Start playing an audio file when application goes in background - ios

Question

It's like "Google Maps" navigate in the background.
I have added "App plays audio" to "Required background modes" already, but it doesn't work.
What's wrong with it?
Here is the sample source code:
-(void)applicationDidEnterBackground:(UIApplication *)application
{
UIDevice* device = [UIDevice currentDevice];
BOOL backgroundSupported = NO;
if ([device respondsToSelector:#selector(isMultitaskingSupported)])
{
backgroundSupported = device.multitaskingSupported;
}
if (backgroundSupported && _bgTask==UIBackgroundTaskInvalid )
{
UIApplication* app = [UIApplication sharedApplication];
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
while (app.applicationState==UIApplicationStateBackground && _bgTask!=UIBackgroundTaskInvalid)
{
[NSThread sleepForTimeInterval:3];
[self playAudio];
}
[app endBackgroundTask:_bgTask];
_bgTask = UIBackgroundTaskInvalid;
});
}
}
-(void)playAudio {
[[AVAudioSession sharedInstance] setCategory:AVAudioSessionCategoryPlayback error:nil];
[[AVAudioSession sharedInstance] setActive: YES error: nil];
NSURL *audioFileLocationURL = [[NSBundle mainBundle] URLForResource:#"sound" withExtension:#"caf"];
NSError *error;
self.audioPlayer = [[AVAudioPlayer alloc] initWithContentsOfURL:audioFileLocationURL error:&error];
[self.audioPlayer setNumberOfLoops:-1];
self.audioPlayer.delegate = self;
[self.audioPlayer prepareToPlay];
[self.audioPlayer play];
}
The url of audio file maybe is from web, and will play multiple audio file in a queue.

Answer 1

From Apple's doc on the subject:
Because it's likely any background tasks you start in
applicationDidEnterBackground: will not run until after that method
exits, you should request additional background execution time before
starting those tasks. In other words, first call
beginBackgroundTaskWithExpirationHandler: and then run the task on a
dispatch queue or secondary thread.
From my own experience of playing background audio, if you don't request extra time you need to set things up beforehand so that [self.audioPlayer play] is literally the first line of code in applicationDidEnterBackground, otherwise the system suspends the app before the audio has a chance to kick in and keep it awake.