为什么MPRemoteCommandCenter播放命令后Swift AVAudioPlayer无法更改状态？

问题描述

此环境为iOS 13.6和Swift5。我有一个非常简单的应用程序，可以成功在前景或背景中播放MP3文件。我添加了MPRemoteCommandCenter播放和暂停命令处理程序。我在前台播放声音文件，然后暂停它。

当我从锁定屏幕上点击“播放”按钮时，我的代码将调用audioPlayer.play（），该返回true。我听到声音再次开始播放，但是播放器的currentTime不会前进。之后，锁定屏幕上的播放和暂停按钮什么也不做。当我再次显示应用程序前景时，播放按钮将从我进入锁定屏幕之前的位置播放。

这是我的AudioPlayer类：

import AVFoundation
import MediaPlayer

class AudioPlayer: RemoteAudioCommandDelegate {
    
    var audioPlayer = AVAudioPlayer()
    
    let remoteCommandHandler = RemoteAudioCommandHandler()
    
    var timer:Timer!

    func play(title: String) {
        let path = Bundle.main.path(forResource: title,ofType: "mp3")!
        let url = URL(fileURLWithPath: path)
            
        do {
            try AVAudioSession.sharedInstance().setCategory(AVAudioSession.Category.playback)
            try AVAudioSession.sharedInstance().setActive(true)

            audioPlayer =  try AVAudioPlayer(contentsOf: url)
            
            remoteCommandHandler.delegate = self
            remoteCommandHandler.enableDisableRemoteCommands(true)
            
            timer = Timer.scheduledTimer(timeInterval: 1.0,target: self,selector: #selector(updateNowPlayingInfo),userInfo: nil,repeats: true)
        } catch let error as NSError {
            print("error = \(error)")
        }
    }
    
    func play() {
        print ("audioPlayer.play() returned \(audioPlayer.play())")
    }
    
    func pause() {
        audioPlayer.pause()
    }
    
    func stop() {
        audioPlayer.stop()
    }
    
    func currentTime() -> TimeInterval {
        return audioPlayer.currentTime
    }
    
    func setCurrentTime(_ time:TimeInterval) {
        audioPlayer.currentTime = time
    }

    @objc func updateNowPlayingInfo() {
        // Hard-code the nowPlayingInfo since this is a simple test app

        var nowPlayingDict =
            [MPMediaItemPropertyTitle: "Tin Man",MPMediaItemPropertyAlbumTitle: "The Complete Greatest Hits",MPMediaItemPropertyAlbumTrackNumber: NSNumber(value: UInt(10) as UInt),MPMediaItemPropertyArtist: "America",MPMediaItemPropertyPlaybackDuration: 208,MPNowPlayingInfoPropertyPlaybackRate: NSNumber(value: 1.0 as Float)] as [String : Any]
        
            nowPlayingDict[MPNowPlayingInfoPropertyElapsedPlaybackTime] = NSNumber(value: audioPlayer.currentTime as Double)
        
        MPNowPlayingInfoCenter.default().nowPlayingInfo = nowPlayingDict
    }
}

这是我的RemoteCommandHandler类：

import Foundation
import MediaPlayer

protocol RemoteAudioCommandDelegate: class {
    func play()
    func pause()
}

class RemoteAudioCommandHandler: NSObject {
    weak var delegate: RemoteAudioCommandDelegate?
    
    var remoteCommandCenter = MPRemoteCommandCenter.shared()

    var playTarget: Any? = nil
    var pauseTarget: Any? = nil
    
    func enableDisableRemoteCommands(_ enabled: Bool) {
        print("Called with enabled = \(enabled)")
        
        remoteCommandCenter.playCommand.isEnabled = enabled
        remoteCommandCenter.pauseCommand.isEnabled = enabled
        
        if enabled {
            addRemoteCommandHandlers()
        } else {
            removeRemoteCommandHandlers()
        }
    }
    
    fileprivate func addRemoteCommandHandlers() {
        print( "Entered")
        
        if playTarget == nil {
            print( "Adding playTarget")
            playTarget = remoteCommandCenter.playCommand.addTarget { (event) -> MPRemoteCommandHandlerStatus in
                print("addRemoteCommandHandlers calling delegate play")
                self.delegate?.play()
                return .success
            }
        }
        
        if pauseTarget == nil {
            print( "Adding pauseTarget")
            pauseTarget = remoteCommandCenter.pauseCommand.addTarget { (event) -> MPRemoteCommandHandlerStatus in
                print("addRemoteCommandHandlers calling delegate pause")
                self.delegate?.pause()
                return .success
            }
        }

    }
    
    fileprivate func removeRemoteCommandHandlers() {
        print( "Entered")
        
        if playTarget != nil {
            print( "Removing playTarget")
            remoteCommandCenter.playCommand.removeTarget(playTarget)
            playTarget = nil
        }
        
        if pauseTarget != nil {
            print( "Removing pauseTarget")
            remoteCommandCenter.pauseCommand.removeTarget(pauseTarget)
            pauseTarget = nil
        }
    }

}

我很乐意提供进一步的必要信息，因为我对为什么这种相对简单的代码（在我看来）不起作用感到困惑。

非常感谢您的帮助！

解决方法

经过更多调试后，我发现AVAudioPlayer开始从锁定屏幕播放声音，但不久后又停止了播放。

我通过添加计时器来缓解此问题。计时器检查用户的最后命令是否正在播放，但声音没有播放。当用户选择暂停或歌曲自然停止播放时，我也会更改状态。

对于这个问题的实际解决方法，我仍然不知所措。