraProviderManager> manager) {
    return initializeImpl(manager);
}

template<typename TProviderPtr>
status_t CameraDeviceClient::initializeImpl(TProviderPtr providerPtr) {
    ATRACE_CALL();
    status_t res;

    res = Camera2ClientBase::initialize(providerPtr);
    if (res != OK) {
        return res;
    }

    String8 threadName;
    mFrameProcessor = new FrameProcessorBase(mDevice);
    threadName = String8::format("CDU-%s-FrameProc", mCameraIdStr.string());
    mFrameProcessor->run(threadName.string());

    mFrameProcessor->registerListener(FRAME_PROCESSOR_LISTENER_MIN_ID,
                                      FRAME_PROCESSOR_LISTENER_MAX_ID,
                                      /*listener*/this,
                                      /*sendPartials*/true);

    return OK;
}
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构造函数和初始化函数都调用 Camera2ClientBase 类的方法.源码如下:
源码路径:@frameworks/av/services/libcameraservcie/common/Camera2ClientBase.cpp

// Interface used by CameraService
//> 构造方法
template <typename TClientBase>
Camera2ClientBase<TClientBase>::Camera2ClientBase(
        const sp<CameraService>& cameraService,
        const sp<TCamCallbacks>& remoteCallback,
        const String16& clientPackageName,
        const String8& cameraId,
        int cameraFacing,
        int clientPid,
        uid_t clientUid,
        int servicePid):
        TClientBase(cameraService, remoteCallback, clientPackageName,
                cameraId, cameraFacing, clientPid, clientUid, servicePid),
        mSharedCameraCallbacks(remoteCallback),
        mDeviceVersion(cameraService->getDeviceVersion(TClientBase::mCameraIdStr)),
        mDeviceActive(false)
{
    ALOGI("Camera %s: Opened. Client: %s (PID %d, UID %d)", cameraId.string(),
            String8(clientPackageName).string(), clientPid, clientUid);

    mInitialClientPid = clientPid;
    mDevice = new Camera3Device(cameraId);
    LOG_ALWAYS_FATAL_IF(mDevice == 0, "Device should never be NULL here.");
}

//> 初始化函数
template <typename TClientBase>
template <typename TProviderPtr>
status_t Camera2ClientBase<TClientBase>::initializeImpl(TProviderPtr providerPtr) {
    ATRACE_CALL();
    ALOGV("%s: Initializing client for camera %s", __FUNCTION__,
          TClientBase::mCameraIdStr.string());
    status_t res;

    // Verify ops permissions
    res = TClientBase::startCameraOps();
    if (res != OK) {
        return res;
    }

    if (mDevice == NULL) {
        ALOGE("%s: Camera %s: No device connected",
                __FUNCTION__, TClientBase::mCameraIdStr.string());
        return NO_INIT;
    }

    res = mDevice->initialize(providerPtr);
    if (res != OK) {
        ALOGE("%s: Camera %s: unable to initialize device: %s (%d)",
                __FUNCTION__, TClientBase::mCameraIdStr.string(), strerror(-res), res);
        return res;
    }

    wp<CameraDeviceBase::NotificationListener> weakThis(this);
    res = mDevice->setNotifyCallback(weakThis);

    return OK;
}
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构造方法中 new Camera3Device(cameraId) 相机,
源码路径:@frameworks/av/services/camera/libcameraservcie/device3/Camera3Device.cpp

//> 构造函数
Camera3Device::Camera3Device(const String8 &id):
        mId(id),
        mOperatingMode(NO_MODE),
        mIsConstrainedHighSpeedConfiguration(false),
        mStatus(STATUS_UNINITIALIZED),
        mStatusWaiters(0),
        mUsePartialResult(false),
        mNumPartialResults(1),
        mTimestampOffset(0),
        mNextResultFrameNumber(0),
        mNextReprocessResultFrameNumber(0),
        mNextShutterFrameNumber(0),
        mNextReprocessShutterFrameNumber(0),
        mListener(NULL),
        mVendorTagId(CAMERA_METADATA_INVALID_VENDOR_ID)
{
    ATRACE_CALL();
    camera3_callback_ops::notify = &sNotify;
    camera3_callback_ops::process_capture_result = &sProcessCaptureResult;
    ALOGV("%s: Created device for camera %s", __FUNCTION__, mId.string());
}
//> 初始化函数
status_t Camera3Device::initialize(sp<CameraProviderManager> manager) {
    ATRACE_CALL();
    Mutex::Autolock il(mInterfaceLock);
    Mutex::Autolock l(mLock);

    ALOGV("%s: Initializing HIDL device for camera %s", __FUNCTION__, mId.string());
    if (mStatus != STATUS_UNINITIALIZED) {
        CLOGE("Already initialized!");
        return INVALID_OPERATION;
    }
    if (manager == nullptr) return INVALID_OPERATION;

    sp<ICameraDeviceSession> session;
    ATRACE_BEGIN("CameraHal::openSession");
    status_t res = manager->openSession(mId.string(), this,
            /*out*/ &session);
    ATRACE_END();
    if (res != OK) {
        SET_ERR_L("Could not open camera session: %s (%d)", strerror(-res), res);
        return res;
    }

    res = manager->getCameraCharacteristics(mId.string(), &mDeviceInfo);
    if (res != OK) {
        SET_ERR_L("Could not retrive camera characteristics: %s (%d)", strerror(-res), res);
        session->close();
        return res;
    }

    std::shared_ptr<RequestMetadataQueue> queue;
    auto requestQueueRet = session->getCaptureRequestMetadataQueue(
        [&queue](const auto& descriptor) {
            queue = std::make_shared<RequestMetadataQueue>(descriptor);
            if (!queue->isValid() || queue->availableToWrite() <= 0) {
                ALOGE("HAL returns empty request metadata fmq, not use it");
                queue = nullptr;
                // don't use the queue onwards.
            }
        });
    if (!requestQueueRet.isOk()) {
        ALOGE("Transaction error when getting request metadata fmq: %s, not use it",
                requestQueueRet.description().c_str());
        return DEAD_OBJECT;
    }

    std::unique_ptr<ResultMetadataQueue>& resQueue = mResultMetadataQueue;
    auto resultQueueRet = session->getCaptureResultMetadataQueue(
        [&resQueue](const auto& descriptor) {
            resQueue = std::make_unique<ResultMetadataQueue>(descriptor);
            if (!resQueue->isValid() || resQueue->availableToWrite() <= 0) {
                ALOGE("HAL returns empty result metadata fmq, not use it");
                resQueue = nullptr;
                // Don't use the resQueue onwards.
            }
        });
    if (!resultQueueRet.isOk()) {
        ALOGE("Transaction error when getting result metadata queue from camera session: %s",
                resultQueueRet.description().c_str());
        return DEAD_OBJECT;
    }
    IF_ALOGV() {
        session->interfaceChain([](
            ::android::hardware::hidl_vec<::android::hardware::hidl_string> interfaceChain) {
                ALOGV("Session interface chain:");
                for (auto iface : interfaceChain) {
                    ALOGV("  %s", iface.c_str());
                }
            });
    }

    mInterface = new HalInterface(session, queue);
    std::string providerType;
    mVendorTagId = manager->getProviderTagIdLocked(mId.string());

    return initializeCommonLocked();
}
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由上面的构建函数和初始化函数、我们看到 ICameraDeviceSession、mResultMetadataQueue 等新特征变量内容,在看看 initializeCommonLocked() 函数内容如下:

status_t Camera3Device::initializeCommonLocked() {

    /** Start up status tracker thread */
    mStatusTracker = new StatusTracker(this);
    status_t res = mStatusTracker->run(String8::format("C3Dev-%s-Status", mId.string()).string());
    if (res != OK) {
        SET_ERR_L("Unable to start status tracking thread: %s (%d)",
                strerror(-res), res);
        mInterface->close();
        mStatusTracker.clear();
        return res;
    }

    /** Register in-flight map to the status tracker */
    mInFlightStatusId = mStatusTracker->addComponent();

    /** Create buffer manager */
    mBufferManager = new Camera3BufferManager();

    mTagMonitor.initialize(mVendorTagId);

    /** Start up request queue thread */
    mRequestThread = new RequestThread(this, mStatusTracker, mInterface);
    res = mRequestThread->run(String8::format("C3Dev-%s-ReqQueue", mId.string()).string());
    if (res != OK) {
        SET_ERR_L("Unable to start request queue thread: %s (%d)",
                strerror(-res), res);
        mInterface->close();
        mRequestThread.clear();
        return res;
    }

    mPreparerThread = new PreparerThread();

    internalUpdateStatusLocked(STATUS_UNCONFIGURED);
    mNextStreamId = 0;
    mDummyStreamId = NO_STREAM;
    mNeedConfig = true;
    mPauseStateNotify = false;

    // Measure the clock domain offset between camera and video/hw_composer
    camera_metadata_entry timestampSource =
            mDeviceInfo.find(ANDROID_SENSOR_INFO_TIMESTAMP_SOURCE);
    if (timestampSource.count > 0 && timestampSource.data.u8[0] ==
            ANDROID_SENSOR_INFO_TIMESTAMP_SOURCE_REALTIME) {
        mTimestampOffset = getMonoToBoottimeOffset();
    }

    // Will the HAL be sending in early partial result metadata?
    camera_metadata_entry partialResultsCount =
            mDeviceInfo.find(ANDROID_REQUEST_PARTIAL_RESULT_COUNT);
    if (partialResultsCount.count > 0) {
        mNumPartialResults = partialResultsCount.data.i32[0];
        mUsePartialResult = (mNumPartialResults > 1);
    }

    camera_metadata_entry configs =
            mDeviceInfo.find(ANDROID_SCALER_AVAILABLE_STREAM_CONFIGURATIONS);
    for (uint32_t i = 0; i < configs.count; i += 4) {
        if (configs.data.i32[i] == HAL_PIXEL_FORMAT_IMPLEMENTATION_DEFINED &&
                configs.data.i32[i + 3] ==
                ANDROID_SCALER_AVAILABLE_STREAM_CONFIGURATIONS_INPUT) {
            mSupportedOpaqueInputSizes.add(Size(configs.data.i32[i + 1],
                    configs.data.i32[i + 2]));
        }
    }

    return OK;
}

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HAL 3 相机子系统将多个运行模式整合为一个统一的视图，您可以使用这种视图实现之前的任何模式以及一些其他模式，例如连拍模式。这样一来，便可以提高用户对聚焦、曝光以及更多后期处理（例如降噪、对比度和锐化）效果的控制能力。此外，这种简化的视图还能够使应用开发者更轻松地使用相机的各种功能。