iOS - AVAudioPlayer在后台时不会继续播放下一首歌曲

Question

所以我有一个应用程序，它可以播放一堆歌曲，而用户可以翻阅一本漫画书。我使用AVAudioPlayer，并将其设置为按设定的顺序播放歌曲。所以当一首歌结束时，下一首就会播放。当应用程序打开时，它可以完美地工作。当应用程序在后台时，问题就会发生。我将应用程序设置为在后台播放，这可以很好地工作。因此，当用户按下主屏幕时，音乐会继续播放。问题发生在歌曲结束时，它应该像打开应用程序时一样播放下一首歌曲。相反，什么都不会发生。根据my NSLog语句，调用了正确的方法，但什么也没有发生。下面是我的代码：

- (void)audioPlayerDidFinishPlaying: (AVAudioPlayer *)player successfully: (BOOL) flag {

NSLog(@"Song finished");

if ([songSelect isEqualToString: @"01icecapades"]) {
    isPlay = @"yes";
    songSelect = @"02sugarcube";
    imageSelect = @"playbanner02";
    [self performSelector:@selector(triggerSong) withObject:nil afterDelay:0];
    [self performSelector:@selector(triggerBanner) withObject:nil afterDelay:0];
}
else if ([songSelect isEqualToString: @"02sugarcube"]) {
    isPlay = @"yes";
    songSelect = @"03bullets";
    imageSelect = @"playbanner03";
    [self performSelector:@selector(triggerSong) withObject:nil afterDelay:0];
    [self performSelector:@selector(triggerBanner) withObject:nil afterDelay:0];
}
else if ([songSelect isEqualToString: @"03bullets"]) {
    isPlay = @"yes";
    songSelect = @"04satanama";
    imageSelect = @"playbanner04";
    [self performSelector:@selector(triggerSong) withObject:nil afterDelay:0];
    [self performSelector:@selector(triggerBanner) withObject:nil afterDelay:0];
}
else if ([songSelect isEqualToString: @"04satanama"]) {
    isPlay = @"yes";
    songSelect = @"05uglyjoke";
    imageSelect = @"playbanner05";
    [self performSelector:@selector(triggerSong) withObject:nil afterDelay:0];
    [self performSelector:@selector(triggerBanner) withObject:nil afterDelay:0];
}
else if ([songSelect isEqualToString: @"05uglyjoke"]) {
    isPlay = @"yes"; 
    songSelect = @"01icecapades";
    imageSelect = @"playbanner01";
    [self performSelector:@selector(triggerSong) withObject:nil afterDelay:0];
    [self performSelector:@selector(triggerBanner) withObject:nil afterDelay:0];
}}

上面的代码识别正在播放的歌曲，并设置下一首正确的歌曲。然后，它会触发另一个设置播放器的方法。

- (void)triggerSong {
NSLog(@"triggerSong called");
NSString       *path;
NSError        *error;
// Path the audio file
path = [[NSBundle mainBundle] pathForResource:songSelect ofType:@"mp3"];
// If we can access the file...
if ([[NSFileManager defaultManager] fileExistsAtPath:path]) 
{    
    // Setup the player
    player = [[AVAudioPlayer alloc] initWithContentsOfURL:[NSURL fileURLWithPath:path] error:&error];
    //player = [initWithContentsOfURL:[NSURL fileURLWithPath:path] error:&error];
    [player setDelegate: self];
    // Set the volume (range is 0 to 1)
    player.volume = 1.0f;     
    [player prepareToPlay];
    [player setNumberOfLoops:0];
    [player play];
    NSLog(@"player play");
    [error release];
    player.delegate = self;
    // schedules an action every second for countdown
    [NSTimer scheduledTimerWithTimeInterval:0.5 target:self selector:@selector(updateTimeLeft) userInfo:nil repeats:YES];
}}

现在我假设这不是最好的方法，但当应用程序处于前台状态时，它工作得很好。我一直在看文档，但似乎找不到这个问题的原因。我希望有人能够看到我的方法中的一个错误。就像我之前说的，triggerSong方法中的两个NSLogs被调用了，所以我不明白为什么AVAudioPlayer (播放器)没有被调用。

另外，我的info.plist中有正确的设置，我的viewDidLoad中也有以下内容：

//Make sure the system follows our playback status
[[AVAudioSession sharedInstance] setCategory:AVAudioSessionCategoryPlayback error:nil];
[[AVAudioSession sharedInstance] setActive: YES error: nil];

谢谢你的见解。非常感谢。

Answer 1

Relevant discussion

简短的回答：

您需要在第一个视图控制器的init或viewDidLoad方法中使用以下代码：

[[UIApplication sharedApplication] beginReceivingRemoteControlEvents];

详细答案W/示例：

下面是我的例子。像你一样，我一开始也有一个应用程序，可以在后台播放音乐，但在第一个剪辑结束后就再也无法继续播放了。我复制了原始的Music.mp3并将其命名为Music2.mp3。我的目的是在Music.mp3结束后立即播放Music2.mp3 (audioPlayerDidFinishPlaying:)。我在后台任务上游手好闲了一段时间，直到我在没有后台任务的情况下让它正常工作：

-(id)init{
    self = [super initWithNibName:@"MediaPlayerViewController" bundle:nil];
    if(self){

        //Need this to play background playlist
        [[UIApplication sharedApplication] beginReceivingRemoteControlEvents];

        //MUSIC CLIP
        //Sets up the first song...
        NSString *musicPath = [[NSBundle mainBundle] pathForResource:@"Music" ofType:@"mp3"];
        if(musicPath){
            NSURL *musicURL = [NSURL fileURLWithPath:musicPath];
            audioPlayer = [[AVAudioPlayer alloc] initWithContentsOfURL:musicURL error:nil]; 
            [audioPlayer setDelegate:self];
        }
    }
    return self;
}


-(IBAction)playAudioFile:(id)sender{

    if([audioPlayer isPlaying]){
        //Stop playing audio and change test of button
        [audioPlayer stop];
        [sender setTitle:@"Play Audio File" forState:UIControlStateNormal];
    }
    else{
        //Start playing audio and change text of button so
        //user can tap to stop playback
        [audioPlayer play];
        [sender setTitle:@"Stop Audio File" forState:UIControlStateNormal];
    }
}


-(void)audioPlayerDidFinishPlaying:(AVAudioPlayer *)player successfully:(BOOL)flag{
    [audioButton setTitle:@"Play Audio File" forState:UIControlStateNormal];
    [playRecordingButton setTitle:@"Play Rec File" forState:UIControlStateNormal];

    //PLAY THE SECOND SONG
    NSString *musicPath2 = [[NSBundle mainBundle] pathForResource:@"Music2" ofType:@"mp3"];
    if(musicPath2){

        NSURL *musicURL2 = [NSURL fileURLWithPath:musicPath2];
        audioPlayer = [[AVAudioPlayer alloc] initWithContentsOfURL:musicURL2 error:nil];
        [audioPlayer setDelegate:self];
        NSLog(@"Play it again: \n%@", musicPath2);
        [audioPlayer play];
    } 
}

最终的结果是，我的应用程序现在正在连续循环播放Music2.mp3，即使应用程序在后台。

Answer 2

为了确认Squatch所说的，这也是Swift中的解决方案：

UIApplication.sharedApplication().beginReceivingRemoteControlEvents()

Answer 3

OS在使用AVAudioPlayer时也出现了同样的问题，然而UIApplication是一个仅用于iOS的构造。OS需要使用NSApplication，但是NSApplication直到应用程序终止才会返回，所以我们需要使用线程。额外的好处是，在NSApplication内部的某个地方有一个assert()，它需要主线程。

此混合C++/Objective C函数是此OS X问题的一种解决方法：

void do_the_dumb (void real_application(void)) {
    std::thread thread ([real_application]() {
        real_application();
        [[NSApplication sharedApplication] terminate: [NSApplication sharedApplication]];
    });
    [[NSApplication sharedApplication] run];
    thread.join();
};