How do I implement camera changing from front to back camera

问题

Below is the code for the camera section, I tried adding in a boolean to detect when the front camera is activated but I receive an error.

import UIKit
import AVFoundation 
class MainCameraCollectionViewCell: UICollectionViewCell {
    @IBOutlet weak var myView: UIView!
    var captureSession = AVCaptureSession()
    var backCamera: AVCaptureDevice?
    var frontCamera: AVCaptureDevice?
    var currentCamera: AVCaptureDevice?
    var photoOutPut: AVCapturePhotoOutput?
    var cameraPreviewLayer: AVCaptureVideoPreviewLayer?
    var image: UIImage?
    var usingFrontCamera = false   
    override func awakeFromNib() {
        super.awakeFromNib()
        setupCaptureSession()
        setupDevice()
        setupInput()
        setupPreviewLayer()
        startRunningCaptureSession()
    }
    func setupCaptureSession(){
        captureSession.sessionPreset = AVCaptureSession.Preset.photo
    }
    func setupDevice(usingFrontCamera:Bool = false){
        let deviceDiscoverySession = AVCaptureDevice.DiscoverySession(deviceTypes: [AVCaptureDevice.DeviceType.builtInWideAngleCamera], mediaType: AVMediaType.video, position: AVCaptureDevice.Position.unspecified)
        let devices = deviceDiscoverySession.devices   
        for device in devices {
            if device.position == AVCaptureDevice.Position.back {
                backCamera = device
            } else if device.position == AVCaptureDevice.Position.front {
                frontCamera = device
            }
        }
        if usingFrontCamera {
            currentCamera = frontCamera
        } else {
            currentCamera = backCamera
        }
    }
    func setupInput() {
        do {
            let captureDeviceInput = try AVCaptureDeviceInput(device: currentCamera!)
            captureSession.addInput(captureDeviceInput)
            photoOutPut = AVCapturePhotoOutput()
            photoOutPut?.setPreparedPhotoSettingsArray([AVCapturePhotoSettings(format:[AVVideoCodecKey: AVVideoCodecType.jpeg])], completionHandler: nil)
            captureSession.addOutput(photoOutPut!)
        } catch {
            print(error)
        }
    }
    func setupPreviewLayer(){
        cameraPreviewLayer = AVCaptureVideoPreviewLayer(session: captureSession)
        cameraPreviewLayer?.videoGravity = AVLayerVideoGravity.resizeAspectFill
        cameraPreviewLayer?.connection?.videoOrientation = AVCaptureVideoOrientation.portrait
        cameraPreviewLayer?.frame = CGRect(x: 0, y: 0, width: UIScreen.main.bounds.width, height: UIScreen.main.bounds.height)
        self.layer.insertSublayer(cameraPreviewLayer!, at: 0)
    }
    func startRunningCaptureSession(){
        captureSession.startRunning()
    }
    @IBAction func cameraButton_TouchUpInside(_ sender: Any) {
        let settings = AVCapturePhotoSettings()
        photoOutPut?.capturePhoto(with: settings, delegate: self as! AVCapturePhotoCaptureDelegate)
    }
    //Flip to front and back camera
    @IBAction func FlipThe_camera(_ sender: UIButton) {
        print("Flip touched")
        if let inputs = captureSession.inputs as? [AVCaptureDeviceInput] {
            for input in inputs {
                captureSession.removeInput(input)
            }
        }
        usingFrontCamera = !usingFrontCamera
        setupCaptureSession()
        setupDevice(usingFrontCamera:usingFrontCamera)
        setupInput()
        setupPreviewLayer()
        startRunningCaptureSession()
    }
}

collectionViewAttemptOfPagingAttempt002[2908:834004] * Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '* -[AVCaptureSession addOutput:] Cannot add output to capture session -> -> because more than one output of the same type is unsupported'

回答1:

Your problem is that you are not checking if you can add input and/or output before adding them to your capture session. Btw you don't need to setup the preview layer again when switching your camera and you need only one captureDevice:

if captureSession.canAddInput(captureDeviceInput) {
    captureSession.addInput(captureDeviceInput)
} 

and

if captureSession.canAddOutput(photoOutPut) {
    captureSession.addOutput(photoOutPut)
}

Your CollectionViewCell code should look something like this:

class MainCameraCollectionViewCell: UICollectionViewCell, AVCapturePhotoCaptureDelegate {
    private var captureSession = AVCaptureSession()
    private var sessionQueue: DispatchQueue!
    private var captureDevice: AVCaptureDevice!
    private  var photoOutPut: AVCapturePhotoOutput!
    private var cameraPreviewLayer: AVCaptureVideoPreviewLayer!
    var image: UIImage?
    var usingFrontCamera = false
    override func awakeFromNib() {
        super.awakeFromNib()
        setupCaptureSession()
        setupDevice()
        setupInput()
        setupPreviewLayer()
        startRunningCaptureSession()
    }
    func setupCaptureSession(){
        captureSession.sessionPreset = .photo
        sessionQueue = DispatchQueue(label: "session queue")
    }
    func setupDevice(usingFrontCamera: Bool = false){
        sessionQueue.async {
            let deviceDiscoverySession = AVCaptureDevice.DiscoverySession(deviceTypes: [.builtInWideAngleCamera], mediaType: .video, position: .unspecified)
            let devices = deviceDiscoverySession.devices
            for device in devices {
                if usingFrontCamera && device.position == .front {
                    self.captureDevice = device
                } else if device.position == .back {
                    self.captureDevice = device
                }
            }
        }
    }
    func setupInput() {
        sessionQueue.async {
            do {
                let captureDeviceInput = try AVCaptureDeviceInput(device: self.captureDevice)
                if self.captureSession.canAddInput(captureDeviceInput) {
                    self.captureSession.addInput(captureDeviceInput)
                }
                self.photoOutPut = AVCapturePhotoOutput()
                self.photoOutPut.setPreparedPhotoSettingsArray([AVCapturePhotoSettings(format:[AVVideoCodecKey: AVVideoCodecType.jpeg])], completionHandler: nil)
                if self.captureSession.canAddOutput(self.photoOutPut) {
                    self.captureSession.addOutput(self.photoOutPut)
                }
            } catch {
                print(error)
            }
        }
    }
    func setupPreviewLayer() {
        cameraPreviewLayer = AVCaptureVideoPreviewLayer(session: captureSession)
        cameraPreviewLayer.videoGravity = .resizeAspectFill
        cameraPreviewLayer.connection?.videoOrientation = .portrait
        cameraPreviewLayer.frame = UIScreen.main.bounds
        layer.insertSublayer(cameraPreviewLayer, at: 0)
    }
    func startRunningCaptureSession() {
        captureSession.startRunning()
    }
    @IBAction func cameraButton_TouchUpInside(_ sender: Any) {
        let settings = AVCapturePhotoSettings()
        photoOutPut.capturePhoto(with: settings, delegate: self)
    }
    //Flip to front and back camera
    @IBAction func FlipThe_camera(_ sender: UIButton) {
        captureSession.beginConfiguration()
        if let inputs = captureSession.inputs as? [AVCaptureDeviceInput] {
            for input in inputs {
                captureSession.removeInput(input)
            }
        }
        usingFrontCamera = !usingFrontCamera
        setupCaptureSession()
        setupDevice(usingFrontCamera: usingFrontCamera)
        setupInput()
        captureSession.commitConfiguration()
        startRunningCaptureSession()
    }
}

Sample project

来源：https://stackoverflow.com/questions/48727766/how-do-i-implement-camera-changing-from-front-to-back-camera