Swift_iOS: 扫描二维码的方法

可以使用AVFoundation框架来启动相机扫描二维码，把一个二维码转换为一个字符串。

如下应用，进入首页看到一个按钮和一个标签。点按钮的话，会触发一次扫描，把扫描到的二维码转换为字符串后，会显示在标签内。代码如下：

import UIKit
@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
    var window: UIWindow?
    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
        self.window = UIWindow(frame: UIScreen.main.bounds)
        let page = Page1()
        self.window!.rootViewController = page
        self.window?.makeKeyAndVisible()
        return true
    }
}
class Page1: UIViewController {
    var count = 0
    var label : UILabel!
    var button1 = UIButton()
    var label1 = UILabel()
    override func viewDidLoad() {
        super.viewDidLoad()
        button1.frame = CGRect(x: 10, y: 130, width: 300, height: 20)
        button1.setTitle("Scan QR code",for: .normal)
        button1.addTarget(self, action: #selector(Success(_:)), for: .touchUpInside)
        view.addSubview(button1)
        label1.frame = CGRect(x: 10, y: 100, width: 300, height: 20)
        label1.text = "QRCode here"
        label1.backgroundColor = .red
        view.addSubview(label1)
    }
    func Success(_ sender: AnyObject) {
        ScannerViewController.Run(self,onFind)
    }
    func onFind(_ qrcode : String){
        label1.text = "QRCode here:\(qrcode)"
    }

}
// for Scan QR code test
import AVFoundation
import UIKit
typealias OnFind = (_ code : String)-> Void

class ScannerViewController: UIViewController, AVCaptureMetadataOutputObjectsDelegate {
    class func Run(_ creator : UIViewController, _ onFind : @escaping OnFind){
        let scanner = ScannerViewController()
        scanner.onFind = onFind
        creator.present(scanner, animated: true)
    }

    var captureSession: AVCaptureSession!
    var previewLayer: AVCaptureVideoPreviewLayer!
    var onFind : OnFind!
    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = UIColor.blue
        captureSession = AVCaptureSession()
        let videoCaptureDevice = AVCaptureDevice.defaultDevice(withMediaType: AVMediaTypeVideo)
        let videoInput: AVCaptureDeviceInput
        do {
            videoInput = try AVCaptureDeviceInput(device: videoCaptureDevice)
        } catch {
            return
        }
        if (captureSession.canAddInput(videoInput)) {
            captureSession.addInput(videoInput)
        } else {
            failed();
            return;
        }
        let metadataOutput = AVCaptureMetadataOutput()
        if (captureSession.canAddOutput(metadataOutput)) {
            captureSession.addOutput(metadataOutput)
            metadataOutput.setMetadataObjectsDelegate(self, queue: DispatchQueue.main)
            metadataOutput.metadataObjectTypes = [AVMetadataObjectTypeQRCode]
        } else {
            failed()
            return
        }
        previewLayer = AVCaptureVideoPreviewLayer(session: captureSession);
        previewLayer.frame = view.layer.bounds;
        previewLayer.videoGravity = AVLayerVideoGravityResizeAspectFill;
        view.layer.addSublayer(previewLayer);
        captureSession.startRunning();
    }
    func failed() {
        let ac = UIAlertController(title: "Scanning not supported", message: "Your device does not support scanning a code from an item. Please use a device with a camera.", preferredStyle: .alert)
        ac.addAction(UIAlertAction(title: "OK", style: .default))
        present(ac, animated: true)
        captureSession = nil
    }
    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        if (captureSession?.isRunning == false) {
            captureSession.startRunning();
        }
    }
    override func viewWillDisappear(_ animated: Bool) {
        super.viewWillDisappear(animated)
        if (captureSession?.isRunning == true) {
            captureSession.stopRunning();
        }
    }
    func captureOutput(_ captureOutput: AVCaptureOutput!, didOutputMetadataObjects metadataObjects: [Any]!, from connection: AVCaptureConnection!) {
        captureSession.stopRunning()
        if let metadataObject = metadataObjects.first {
            let readableObject = metadataObject as! AVMetadataMachineReadableCodeObject;
            AudioServicesPlaySystemSound(SystemSoundID(kSystemSoundID_Vibrate))
            found(code: readableObject.stringValue);
        }
        dismiss(animated: true)
    }
    func found(code: String) {
        self.onFind(code)
        self.dismiss(animated: true, completion: nil)
    }
    override var prefersStatusBarHidden: Bool {
        return true
    }
    override var supportedInterfaceOrientations: UIInterfaceOrientationMask {
        return .portrait
    }
}

需要注意的是，首先必须在info.plist内指定NSCameraUsageDescription，如下：

 <key>NSCameraUsageDescription</key>
<string>scan QR</string>

否则会报错：

 [access] This app has crashed because it attempted to access privacy-sensitive data without a usage description.  The app's Info.plist must contain an NSCameraUsageDescription key with a string value explaining to the user how the app uses this data