xcode9 swift4致命错误：展开一个可选值时意外发现nil

Question

应用启动时，我试图播放mp3文件（5秒），但不断出现错误：

线程1：致命错误：展开一个可选值时意外地找到零

import UIKit
import AudioToolbox

class ViewController: UIViewController {

override func viewDidLoad() {
    super.viewDidLoad()
    // Do any additional setup after loading the view, typically from a nib.

    //create SystemSoundID
    var soundID:SystemSoundID = 0
    let path = Bundle.main.path(forResource: "iPhone100Voice", ofType: "mp3")
    let baseURL = NSURL(fileURLWithPath: path!)
    AudioServicesCreateSystemSoundID(baseURL, &soundID)
    //play
    AudioServicesPlaySystemSound(soundID)

    }

override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
    // Dispose of any resources that can be recreated.
    }

}

谁能解释为什么以及如何解决这个问题？

Answer 1

检查以下几点，所有条件都必须为真：

• 该文件是否在项目导航器中？
• 在文件检查器中是否选中了“ Target Membership复选框（要获取它，请选择文件并按⌥⌘1 ）
• 文件名的拼写是否正确（ iPhone100Voice.mp3 ）？

Answer 2

请检查您的文件名，也许文件名不正确或捆绑包中没有

因此，请检查带有扩展名的文件名。

if let或guard unwrap路径，请使用：

override func viewDidLoad() {

    super.viewDidLoad()
    var soundID:SystemSoundID = 0

    if let path = Bundle.main.path(forResource: "iPhone100Voice", ofType: "mp3") {
       let baseURL = NSURL(fileURLWithPath: path)
       AudioServicesCreateSystemSoundID(baseURL, &soundID)
       AudioServicesPlaySystemSound(soundID)
    } else {
        let alert = UIAlertController(title: "Alert", message: "File not found in your bundle", preferredStyle: UIAlertControllerStyle.alert)
        alert.addAction(UIAlertAction(title: "OK", style: UIAlertActionStyle.cancel, handler: nil))
        self.present(alert, animated: true, completion: nil)

   }
}

Answer 3

播放视频可能对您有所帮助。 将视频文件添加到项目中时，请确保在文件检查器中选择了“目标”复选框。

override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(true)


        if  let path = Bundle.main.path(forResource: "video_1523966087401 (1)", ofType: ".mp4"){
            let player = AVPlayer(url: URL(fileURLWithPath: path))
            let playerController = AVPlayerViewController()
            playerController.player = player
            present(playerController, animated: true) {
                player.play()
            }
        }else{
            //show dialog to user

            print("Error: File not found")
        }
    }

Answer 4

! 表示崩溃运算符，它解开了可选值。 比如说，那里的价值就像Optional（53）一样。 因此，当path!时，变量应包含nil以外的值path! 尝试解开可选内容并在路径变量中附加值53。

为了解决这个问题，我们可以使用两种方法

使用警卫

guard let pathValue = path else {
        return
}

let baseURL = NSURL(fileURLWithPath: pathValue) //no need to use pathValue, if you use then you have to use path!, although it makes no crash now.  
AudioServicesCreateSystemSoundID(baseURL, &soundID)
//play
AudioServicesPlaySystemSound(soundID)

如果path为nil，则进入该块并返回；如果它具有value，则继续到下一行。

使用if-let，了解optional binding

if let pathValue = path {
    let baseURL = NSURL(fileURLWithPath: pathValue) //same comment
    AudioServicesCreateSystemSoundID(baseURL, &soundID)
    //play
    AudioServicesPlaySystemSound(soundID)
} 

现在，如果path是可选的，则无法到达块并传递到下一行