How to trigger AVAudioPlayer playback using a remote push notification? - objective-c

Question

I'm creating an app that has a remotely-triggered alarm. Essentially, I'm trying to trigger a looping MP3 file to play (while the app is backgrounded) when a remote push notification arrives with a particular payload.
I've tried using didReceiveRemoteNotification: fetchCompletionHandler:, so that code can be run as a result of receiving a remote notification with a particular userInfo payload.
Here is my attempted didReceiveRemoteNotification: fetchCompletionHandler: from my AppDelegate.m:
- (void)application:(UIApplication *)application didReceiveRemoteNotification:(NSDictionary *)userInfo fetchCompletionHandler:(void (^)(UIBackgroundFetchResult))completionHandler
{
NSString *command = [userInfo valueForKeyPath:#"custom.a.command"];
if (command) {
UIApplicationState applicationState = [[UIApplication sharedApplication] applicationState];
if ([command isEqualToString:#"alarm"] && applicationState != UIApplicationStateActive) {
// Play alarm sound on loop until app is opened by user
NSLog(#"playing alarm.mp3");
NSString *soundFilePath = [[NSBundle mainBundle] pathForResource:#"alarm" ofType:#"mp3"];
NSURL *soundFileURL = [NSURL fileURLWithPath:soundFilePath];
NSError *error;
self.player = nil;
self.player = [[AVAudioPlayer alloc] initWithContentsOfURL:soundFileURL error:&error];
self.player.numberOfLoops = -1; // Infinitely loop while self.player is playing
self.player.delegate = self;
[self.player play];
}
}
completionHandler(UIBackgroundFetchResultNewData);
}
I expected the looping audio file to start playing as soon as the push notification arrived (with the app inactive or backgrounded), but it didn't. Instead, the audio playback surprisingly began when I then bring the app to the foreground.
What is missing from this approach, and/or can a different way work better?

Answer 1

You cannot start an audio session with the app in background. Audio sessions have to be initialized/started while the app is in foreground. An audio session properly initialized and running can continue if the app is pushed to background provided another app in foreground does not interrupt it.
Based on this information, I would say that your application likely has to start an audio session while you are in control and in foreground, keep the audio session alive while in background. Upon receiving the push notification, use the existing opened audio session to deliver audio out.
This has serious limitations since any other app, like Netflix, that uses a dedicated audio session may interrupt your app's audio session and prevent it from being able to play the MP3 when it arrives.
You may want to consider pre-packaging and/or downloading the MP3 ahead of time, and refer to them direcly in the Sound parameters of your push notification.
You may follow this tutorial to see how you can play custom sounds using push notifications: https://medium.com/#dmennis/the-3-ps-to-custom-alert-sounds-in-ios-push-notifications-9ea2a2956c11
func pushNotificationHandler(userInfo: Dictionary<AnyHashable,Any>) {
// Parse the aps payload
let apsPayload = userInfo["aps"] as! [String: AnyObject]
// Play custom push notification sound (if exists) by parsing out the "sound" key and playing the audio file specified
// For example, if the incoming payload is: { "sound":"tarzanwut.aiff" } the app will look for the tarzanwut.aiff file in the app bundle and play it
if let mySoundFile : String = apsPayload["sound"] as? String {
playSound(fileName: mySoundFile)
}
}
// Play the specified audio file with extension
func playSound(fileName: String) {
var sound: SystemSoundID = 0
if let soundURL = Bundle.main.url(forAuxiliaryExecutable: fileName) {
AudioServicesCreateSystemSoundID(soundURL as CFURL, &sound)
AudioServicesPlaySystemSound(sound)
}
}