如何使用 Swift 在 Xcode 中使用切换开关关闭背景音乐？

Question

应用启动后，我正在通过应用委托在我的应用中播放背景音乐。现在，在我的第二个 V.C.我设置了一个拨动开关来关闭/关闭背景音乐。但是，每当我运行以下代码时，我的应用程序都会崩溃并给我这个错误：-

Fatal error: Unexpectedly found nil while implicitly unwrapping an Optional value

谁能告诉我如何在我的 V.C. 中实现以下任务。感谢您的帮助！谢谢：）

**App Delegate**

let vc = SecondViewController()

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
    // Override point for customization after application launch.
    
    vc.playMusic()
    
    return true
}

**Second View Controller**

import UIKit
import AVFoundation

class SecondViewController: UIViewController {

@IBOutlet weak var musicToggleSwitch: UISwitch!

var music: AVAudioPlayer!

let vc_1 = ViewController()

override func viewDidLoad() {
    super.viewDidLoad()
  
    self.musicToggleSwitch.setOn(UserDefaults.standard.bool(forKey: "musicToggleState"), animated: true)
    
   }

@IBAction func musicToggleSwitch(_ sender: UISwitch) {
    
    if (musicToggleSwitch.isOn == true) {
        
        if (music.isPlaying == false) {
            
            music.play()
            
        }
        
    }
    else {
        
        if (music.isPlaying == true) {
            
            music.stop()
            
        }
        
    }
    
    UserDefaults.standard.set(sender.isOn, forKey: "musicToggleState")

}

func playMusic() {
    
    if let musicURL = Bundle.main.url(forResource: "Music", withExtension: "mp3") {
        if let audioPlayer = try? AVAudioPlayer(contentsOf: musicURL) {
            music = audioPlayer
            music.numberOfLoops = -1
            music.play()
        }
    }
    
}

}

Answer 1

这应该可以解决您的问题。

如果还是不播放，在music变量的创建块中加一些断点看看哪里出错了。

class SecondViewController: UIViewController {
    
    @IBOutlet weak var musicToggleSwitch: UISwitch!
    
    var music: AVAudioPlayer? = {
        guard let musicURL = Bundle.main.url(forResource: "Music", withExtension: "mp3") else {
            return nil
        }
        
        let audioPlayer = try? AVAudioPlayer(contentsOf: musicURL)
        audioPlayer?.numberOfLoops = -1
        return audioPlayer
    }()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        self.musicToggleSwitch.setOn(UserDefaults.standard.bool(forKey: "musicToggleState"), animated: true)
    }
    
    @IBAction func musicToggleSwitch(_ sender: UISwitch) {
        guard let music = music, sender.isOn != music.isPlaying else {
            return
        }
        
        if sender.isOn {
            music.play()
        } else {
            music.stop()
        }
        
        UserDefaults.standard.set(sender.isOn, forKey: "musicToggleState")
    }
    
    func playMusic() {
        music?.play()
    }
}

Answer 2

问题出在这一行：

let vc = SecondViewController()

这与您在界面中看到的 SecondViewController 不同。所以playMusic在错误的视图控制器上被调用，在视图控制器上你可以看到music仍然是nil然后你崩溃了。