(1)Framework CameraManager ：/frameworks/base/core/java/android/hardware/camera2/CameraManager.java

@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);
}

public void openCameraForUid(@NonNull String cameraId,
        @NonNull final CameraDevice.StateCallback callback, @Nullable Handler handler,
        int clientUid)
        throws CameraAccessException {
    /* Do something in*/
    ......
    /* Do something out*/
    openCameraDeviceUserAsync(cameraId, callback, handler, clientUid);
}

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;

        android.hardware.camera2.impl.CameraDeviceImpl deviceImpl =   //实例化一个 CameraDeviceImpl。构造时传入了 CameraDevice.StateCallback 以及 Handler。
                new android.hardware.camera2.impl.CameraDeviceImpl(  
                    cameraId,
                    callback,
                    handler,
                    characteristics,
                    mContext.getApplicationInfo().targetSdkVersion);

        ICameraDeviceCallbacks callbacks = deviceImpl.getCallbacks(); //获取 CameraDeviceCallback 实例，这是提供给远端连接到 CameraDeviceImpl 的接口。

       try {
            if (supportsCamera2ApiLocked(cameraId)) {  //HAL3 中走的是这一部分逻辑，主要是从 CameraManagerGlobal 中获取 CameraService 的本地接口，通过它远端调用（采用 Binder 机制） connectDevice 方法连接到相机设备。
　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　//注意返回的 cameraUser 实际上指向的是远端 CameraDeviceClient 的本地接口。
                // 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);
            } else {
                // 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);
                }

                Log.i(TAG, "Using legacy camera HAL.");
                cameraUser = CameraDeviceUserShim.connectBinderShim(callbacks, id);
            }
        } catch (ServiceSpecificException e) {
            /* Do something in */
            ......
            /* Do something out */
        }

        // 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); //将 CameraDeviceClient 设置到 CameraDeviceImpl 中进行管理。
        device = deviceImpl;
    }

    return device;
}

/**
 * Set remote device, which triggers initial onOpened/onUnconfigured callbacks
 *
 * <p>This function may post onDisconnected and throw CAMERA_DISCONNECTED if remoteDevice dies
 * during setup.</p>
 *
 */
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); //通过 ICameraDeviceUserWrapper 给远端设备实例加上一层封装。

        IBinder remoteDeviceBinder = remoteDevice.asBinder(); //使用 Binder 机制的一些基本设置。
        // For legacy camera device, remoteDevice is in the same process, and
        // asBinder returns NULL.
        if (remoteDeviceBinder != null) {
            try {
                remoteDeviceBinder.linkToDeath(this, /*flag*/ 0); //如果这个binder消失，为标志信息注册一个接收器。
            } catch (RemoteException e) {
                CameraDeviceImpl.this.mDeviceHandler.post(mCallOnDisconnected);

                throw new CameraAccessException(CameraAccessException.CAMERA_DISCONNECTED,
                        "The camera device has encountered a serious error");
            }
        }

        mDeviceHandler.post(mCallOnOpened); //需此处触发 onOpened 与 onUnconfigured 这两个回调，每个回调都是通过 mDeviceHandler 启用一个新线程来调用的。
        mDeviceHandler.post(mCallOnUnconfigured);
    }
}

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;

　　 //此处调用的 connectHelper 方法才真正实现了连接逻辑（HAL1 时最终也调用到这个方法）。需要注意的是，设定的模板类型是 ICameraDeviceCallbacks 以及 CameraDeviceClient。
    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; //client 指向的类型是 CameraDeviceClient，其实例则是最终的返回结果。
    return ret;
}

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);

    /* Do something in */
    ......
    /* Do something out */

        sp<BasicClient> tmp = nullptr;
        //调用 makeClient 生成 CameraDeviceClient 实例。
        if(!(ret = makeClient(this, cameraCb, clientPackageName, cameraId, facing, clientPid,
                clientUid, getpid(), legacyMode, halVersion, deviceVersion, effectiveApiLevel,
                /*out*/&tmp)).isOk()) {
            return ret;
        }
        //初始化 CLIENT 实例。注意此处的模板类型 CLIENT 即是 CameraDeviceClient，传入的参数 mCameraProviderManager 则是与 HAL service 有关。 
        client = static_cast<CLIENT*>(tmp.get());

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

        err = client->initialize(mCameraProviderManager);

    /* Do something in */
    ......
    /* Do something out */

    // 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;
}    

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:
            /* Do something in */
            ......
            /* Do something out */
          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 : 实例化了 CameraDeviceClient 类作为 Client（注意此处构造传入了 ICameraDeviceCallbacks，这是连接到 CameraDeviceImpl 的远端回调）
                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 {
        /* Do something in */
        ......
        /* Do something out */
    }
    return Status::ok();
}

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),  //继承它的父类 Camera2ClientBase 
    mInputStream(),
    mStreamingRequestId(REQUEST_ID_NONE),
    mRequestIdCounter(0),
    mPrivilegedClient(false) {

    char value[PROPERTY_VALUE_MAX];
    property_get("persist.camera.privapp.list", value, "");
    String16 packagelist(value);
    if (packagelist.contains(clientPackageName.string())) {
        mPrivilegedClient = true;
    }

    ATRACE_CALL();
    ALOGI("CameraDeviceClient %s: Opened", cameraId.string());
}

//对外提供调用的初始化函数接口 initialize。
status_t CameraDeviceClient::initialize(sp<CameraProviderManager> manager) {
    return initializeImpl(manager);
}

//初始化的具体实现函数，模板 TProviderPtr 在此处即是 CameraProviderManager 类。
template<typename TProviderPtr>
//首先将父类初始化，注意此处传入了 CameraProviderManager。
status_t CameraDeviceClient::initializeImpl(TProviderPtr providerPtr) {
    ATRACE_CALL();
    status_t res;

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

    //这里是关于 FrameProcessor 的创建与初始化配置等等
    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;
}

template <typename TClientBase>　//模板 TClientBase，在 CameraDeviceClient 继承 Camera2ClientBase 时被指定为 CameraDeviceClientBase。
Camera2ClientBase<TClientBase>::Camera2ClientBase( //构造的相关参数，以及初始化列表，这里面需要注意 TCamCallbacks 在 CameraDeviceClientBase 中被指定为了 ICameraDeviceCallbacks。 
        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); //创建了一个 Camera3Device。
    LOG_ALWAYS_FATAL_IF(mDevice == 0, "Device should never be NULL here.");
}

template <typename TClientBase> //初始化函数接口，真正的实现部分在 initializeImpl 中。
status_t Camera2ClientBase<TClientBase>::initialize(sp<CameraProviderManager> manager) {
    return initializeImpl(manager);
}

//TClientBase 对应 CameraDeviceClientBase，而 TProviderPtr 对应的是 CameraProviderManager。
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(); //调用 CameraDeviceClientBase 的 startCameraOps 方法，检查 ops 的权限。
    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); //初始化 Camera3Device 的实例，注意此处传入了 CameraProviderManager。
    if (res != OK) {
        ALOGE("%s: Camera %s: unable to initialize device: %s (%d)",
                __FUNCTION__, TClientBase::mCameraIdStr.string(), strerror(-res), res);
        return res;
    }

　　 //在 Camera3Device 实例中设置 Notify 回调。
    wp<CameraDeviceBase::NotificationListener> weakThis(this);
    res = mDevice->setNotifyCallback(weakThis);

    return OK;
}

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();
　　//在这个观察构造函数中设定了两个回调接口：
    camera3_callback_ops::notify = &sNotify;
    camera3_callback_ops::process_capture_result = &sProcessCaptureResult;
    ALOGV("%s: Created device for camera %s", __FUNCTION__, mId.string());
}

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, //调用CameraProviderManager的openSession方法，开启了远端的Session
            /*out*/ &session);
    ATRACE_END();
    if (res != OK) {
        SET_ERR_L("Could not open camera session: %s (%d)", strerror(-res), res);
        return res;
    }

    /* Do something in */
    ......
    /* Do something out */

    return initializeCommonLocked();
}

status_t CameraProviderManager::openSession(const std::string &id,
        const sp<hardware::camera::device::V3_2::ICameraDeviceCallback>& callback,
        /*out*/
        sp<hardware::camera::device::V3_2::ICameraDeviceSession> *session) {

    std::lock_guard<std::mutex> lock(mInterfaceMutex);

    auto deviceInfo = findDeviceInfoLocked(id, //首先调用 findDeviceInfoLocked，获取 HAL3 相关的 DeviceInfo3
            /*minVersion*/ {3,0}, /*maxVersion*/ {4,0});
    if (deviceInfo == nullptr) return NAME_NOT_FOUND;

    auto *deviceInfo3 = static_cast<ProviderInfo::DeviceInfo3*>(deviceInfo);

    Status status;
    hardware::Return<void> ret;
　　 //通过远端调用 CameraDevice 的 open 方法，创建 CameraDeviceSession 实例并将其本地调用接口通过入参 session 返回。
    ret = deviceInfo3->mInterface->open(callback, [&status, &session]
            (Status s, const sp<device::V3_2::ICameraDeviceSession>& cameraSession) {
                status = s;
                if (status == Status::OK) {
                    *session = cameraSession;
                }
            });
    if (!ret.isOk()) {
        ALOGE("%s: Transaction error opening a session for camera device %s: %s",
                __FUNCTION__, id.c_str(), ret.description().c_str());
        return DEAD_OBJECT;
    }
    return mapToStatusT(status);
}

Return<void> CameraDevice::open(const sp<ICameraDeviceCallback>& callback, open_cb _hidl_cb)  {
    Status status = initStatus();
    sp<CameraDeviceSession> session = nullptr;

    if (callback == nullptr) {
        ALOGE("%s: cannot open camera %s. callback is null!",
                __FUNCTION__, mCameraId.c_str());
        _hidl_cb(Status::ILLEGAL_ARGUMENT, nullptr);
        return Void();
    }

    if (status != Status::OK) {
        /* Do something in */
        ......
        /* Do something out */
    } else {
        mLock.lock();

        /* Do something in */
        ......
        /* Do something out */

        /** Open HAL device */
        status_t res;
        camera3_device_t *device;

        ATRACE_BEGIN("camera3->open");
        res = mModule->open(mCameraId.c_str(), //注意 mModule 是在 HAL Service 初始化时就已经配置好的，它对从libhardware库中加载的 Camera HAL 接口进行了一层封装，从这里往下就会一路走到 QCamera3HWI 的构造流程去。
                reinterpret_cast<hw_device_t**>(&device));
        ATRACE_END();

        /* Do something in */
        ......
        /* Do something out */

　　　　 //创建 session 并让内部成员 mSession 持有，具体实现的函数为 creatSession。
        session = createSession(
                device, info.static_camera_characteristics, callback);

        /* Do something in */
        ......
        /* Do something out */

        mSession = session;

        IF_ALOGV() {
            session->getInterface()->interfaceChain([](
                ::android::hardware::hidl_vec<::android::hardware::hidl_string> interfaceChain) {
                    ALOGV("Session interface chain:");
                    for (auto iface : interfaceChain) {
                        ALOGV("  %s", iface.c_str());
                    }
                });
        }
        mLock.unlock();
    }
    _hidl_cb(status, session->getInterface());
    return Void();
}

int CameraModule::open(const char* id, struct hw_device_t** device) {
    int res;
    ATRACE_BEGIN("camera_module->open");
    res = filterOpenErrorCode(mModule->common.methods->open(&mModule->common, id, device));
    ATRACE_END();
    return res;
}

/*===========================================================================
 * FUNCTION   : camera_device_open
 *
 * DESCRIPTION: static function to open a camera device by its ID
 *
 * PARAMETERS :
 *   @camera_id : camera ID
 *   @hw_device : ptr to struct storing camera hardware device info
 *
 * RETURN     : int32_t type of status
 *              NO_ERROR  -- success
 *              none-zero failure code
 *==========================================================================*/
int QCamera2Factory::camera_device_open(
    const struct hw_module_t *module, const char *id,
    struct hw_device_t **hw_device)
{
    /* Do something in */
    ......
    /* Do something out */

#ifdef QCAMERA_HAL1_SUPPORT //注意到这里通过宏定义添加了对 HAL1 的兼容操作。实际上是要调用 cameraDeviceOpen 来进行下一步操作。
    if(gQCameraMuxer)
        rc =  gQCameraMuxer->camera_device_open(module, id, hw_device);
    else
#endif
        rc = gQCamera2Factory->cameraDeviceOpen(atoi(id), hw_device);
    return rc;
}

struct hw_module_methods_t QCamera2Factory::mModuleMethods = {
    .open = QCamera2Factory::camera_device_open, //这里就将前面所说的 open 函数指针指定为了 camera_device_open 这个方法。
};

/*===========================================================================
 * FUNCTION   : cameraDeviceOpen
 *
 * DESCRIPTION: open a camera device with its ID
 *
 * PARAMETERS :
 *   @camera_id : camera ID
 *   @hw_device : ptr to struct storing camera hardware device info
 *
 * RETURN     : int32_t type of status
 *              NO_ERROR  -- success
 *              none-zero failure code
 *==========================================================================*/
int QCamera2Factory::cameraDeviceOpen(int camera_id,
                    struct hw_device_t **hw_device)
{
    /* Do something in */
    ......
    /* Do something out */

    if ( mHalDescriptors[camera_id].device_version == CAMERA_DEVICE_API_VERSION_3_0 ) {
        QCamera3HardwareInterface *hw = new QCamera3HardwareInterface(mHalDescriptors[camera_id].cameraId, //首先创建了 QCamera3HardwareInterface 的实例。
                mCallbacks);
        if (!hw) {
            LOGE("Allocation of hardware interface failed");
            return NO_MEMORY;
        }
        rc = hw->openCamera(hw_device); //调用实例的 openCamera 方法。
        if (rc != 0) {
            delete hw;
        }
    }

    /* Do something in */
    ......
    /* Do something out */

    return rc;
}

camera3_device_ops_t QCamera3HardwareInterface::mCameraOps = {
    .initialize                         = QCamera3HardwareInterface::initialize,
    .configure_streams                  = QCamera3HardwareInterface::configure_streams,
    .register_stream_buffers            = NULL,
    .construct_default_request_settings = QCamera3HardwareInterface::construct_default_request_settings,
    .process_capture_request            = QCamera3HardwareInterface::process_capture_request,
    .get_metadata_vendor_tag_ops        = NULL,
    .dump                               = QCamera3HardwareInterface::dump,
    .flush                              = QCamera3HardwareInterface::flush,
    .reserved                           = {0},
};

int QCamera3HardwareInterface::openCamera(struct hw_device_t **hw_device)
{
    /* Do something in */
    ......
    /* Do something out */

    rc = openCamera(); //调用另一个 openCamera 方法，这是具体实现的部分。
    if (rc == 0) {
        *hw_device = &mCameraDevice.common; //打开相机成功后，将设备结构中的 common 部分通过双重指针 hw_device 返回。
    } else
        *hw_device = NULL;

    /* Do something in */
    ......
    /* Do something out */
    return rc;
}

int QCamera3HardwareInterface::openCamera()
{
    /* Do something in */
    ......
    /* Do something out */

    rc = camera_open((uint8_t)mCameraId, &mCameraHandle);  //这里就开始进入接口层了，调用的是接口层中的 camera_open 接口。注意此处获取到了 mCameraHandle.

    /* Do something in */
    ......
    /* Do something out */

    rc = mCameraHandle->ops->register_event_notify(mCameraHandle->camera_handle, //注意这里传入了一个 camEvtHandle
            camEvtHandle, (void *)this);

    /* Do something in */
    ......
    /* Do something out */ 

    rc = mCameraHandle->ops->get_session_id(mCameraHandle->camera_handle, 
        &sessionId[mCameraId]);

    /* Do something in */
    ......
    /* Do something out */

    return NO_ERROR;
}

int QCamera3HardwareInterface::initialize(const struct camera3_device *device,
                                  const camera3_callback_ops_t *callback_ops)
{
    LOGD("E");
    QCamera3HardwareInterface *hw =
        reinterpret_cast<QCamera3HardwareInterface *>(device->priv);
    if (!hw) {
        LOGE("NULL camera device");
        return -ENODEV;
    }

    int rc = hw->initialize(callback_ops); //调用了真正实现的初始化逻辑的函数
    LOGD("X");
    return rc;
}

int QCamera3HardwareInterface::initialize(
        const struct camera3_callback_ops *callback_ops)
{
    ATRACE_CALL();
    int rc;

    LOGI("E :mCameraId = %d mState = %d", mCameraId, mState);
    pthread_mutex_lock(&mMutex);

    // Validate current state
    switch (mState) {
        case OPENED:
            /* valid state */
            break;
        default:
            LOGE("Invalid state %d", mState);
            rc = -ENODEV;
            goto err1;
    }

    rc = initParameters(); //参数（mParameters）初始化，注意这里的参数和 CameraParameter 是不同的，它是 metadata_buffer 相关参数的结构。
    if (rc < 0) {
        LOGE("initParamters failed %d", rc);
        goto err1;
    }
    mCallbackOps = callback_ops; //这里将 camera3_call_back_ops 与 mCallbackOps 关联了起来。

    mChannelHandle = mCameraHandle->ops->add_channel( //获取 mChannelHandle 这一句柄，调用的方法实际是 mm_camera_interface.c 中的 mm_camera_intf_add_channel。
            mCameraHandle->camera_handle, NULL, NULL, this);
    if (mChannelHandle == 0) {
        LOGE("add_channel failed");
        rc = -ENOMEM;
        pthread_mutex_unlock(&mMutex);
        return rc;
    }

    pthread_mutex_unlock(&mMutex);
    mCameraInitialized = true;
    mState = INITIALIZED;
    LOGI("X");
    return 0;

err1:
    pthread_mutex_unlock(&mMutex);
    return rc;
}

int32_t camera_open(uint8_t camera_idx, mm_camera_vtbl_t **camera_vtbl)
{
    int32_t rc = 0;
    mm_camera_obj_t *cam_obj = NULL;

    /* Do something in */
    ......
    /* Do something out */

    rc = mm_camera_open(cam_obj);

    /* Do something in */
    ......
    /* Do something out */
}

static uint32_t mm_camera_intf_add_channel(uint32_t camera_handle,
                                           mm_camera_channel_attr_t *attr,
                                           mm_camera_buf_notify_t channel_cb,
                                           void *userdata)
{
    uint32_t ch_id = 0;
    mm_camera_obj_t * my_obj = NULL;

    LOGD("E camera_handler = %d", camera_handle);
    pthread_mutex_lock(&g_intf_lock);
    my_obj = mm_camera_util_get_camera_by_handler(camera_handle);

    if(my_obj) {
        pthread_mutex_lock(&my_obj->cam_lock);
        pthread_mutex_unlock(&g_intf_lock);
        ch_id = mm_camera_add_channel(my_obj, attr, channel_cb, userdata); //通过调用实现层的 mm_camera_add_channel 来获取一个 channel id，也就是其句柄。
    } else {
        pthread_mutex_unlock(&g_intf_lock);
    }
    LOGD("X ch_id = %d", ch_id);
    return ch_id;
}

int32_t mm_camera_open(mm_camera_obj_t *my_obj)
{
    char dev_name[MM_CAMERA_DEV_NAME_LEN];
    int32_t rc = 0;
    int8_t n_try=MM_CAMERA_DEV_OPEN_TRIES;
    uint8_t sleep_msec=MM_CAMERA_DEV_OPEN_RETRY_SLEEP;
    int cam_idx = 0;
    const char *dev_name_value = NULL;
    int l_errno = 0;
    pthread_condattr_t cond_attr;

    LOGD("begin\n");

    if (NULL == my_obj) {
        goto on_error;
    }
    dev_name_value = mm_camera_util_get_dev_name(my_obj->my_hdl); //此处调用的函数是为了获取 my_obj 的句柄，这里不深入分析。
    if (NULL == dev_name_value) {
        goto on_error;
    }
    snprintf(dev_name, sizeof(dev_name), "/dev/%s",
             dev_name_value);
    sscanf(dev_name, "/dev/video%d", &cam_idx);
    LOGD("dev name = %s, cam_idx = %d", dev_name, cam_idx);

    do{
        n_try--;
        errno = 0;
        my_obj->ctrl_fd = open(dev_name, O_RDWR | O_NONBLOCK); //读取设备文件的文件描述符，存到 my_obj->ctrl_fd 中。
        l_errno = errno;
        LOGD("ctrl_fd = %d, errno == %d", my_obj->ctrl_fd, l_errno);
        if((my_obj->ctrl_fd >= 0) || (errno != EIO && errno != ETIMEDOUT) || (n_try <= 0 )) {
            break;
        }
        LOGE("Failed with %s error, retrying after %d milli-seconds",
              strerror(errno), sleep_msec);
        usleep(sleep_msec * 1000U);
    }while (n_try > 0);

    if (my_obj->ctrl_fd < 0) {
        LOGE("cannot open control fd of ''%s'' (%s)\n",
                  dev_name, strerror(l_errno));
        if (l_errno == EBUSY)
            rc = -EUSERS;
        else
            rc = -1;
        goto on_error;
    } else {
        mm_camera_get_session_id(my_obj, &my_obj->sessionid); //成功获取到文件描述符后，就要获取 session 的 id 了。
        LOGH("Camera Opened id = %d sessionid = %d", cam_idx, my_obj->sessionid);
    }

    /* Do something in */
    ......
    /* Do something out */

    /* unlock cam_lock, we need release global intf_lock in camera_open(),
     * in order not block operation of other Camera in dual camera use case.*/
    pthread_mutex_unlock(&my_obj->cam_lock);
    return rc;
}

uint32_t mm_camera_add_channel(mm_camera_obj_t *my_obj,
                               mm_camera_channel_attr_t *attr,
                               mm_camera_buf_notify_t channel_cb,
                               void *userdata)
{
    mm_channel_t *ch_obj = NULL;
    uint8_t ch_idx = 0;
    uint32_t ch_hdl = 0;

　　 //从现有的 Channel 中找到第一个状态为 NOTUSED 的，获取到 ch_obj 中
    for(ch_idx = 0; ch_idx < MM_CAMERA_CHANNEL_MAX; ch_idx++) {
        if (MM_CHANNEL_STATE_NOTUSED == my_obj->ch[ch_idx].state) {
            ch_obj = &my_obj->ch[ch_idx];
            break;
        }
    }

　　/*初始化 ch_obj 结构。首先调用 mm_camera_util_generate_handler 为其生成一个句柄（也是该函数的返回值），
　　 *然后将状态设置为 STOPPED，注意这里还保存了 my_obj 的指针及其 session id，最后调用 mm_channel_init 完成了 Channel 的初始化。*/
    if (NULL != ch_obj) {
        /* initialize channel obj */
        memset(ch_obj, 0, sizeof(mm_channel_t));
        ch_hdl = mm_camera_util_generate_handler(ch_idx);
        ch_obj->my_hdl = ch_hdl;
        ch_obj->state = MM_CHANNEL_STATE_STOPPED;
        ch_obj->cam_obj = my_obj;
        pthread_mutex_init(&ch_obj->ch_lock, NULL);
        ch_obj->sessionid = my_obj->sessionid;
        mm_channel_init(ch_obj, attr, channel_cb, userdata);
    }

    pthread_mutex_unlock(&my_obj->cam_lock);

    return ch_hdl;
}

open 流程（黑色箭头线条）
    CameraManager注册AvailabilityCallback回调，用于接收相机设备的可用性状态变更的通知。
    CameraManager通过调用getCameraIdList()获取到当前可用的camera id，通过getCameraCharacteristcs()函数获取到指定相机设备的特性。
    CameraManager调用openCamera()打开指定相机设备，并返回一个CameraDevice对象，后续通过该CameraDevice对象操控具体的相机设备。
    使用CameraDevice对象的createCaptureSession()创建一个session，数据请求（预览、拍照等）都是通过session进行。在创建session时，需要提供Surface作为参数，用于接收返回的图像。

configure stream流程（蓝色箭头线条）
    申请Surface，如上图的OUTPUT STREAMS DESTINATIONS框，用于在创建session时作为参数，接收session返回的图像。
    创建session后，surface会被配置成框架的stream。在框架中，stream定义了图像的size及format。
    每个request都需要携带target surface用于指定返回的图像是归属到哪个被configure的stream的。

request处理流程（橙色箭头线条）
    CameraDevice对象通过createCaptureRequest()来创建request，每个reqeust都需要有surface和settings（settings就是metadata，request包含的所有配置信息都是放在metadata中的）。
    使用session的capture()、captureBurst()、setStreamingRequest()、setStreamingBurst()等api可以将request发送到框架。
    预览的request，通过setStreamingRequest()、setStreamingBurst()发送，仅调用一次。将request set到repeating request list里面。只要pending request queue里面没有request，就将repeating list里面的request copy到pending queue里面。
    拍照的request，通过capture()、captureBurst()发送，每次需要拍照都会调用。每次触发，都会直接将request入到pending request queue里面，所以拍照的request比预览的request的优先级更高。
    in-progress queue代表当前正在处理的request的queue，每处理完一个，都会从pending queue里面拿出来一个新的request放到这里。

数据返回流程（紫色箭头线条）
    硬件层面返回的数据会放到result里面返回，会通过session的capture callback回调响应。

request处理流程（黑色箭头线条）
    framework异步地submit request到hal，hal依次处理，并返回result。
    每个被submit到hal的request都必须携带stream。stream分为input stream和output stream：input stream对应的buffer是已有图像数据的buffer，hal对这些buffer进行reprocess；output stream对应的buffer是empty buffer，hal将生成的图像数据填充的这些buffer里面。

input stream处理流程（图像的INPUT STREAM 1）
    request携带input stream及input buffer到hal。
    hal进行reprocess，然后新的图像数据重新填充到buffer里面，返回到framework。

output stream处理流程（图像的OUTPUT STREAM 1…N）
    request携带output stream及output buffer到hal。
    hal经过一系列模块的的处理，将图像数据写到buffer中，返回到frameowork。

由于最近需要使用相机拍照等功能，鉴于老旧的相机API问题多多，而且新的设备都是基于安卓5.0以上的，于是本人决定研究一下安卓5.0新引入的Camera2 API 来实现 Camera2API地址

首先我们来熟悉一下官方给的这几个图：

这里引用了管道的概念将安卓设备和摄像头之间联通起来，系统向摄像头发送 Capture 请求，而摄像头会返回 CameraMetadata。这一切建立在一个叫作 CameraCaptureSession 的会话中。

其中 CameraManager 是那个站在高处统管所有摄像投设备（CameraDevice）的管理者，而每个 CameraDevice 自己会负责建立 CameraCaptureSession 以及建立 CaptureRequest。CameraCharacteristics 是 CameraDevice 的属性描述类，非要做个对比的话，那么它与原来的 CameraInfo 有相似性。 类图中有着三个重要的 callback，虽然这增加了阅读代码的难度，但是你必须要习惯，因为这是新包的风格。其中 CameraCaptureSession.CaptureCallback 将处理预览和拍照图片的工作，需要重点对待。这些类是如何相互配合的？下面是简单的流程图。

有了这三张图，那么接下来就好了理解了，我们按照拍照流程的指示，来吧整个过程走一遍

1首先定义一个SufaceView 用来实现预览照片用

surfaceView=(SurfaceView) findViewById(R.id.surfaceView);

surfaceHolder.addCallback(new SurfaceHolder.Callback() {
    @Override
    public void surfaceCreated(SurfaceHolder holder) {
        initCamera();
    }

    @Override
    public void surfaceChanged(SurfaceHolder holder, int format, int width, int height) {

    }

    @Override
    public void surfaceDestroyed(SurfaceHolder holder) {

    }
});

其中initCamera2()方法是用来初始化相机的方法

2获取Camera ID,该ID是用来打开相机的关键，一般后置摄像头是0，前置摄像头是1,这里我们选择后置摄像头做详解

mCameraID = "" + CameraCharacteristics.LENS_FACING_BACK;//后摄像头

3通过Camera ID 来打开摄像头，这里我们需要使用CamerManager，这是类是一个管理服务类，值得注意的是，打开摄像头是一个相当复杂的过程，不能直接在主线程中直接执行，其核心代码为：

HandlerThread handlerThread=new HandlerThread("Camera2");
handlerThread.start();
childHandler=new Handler(handlerThread.getLooper());
mainHandler=new Handler(getMainLooper());

mImageReader.setOnImageAvailableListener(new ImageReader.OnImageAvailableListener() { //可以在这里处理拍照得到的临时照片 例如，写入本地
    @Override
    public void onImageAvailable(ImageReader reader) {
        mCameraDevice.close();
        surfaceView.setVisibility(View.GONE);
        iv_show.setVisibility(View.VISIBLE);
        // 拿到拍照照片数据
        Image image = reader.acquireNextImage();
        ByteBuffer buffer = image.getPlanes()[0].getBuffer();
        byte[] bytes = new byte[buffer.remaining()];
        buffer.get(bytes);//由缓冲区存入字节数组
        final Bitmap bitmap = BitmapFactory.decodeByteArray(bytes, 0, bytes.length);
        if (bitmap != null) {
            iv_show.setImageBitmap(bitmap);
        }
    }
}, mainHandler);

cameraManager = (CameraManager) getSystemService(Context.CAMERA_SERVICE);
try {
    if (ActivityCompat.checkSelfPermission(this, Manifest.permission.CAMERA) != PackageManager.PERMISSION_GRANTED) {
        return;
    }
    //打开摄像头
    cameraManager.openCamera(mCameraID, stateCallback, mainHandler);
} catch (CameraAccessException e) {
    e.printStackTrace();
}

<uses-permission android:name="android.permission.CAMERA"></uses-permission>

4 开启相机后有一个回调，stateCallback,该回调是用来返回相机是否正常打开的状态的

private CameraDevice.StateCallback stateCallback = new CameraDevice.StateCallback() {
    @Override
    public void onOpened(CameraDevice camera) {//打开摄像头
        mCameraDevice = camera;
        //开启预览
        takePreview();
    }

    @Override
    public void onDisconnected(CameraDevice camera) {//关闭摄像头
        if (null != mCameraDevice) {
            mCameraDevice.close();
            MainActivity.this.mCameraDevice = null;
        }
    }

    @Override
    public void onError(CameraDevice camera, int error) {//发生错误
        Toast.makeText(MainActivity.this, "摄像头开启失败", Toast.LENGTH_SHORT).show();
    }
};

5 相机开启成功后，执行回调中的onOpen方法，在该方法中，我们实现让图像显示在界面上

/**
 * 开始预览
 */
private void takePreview() {
    try {
        // 创建预览需要的CaptureRequest.Builder
        final CaptureRequest.Builder previewRequestBuilder = mCameraDevice.createCaptureRequest(CameraDevice.TEMPLATE_PREVIEW);
        // 将SurfaceView的surface作为CaptureRequest.Builder的目标
        previewRequestBuilder.addTarget(surfaceHolder.getSurface());
        // 创建CameraCaptureSession，该对象负责管理处理预览请求和拍照请求
        mCameraDevice.createCaptureSession(Arrays.asList(surfaceHolder.getSurface(), mImageReader.getSurface()), new CameraCaptureSession.StateCallback() // ③
        {
            @Override
            public void onConfigured(CameraCaptureSession cameraCaptureSession) {
                if (null == mCameraDevice) return;
                // 当摄像头已经准备好时，开始显示预览
                mCameraCaptureSession = cameraCaptureSession;
                try {
                    // 自动对焦
                    previewRequestBuilder.set(CaptureRequest.CONTROL_AF_MODE, CaptureRequest.CONTROL_AF_MODE_CONTINUOUS_PICTURE);
                    // 打开闪光灯
                    previewRequestBuilder.set(CaptureRequest.CONTROL_AE_MODE, CaptureRequest.CONTROL_AE_MODE_ON_AUTO_FLASH);
                    // 显示预览
                    CaptureRequest previewRequest = previewRequestBuilder.build();
                    mCameraCaptureSession.setRepeatingRequest(previewRequest, null, childHandler);
                } catch (CameraAccessException e) {
                    e.printStackTrace();
                }
            }

            @Override
            public void onConfigureFailed(CameraCaptureSession cameraCaptureSession) {
                Toast.makeText(MainActivity.this, "配置失败", Toast.LENGTH_SHORT).show();
            }
        }, childHandler);
    } catch (CameraAccessException e) {
        e.printStackTrace();
    }
}

6，预览完成后，接下来就是拍照了，注意某些手机的摄像头会和正常手机的摄像头另类，成像会成180度的倒立像，比如nexus 5x，这时候只需要设置

rORIENTATIONS的值来调整角度就可以

/**
 * 拍照
 */
private void takePicture() {
    if (mCameraDevice == null) return;
    // 创建拍照需要的CaptureRequest.Builder
    final CaptureRequest.Builder captureRequestBuilder;
    try {
        captureRequestBuilder = mCameraDevice.createCaptureRequest(CameraDevice.TEMPLATE_STILL_CAPTURE);
        // 将imageReader的surface作为CaptureRequest.Builder的目标
        captureRequestBuilder.addTarget(mImageReader.getSurface());
        // 自动对焦
        captureRequestBuilder.set(CaptureRequest.CONTROL_AF_MODE, CaptureRequest.CONTROL_AF_MODE_CONTINUOUS_PICTURE);
        // 自动曝光
        captureRequestBuilder.set(CaptureRequest.CONTROL_AE_MODE, CaptureRequest.CONTROL_AE_MODE_ON_AUTO_FLASH);
        // 获取手机方向
        int rotation = getWindowManager().getDefaultDisplay().getRotation();
        // 根据设备方向计算设置照片的方向
        captureRequestBuilder.set(CaptureRequest.JPEG_ORIENTATION, ORIENTATIONS.get(rotation)+rORIENTATIONS);
        //拍照
        CaptureRequest mCaptureRequest = captureRequestBuilder.build();
        mCameraCaptureSession.capture(mCaptureRequest, null, childHandler);
    } catch (CameraAccessException e) {
        e.printStackTrace();
    }
}