iOS - AVAudioPlayer doesn't continue to next song while in background

Question

So I have an app that plays a bunch of songs while the user can flip through a comic book. I use AVAudioPlayer and I have it set up to play the songs in a set order. So when one song finishes, the next one will play. This works flawlessly when the app is open. The problem occurs when the app is in the background. I set up the app to play in the background, and that works fine. So when the user presses the home screen the music continues to play. The problem occurs when the song ends, it is suppose to play the next song like it does when the app is open. Instead nothing happens. According to the my NSLog statements the correct methods are being called but nothing happens. Here is my code:

- (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];
}}

Above is the code that recognizes which song is playing, and sets the correct song next. Then it triggers another method that sets up the player.

- (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];
}}

Now I assuming this is not the best way to do this, but it works great when the app is in the foreground state. I've been looking through the documentation and I can't seem to find the cause of this problem. I was hoping somebody might be able to see an error to my approach. Like I said before, the two NSLogs in the triggerSong method are being called so I can't see why the AVAudioPlayer (player) is not being called.

Also I have the correct setting in my info.plist and I have this in my viewDidLoad:

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

Thanks for any insight. Much appreciated.

Answer 1

Relevant discussion

SHORT ANSWER:

You need this code in either your first view controller's init or viewDidLoad method:

[[UIApplication sharedApplication] beginReceivingRemoteControlEvents];

---

LONG ANSWER W/ SAMPLE:

Here is my example. Like you, I began with an app that would play music in the background but could never continue playing after the first clip ended. I made a copy of the original Music.mp3 and named it Music2.mp3. My intention was to play Music2.mp3 as soon as Music.mp3 ended (audioPlayerDidFinishPlaying:). I goofed around with the background tasks for awhile until I got this working WITHOUT the background task:

-(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];
    } 
}

The end result is that my app is now playing Music2.mp3 on a continuous loop, even if the app is in the background.

Answer 2

Just to confirm what Squatch said, this is also the solution in Swift:

UIApplication.sharedApplication().beginReceivingRemoteControlEvents()

Answer 3

OS X exhibits the same problem using AVAudioPlayer, however UIApplication is an iOS-only construct. OS X requires using NSApplication instead, but NSApplication doesn't return until the application is terminating so we need to use threads. As a bonus, there's an assert() somewhere in the depths of NSApplication that demands the main thread.

This hybrid C++/Objective C function is one workaround for this OS X issue:

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();
};