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在iOS13.0开始支持多摄像头预览AVCaptureMultiCamSession，然后iOS15.0增加支持摄像头画中画预览。在使用之前，我们通过isMultiCamSupported()判断是否支持多Camera同时预览。让我们先看下效果：

一、Camera架构

1、Camera流水线

Camera由AVCaptureDeviceInput、AVCaptureSession、AVCaptureOutput构成。如下图所示：

2、单Camera架构

单个Camera架构意味只有一个AVCaptureDeviceInput，同步输出VideoData和DepthData，支持预览和输出文件。如下图所示：

3、多Camera架构

与单Camera架构相比，多Camera架构包括多个输入源AVCaptureDeviceInput，多个摄像头同时预览，如下图所示：

二、Camera类图结构

 Camera类图包括AVCaptureDeviceInput、AVCaptureMultiCamSession、AVCaptureVideoDataOutput、AVCaptureVideoPreviewLayer、AVAssetWriter。如下图所示：

三、Camera输入输出

Camera的输入包括：前置Camera、后置Camera、麦克风，输出包括：预览数据、图片、文件、Metadata，由AVCaptureMultiCamSession进行管理。如下图所示：

四、MultiCamera流同步

多个Camera同时预览，它们共享分辨率和帧率。也需要进行流同步，包括如下：

• 曝光
• 对焦
• 白平衡

五、Camera画中画预览

 1、初始化

初始化阶段，主要设置预览图层、配置capture session，示例代码如下：

	override func viewDidLoad() {
		super.viewDidLoad()
		
		// 设置前置、后置camera预览图层
		backCameraVideoPreviewView.videoPreviewLayer.setSessionWithNoConnection(session)
		frontCameraVideoPreviewView.videoPreviewLayer.setSessionWithNoConnection(session)
		
		// 配置 capture session
		sessionQueue.async {
			self.configureSession()
		}
	}

2、配置session

 配置capture session的示例代码如下：

	private func configureSession() {
		guard setupResult == .success else { return }
		
		guard AVCaptureMultiCamSession.isMultiCamSupported else {
			print("MultiCam not supported on this device")
			setupResult = .multiCamNotSupported
			return
		}

		session.beginConfiguration()
		defer {
			session.commitConfiguration()
			if setupResult == .success {
				checkSystemCost()
			}
		}

		guard configureBackCamera() else {
			setupResult = .configurationFailed
			return
		}
		
		guard configureFrontCamera() else {
			setupResult = .configurationFailed
			return
		}
	}

3、配置后置Camera 

配置流程包括：查找后置Camera、添加到session、连接输入设备到输出数据、连接输入设备到预览图层等，示例代码如下：

	private func configureBackCamera() -> Bool {
		session.beginConfiguration()
		defer {
			session.commitConfiguration()
		}
		
		// 查找后置camera
		guard let backCamera = AVCaptureDevice.default(.builtInWideAngleCamera, for: .video, position: .back) else {
			print("Could not find the back camera")
			return false
		}
		
		// 添加后置camera到session
		do {
			backCameraDeviceInput = try AVCaptureDeviceInput(device: backCamera)
			
			guard let backCameraDeviceInput = backCameraDeviceInput,
				session.canAddInput(backCameraDeviceInput) else {
					return false
			}
			session.addInputWithNoConnections(backCameraDeviceInput)
		} catch {
			print("Could not create back camera device input: (error)")
			return false
		}
		
		// 查找后置camera输入视频端口
		guard let backCameraDeviceInput = backCameraDeviceInput,
			let backCameraVideoPort = backCameraDeviceInput.ports(for: .video,
                                      sourceDeviceType: backCamera.deviceType,
                                      sourceDevicePosition: backCamera.position).first else {
                                        return false
		}
		
		// 添加后置camera输出视频数据
		guard session.canAddOutput(backCameraVideoDataOutput) else {
			print("Could not add the back camera video data output")
			return false
		}
		session.addOutputWithNoConnections(backCameraVideoDataOutput)
        
        backCameraVideoDataOutput.videoSettings = [kCVPixelBufferPixelFormatTypeKey as String: Int(kCVPixelFormatType_32BGRA)]
		backCameraVideoDataOutput.setSampleBufferDelegate(self, queue: dataOutputQueue)
		
		// 连接后置camera输入到数据输出
		let backCameraVideoDataOutputConnection = AVCaptureConnection(inputPorts: [backCameraVideoPort],
                                                                      output: backCameraVideoDataOutput)
		guard session.canAddConnection(backCameraVideoDataOutputConnection) else {
			print("Could not add a connection to the back camera video data output")
			return false
		}
		session.addConnection(backCameraVideoDataOutputConnection)
		backCameraVideoDataOutputConnection.videoOrientation = .portrait

		// 连接后置camera输入到预览图层
		guard let backCameraVideoPreviewLayer = backCameraVideoPreviewLayer else {
			return false
		}
		let backCameraVideoPreviewLayerConnection = AVCaptureConnection(inputPort: backCameraVideoPort, videoPreviewLayer: backCameraVideoPreviewLayer)
		guard session.canAddConnection(backCameraVideoPreviewLayerConnection) else {
			print("Could not add a connection to the back camera video preview layer")
			return false
		}
		session.addConnection(backCameraVideoPreviewLayerConnection)
		
		return true
	}

4、配置前置Camera

前置Camera与后置的配置流程类似，只是把back换成front。另外，前置Camera开启镜像。示例代码如下：

    private func configureFrontCamera() -> Bool {
		session.beginConfiguration()
		defer {
			session.commitConfiguration()
		}
		
		// 查找前置camera
		guard let frontCamera = AVCaptureDevice.default(.builtInWideAngleCamera, for: .video, position: .front) else {
			print("Could not find the front camera")
			return false
		}
		
		// 添加前置camera到session
		do {
			frontCameraDeviceInput = try AVCaptureDeviceInput(device: frontCamera)
			
			guard let frontCameraDeviceInput = frontCameraDeviceInput,
				session.canAddInput(frontCameraDeviceInput) else {
					return false
			}
			session.addInputWithNoConnections(frontCameraDeviceInput)
		} catch {
			print("Could not create front camera device input: (error)")
			return false
		}
		
		// 查找前置camera输入视频端口
		guard let frontCameraDeviceInput = frontCameraDeviceInput,
			let frontCameraVideoPort = frontCameraDeviceInput.ports(for: .video,
                                       sourceDeviceType: frontCamera.deviceType,
                                       sourceDevicePosition: frontCamera.position).first else {
                                       return false
		}
		
		// 添加前置camera输出视频数据
		guard session.canAddOutput(frontCameraVideoDataOutput) else {
			print("Could not add the front camera video data output")
			return false
		}
		session.addOutputWithNoConnections(frontCameraVideoDataOutput)
		
        frontCameraVideoDataOutput.videoSettings = [kCVPixelBufferPixelFormatTypeKey as String: Int(kCVPixelFormatType_32BGRA)]
		frontCameraVideoDataOutput.setSampleBufferDelegate(self, queue: dataOutputQueue)
		
		// 连接前置camera输入到数据输出
		let frontCameraVideoDataOutputConnection = AVCaptureConnection(inputPorts: [frontCameraVideoPort],
                                                                       output: frontCameraVideoDataOutput)
		guard session.canAddConnection(frontCameraVideoDataOutputConnection) else {
			print("Could not add a connection to the front camera video data output")
			return false
		}
		session.addConnection(frontCameraVideoDataOutputConnection)
		frontCameraVideoDataOutputConnection.videoOrientation = .portrait

		// 连接前置camera输入到预览图层
		guard let frontCameraVideoPreviewLayer = frontCameraVideoPreviewLayer else {
			return false
		}
		let frontCameraVideoPreviewLayerConnection = AVCaptureConnection(inputPort: frontCameraVideoPort, videoPreviewLayer: frontCameraVideoPreviewLayer)
		guard session.canAddConnection(frontCameraVideoPreviewLayerConnection) else {
			print("Could not add a connection to the front camera video preview layer")
			return false
		}
		session.addConnection(frontCameraVideoPreviewLayerConnection)
        // 前置camera开启镜像
		frontCameraVideoPreviewLayerConnection.isVideoMirrored = true
        frontCameraVideoPreviewLayerConnection.automaticallyAdjustsVideoMirroring = false
		
		return true
	}

5、配置双向麦克风

 除了提供画中画Camera，还提供前后双向麦克风。示例代码如下：

	private func configureMicrophone() -> Bool {
		session.beginConfiguration()
		defer {
			session.commitConfiguration()
		}
		
		// 查找麦克风
		guard let microphone = AVCaptureDevice.default(for: .audio) else {
			print("Could not find the microphone")
			return false
		}
		
		// 添加麦克风到session
		do {
			microphoneDeviceInput = try AVCaptureDeviceInput(device: microphone)
			
			guard let microphoneDeviceInput = microphoneDeviceInput,
				session.canAddInput(microphoneDeviceInput) else {
					return false
			}
			session.addInputWithNoConnections(microphoneDeviceInput)
		} catch {
			print("Could not create microphone input: (error)")
			return false
		}
		
		// 查找输入设备的后置音频端口
		guard let microphoneDeviceInput = microphoneDeviceInput,
			let backMicrophonePort = microphoneDeviceInput.ports(for: .audio,
                                     sourceDeviceType: microphone.deviceType,
                                     sourceDevicePosition: .back).first else {
                                        return false
		}
		
		// 查找输入设备的前置音频端口
		guard let frontMicrophonePort = microphoneDeviceInput.ports(for: .audio,
                                        sourceDeviceType: microphone.deviceType,
                                        sourceDevicePosition: .front).first else {
			return false
		}
		
		// 添加后置麦克风到输出数据
		guard session.canAddOutput(backMicrophoneAudioDataOutput) else {
			print("Could not add the back microphone audio data output")
			return false
		}
		session.addOutputWithNoConnections(backMicrophoneAudioDataOutput)
		backMicrophoneAudioDataOutput.setSampleBufferDelegate(self, queue: dataOutputQueue)
		
		// 添加前置麦克风到输出数据
		guard session.canAddOutput(frontMicrophoneAudioDataOutput) else {
			print("Could not add the front microphone audio data output")
			return false
		}
		session.addOutputWithNoConnections(frontMicrophoneAudioDataOutput)
		frontMicrophoneAudioDataOutput.setSampleBufferDelegate(self, queue: dataOutputQueue)
		
		// 连接后置麦克风到输出数据
		let backMicrophoneAudioDataOutputConnection = AVCaptureConnection(inputPorts: [backMicrophonePort],
                                                                          output: backMicrophoneAudioDataOutput)
		guard session.canAddConnection(backMicrophoneAudioDataOutputConnection) else {
			print("Could not add a connection to the back microphone audio data output")
			return false
		}
		session.addConnection(backMicrophoneAudioDataOutputConnection)
		
		// 连接前置麦克风到输出数据
		let frontMicrophoneAudioDataOutputConnection = AVCaptureConnection(inputPorts: [frontMicrophonePort],
                                                                           output: frontMicrophoneAudioDataOutput)
		guard session.canAddConnection(frontMicrophoneAudioDataOutputConnection) else {
			print("Could not add a connection to the front microphone audio data output")
			return false
		}
		session.addConnection(frontMicrophoneAudioDataOutputConnection)
		
		return true
	}

六、降低功耗

iOS提供API获取硬件功耗：

var hardwareCost: Float { get } // 取值[0.0, 1.0]

同时提供API获取系统压力功耗：

var systemPressureCost: Float { get } // 取值[0.0, 1.0]

关于降低功耗的可行方案如下：

• 设置最大帧率
• 降低Camera分辨率
• 选择低精度像素格式