Start playing sound in background on iOS?

Question

I'm trying to do something similar to Tile.app. When it shows a notification, it plays a sound. That seems simple enough-- use UILocalNotification and include the sound file name. But local notifications limit sounds to no more than 30 seconds, and Tile's sound keeps playing for a whole lot longer than that. I expect it's looping, and it continues until I can't stand the noise any more.

The sound also plays even if the phone's mute switch is on, which doesn't happen with local notifications. For these reasons, UILocalNotification appears to be out.

I thought maybe I could post a text-only local notification and have my app play the sound. But using AVAudioPlayer, the play method returns NO and the sound doesn't play.

Other questions have suggested that this is by design, that apps aren't supposed to be able to play sounds from the background-- only continue sound that's already playing. I'd accept that reasoning except that Tile does it, so it's obviously possible. One suggested workaround violates Apple's guidelines in a way that I'm pretty sure Apple checks for now.

Some details that may be relevant:

• I have the audio background mode in Info.plist
• I use AVAudioSessionCategoryPlayback on the audio session, and the session should be active (at least, setActive:error: claims to succeed).
• This app uses Bluetooth but does not currently use the bluetooth-central background mode.

Answer 1

Actually you can use one of AudioServices... functions

• AudioServicesPlaySystemSound
• AudioServicesPlayAlertSound
• AudioServicesPlaySystemSoundWithCompletion

to start playing sound in background.

I'm not sure about length of sound that can be submitted to AudioServicesCreateSystemSoundID because I'm checked only with short looped sound for ringing but this API not tied to notification so possible can overpass 30 second limit.

EDIT: App still requires audio in UIBackgroundModes to play in background

Cut from my test project appDelegate, iOS 9.3.1

func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?) -> Bool {
    // test sound in foreground.
    registerAndPlaySound()

    return true
}

func applicationDidEnterBackground(application: UIApplication) {
    var task =  UIBackgroundTaskInvalid
    task = application.beginBackgroundTaskWithName("time for music") {
        application.endBackgroundTask(task)
    }
    // dispatch not required, it's just to simulate "deeper" background  
    let delayTime = dispatch_time(DISPATCH_TIME_NOW, Int64(5 * Double(NSEC_PER_SEC)))

    dispatch_after(delayTime, dispatch_get_main_queue()) {
        self.registerAndPlaySound()
    }
}

func registerAndPlaySound() {
    var soundId: SystemSoundID = 0

    let bundle = NSBundle.mainBundle()
    guard let soundUrl = bundle.URLForResource("InboundCallRing", withExtension: "wav") else {
        return
    }
    AudioServicesCreateSystemSoundID(soundUrl, &soundId)

    AudioServicesPlayAlertSound(soundId)
}

Update

There I used beginBackgroundTask to start sound only as simplest way to execute code in background in another project similar code to registerAndPlaySound was called from PushKit callback without background task.

Answer 2

From official Apple documentation:

"Custom sounds must be under 30 seconds when played. If a custom sound is over that limit, the default system sound is played instead."

There is no way that you can make UILocalNotication to loop the sound. You can schedule it again with time intervals for 30s, but that one will be a schedulation of another alarm.

To setup your custom notification sound:

notif.soundName = @"sound.caf";

Make sure the sound is actually in your app’s bundle, is in the correct format (linear PCM or IMA4—pretty much anywhere that explains how to convert sounds for iOS will tell you how to do that), and is under 30 seconds.