ios - 如何检测股票音乐应用程序是否已终止？

Question

我正在使用 MPMusicPlayerController.systemMusicPlayer() 制作一个音乐播放器来学习 Swift，除了一个小问题外，一切都很顺利；每当从我的应用程序播放音乐，然后我切换到音乐应用程序并通过多任务处理终止它时，音乐会按预期停止，但当我切换到它时我的应用程序崩溃了。我在想这可能是因为我没有为音乐终止时应该发生的事情设定条件，而且我似乎无法在 Apple Docs 中找到任何真正有用的东西。我试过了

if (musicPlayer.playbackState == MPMusicPlaybackState.Interrupted) || (musicPlayer.playbackState == MPMusicPlaybackState.Stopped)

没有用。谁能提供一些关于为什么会发生这种情况和/或解决方案的见解？这是我的代码示例:

override func viewDidLoad() {
        super.viewDidLoad()
        var notificationCenter = NSNotificationCenter.defaultCenter()

        notificationCenter.addObserver(self, selector: "handlePlayingItemChanged", name: MPMusicPlayerControllerNowPlayingItemDidChangeNotification, object: musicPlayer)
        notificationCenter.addObserver(self, selector: "handlePlayState", name: MPMusicPlayerControllerPlaybackStateDidChangeNotification, object: musicPlayer)

        musicPlayer.beginGeneratingPlaybackNotifications()

        if (musicPlayer.playbackState == MPMusicPlaybackState.Playing) {
            playButton.setImage(pause, forState: UIControlState.Normal)
            setupCurrentMediaItem()
            handleShuffleAndReplayMode()
            self.isPlay = true
        } else if (musicPlayer.playbackState == MPMusicPlaybackState.Paused) {
            playButton.setImage(play, forState: UIControlState.Normal)
            self.isPlay = false
        } else if (musicPlayer.playbackState == MPMusicPlaybackState.Interrupted) || (musicPlayer.playbackState == MPMusicPlaybackState.Stopped) {
            playButton.setImage(play, forState: UIControlState.Normal)
            self.isPlay = false
        }
    }

Answer 1

如果你不移除观察者，你将始终抛出异常:

您无条件地添加了观察者以将其移除:

notificationCenter.addObserver...

移除观察者:

notificationCenter.removeObserver...

在这里查看更多信息 - Key-Value Observing Programming Guide