Error handling of AVAudioPlayer's contentsOfURL:error: in Swift 2

Question

我遵循了一个教程：how to create an MP3 Player in Swift，我遇到了语法在 Swift 1.2 和 Swift 2.0 之间发生变化的地方。

我遇到了以下方法的错误处理问题：

player = AVAudioPlayer(contentsOfURL: url, error: &error)

我知道我需要使用 try 和 catch 来 "Swift2-ify" 它。我已经完成了 Swift 1.2 代码的 "apples to oranges" 翻译，但我很难做到 "apples to apples"。

这里是 Swift 1.2.

教程中的相关 methods/declarations

var player: AVAudioPlayer?

func queueTrack(){
    if (player != nil) {
        player = nil
    }

    var error:NSError?
    let url = NSURL.fileURLWithPath(tracks[currentTrackIndex] as String)
    player = AVAudioPlayer(contentsOfURL: url, error: &error)

    if let hasError = error {
        //TODO: SHOW ALERT HERE
    } else {
        player?.delegate = self
        player?.prepareToPlay()
    }
}

这是我在 Swift 2.0 中尝试的。它运行，但我收到警告。

func queueTrack() {
    if (player != nil) {
        player =  nil
    }

    let url = NSURL.fileURLWithPath(tracks[currentTrackIndex] as String)
    // I get a warning to make 'var error' to 'let error' here
    // If I do what the compiler says, I get a warning the error isn't
    // initialized after 'catch' outside the curly braces
    var error: NSError? // TODO: figure out how to remove this warning


    do {
        player = try AVAudioPlayer(contentsOfURL: url)
    } catch {
        NSLog("Unresolved error \(error)")
        // SHOW ALERT OR SOMETHING
    }

    // Immutable value 'hasError' was never used; consider replacing
    // with '_' or removing it
    // If earlier declaration of error is changed to let, the warning turns
    // into an compiler error

    if let hasError = error {
        // show alert
    } else {
        player?.delegate = self
        player?.prepareToPlay()
    }
}

我的翻译有什么错误吗？

Answer 1

您不再需要var error: NSError?完全，删除它和相关行。

现在您处理 catch 块中可能出现的错误。

func queueTrack() {

    let url = NSURL.fileURLWithPath(tracks[currentTrackIndex] as String)

    do {
        player = try AVAudioPlayer(contentsOfURL: url)
        player?.delegate = self
        player?.prepareToPlay()
    } catch {
        NSLog("Unresolved error \(error)")
        // SHOW ALERT OR SOMETHING
    }

}

请注意 catch 块中的这个 error 变量不是与之前相同的变量，它是一个新变量（类型 ErrorType) 由 catch 块生成。

catch 块还有另一种语法：

    do {
        player = try AVAudioPlayer(contentsOfURL: url)
        player?.delegate = self
        player?.prepareToPlay()
    } catch let error as NSError {
        NSLog("Unresolved error \(error.debugDescription)")
        // SHOW ALERT OR SOMETHING
    }

这里的error不会是ErrorType而是像往常一样NSError。

Error handling of AVAudioPlayer's contentsOfURL:error: in Swift 2

Error handling of AVAudioPlayer's contentsOfURL:error: in Swift 2

avaudioplayer

ios

swift

swift2