在ios7之前，要掃描QR code必須要用到第三方程式，但現在ios已經內建這項功能。
這次我們要用到 “AVCaptureSession” 這個類別並且偵測裝置中的相機

1. 在 < info.plist > 中建立相機、相簿的使用權限
   

2. 建立一個新的類別: “DeviceInput”，並且繼承 “NSObject”
   

3. 在 < DeviceInput.swift >
   匯入 “AVFoundation”

import AVFoundation

宣告 input 相關輸入方式

class DeviceInput: NSObject {
    //declared the input device
    var frontWildAngleCamera : AVCaptureDeviceInput? //frontwildAngleCamera
    var backWildAngleCamera: AVCaptureDeviceInput? // backwildAngleCamera
    var backTelephotoCamera: AVCaptureDeviceInput? //backtelephotoCamera
    var backDualCamera: AVCaptureDeviceInput? // dual camera

4. 取得所有 camera 相關裝置，並且放到對應變數中

//get all the camera type
func getAllCameras() {
    
    let cameraDevices = AVCaptureDevice.DiscoverySession(deviceTypes: [.builtInWideAngleCamera, .builtInTelephotoCamera, .builtInDualCamera],mediaType: .video,  position: .unspecified).devices
            for camera in cameraDevices {
                let inputDevice = try! AVCaptureDeviceInput(device: camera)
                
                if camera.deviceType == .builtInWideAngleCamera, camera.position == .front {
                    frontWildAngleCamera = inputDevice
                }
                if camera.deviceType == .builtInWideAngleCamera, camera.position == .back {
                    backWildAngleCamera = inputDevice
                }
                if camera.deviceType == .builtInTelephotoCamera {
                    backTelephotoCamera = inputDevice
                }
                if camera.deviceType == .builtInDualCamera {
                    backDualCamera = inputDevice
                }
            }
        }

5. 重新複寫初始化設定

override init() {
        super.init()
        getAllCameras()
}