How to resume audio after interruption in Swift?

Question

I am following instructions here, I've put together this test project to handle interruptions to audio play. Specifically, I'm using the alarm from the default iphone clock app as interruption. It appears that the interruption handler is getting called but is not getting past the let = interruptionType line as "wrong type" showed up twice.

import UIKit
import AVFoundation

class ViewController: UIViewController {

    var player = AVAudioPlayer()

    let audioPath = NSBundle.mainBundle().pathForResource("rachmaninov-romance-sixhands-alianello", ofType: "mp3")!

    func handleInterruption(notification: NSNotification) {

        guard let interruptionType = notification.userInfo?[AVAudioSessionInterruptionTypeKey] as? AVAudioSessionInterruptionType else { print("wrong type"); return }

        switch interruptionType {

        case .Began:
            print("began")
            // player is paused and session is inactive. need to update UI)
            player.pause()
            print("audio paused")

        default:
            print("ended")
            /**/
            if let option = notification.userInfo?[AVAudioSessionInterruptionOptionKey] as? AVAudioSessionInterruptionOptions where option == .ShouldResume {
                // ok to resume playing, re activate session and resume playing
                // need to update UI
                player.play()
                print("audio resumed")
            }
            /**/
        }
    }

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.

        do {
            try player = AVAudioPlayer(contentsOfURL: NSURL(fileURLWithPath: audioPath))
            player.numberOfLoops = -1 // play indefinitely
            player.prepareToPlay()
            //player.delegate = player

        } catch {
            // process error here
        }

        // enable play in background https://stackoverflow.com/a/30280699/1827488 but this audio still gets interrupted by alerts
        do {
            try AVAudioSession.sharedInstance().setCategory(AVAudioSessionCategoryPlayback)
            print("AVAudioSession Category Playback OK")
            do {
                try AVAudioSession.sharedInstance().setActive(true)
                print("AVAudioSession is Active")
            } catch let error as NSError {
                print(error.localizedDescription)
            }
        } catch let error as NSError {
            print(error.localizedDescription)
        }

        // add observer to handle audio interruptions
        // using 'object: nil' does not have a noticeable effect
        let theSession = AVAudioSession.sharedInstance()
        NSNotificationCenter.defaultCenter().addObserver(self, selector: #selector(ViewController.handleInterruption(_:)), name: AVAudioSessionInterruptionNotification, object: theSession)

        // start playing audio
        player.play()
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }
}

Furthermore, following an idea here, I have modified the handler to

func handleInterruption(notification: NSNotification) {

        //guard let interruptionType = notification.userInfo?[AVAudioSessionInterruptionTypeKey] as? AVAudioSessionInterruptionType else { print("wrong type"); return }

        if notification.name != AVAudioSessionInterruptionNotification
            || notification.userInfo == nil{
            return
        }

        var info = notification.userInfo!
        var intValue: UInt = 0
        (info[AVAudioSessionInterruptionTypeKey] as! NSValue).getValue(&intValue)
        if let interruptionType = AVAudioSessionInterruptionType(rawValue: intValue) {

            switch interruptionType {

            case .Began:
                print("began")
                // player is paused and session is inactive. need to update UI)
                player.pause()
                print("audio paused")

            default:
                print("ended")
                /** /
                if let option = notification.userInfo?[AVAudioSessionInterruptionOptionKey] as? AVAudioSessionInterruptionOptions where option == .ShouldResume {
                    // ok to resume playing, re activate session and resume playing
                    // need to update UI
                    player.play()
                    print("audio resumed")
                }
                / **/
                player.play()
                print("audio resumed")
            }
        }
    }

Results are that all of "began", "audio paused", "ended" and "audio resumed" show up in console but audio play is not actually resumed.

Note: I moved the player.play() outside of the commented out where option == .ShouldResume if statement because that if condition is not true when the .Ended interruption occurs.

Answer 1

(Posted on behalf of the question author, after it was posted in the question).

Solution found! Following discussion here, inserted this in viewDidLoad()

do {
    try AVAudioSession.sharedInstance().setCategory(AVAudioSessionCategoryPlayback, withOptions: AVAudioSessionCategoryOptions.MixWithOthers)
} catch {        
}

After clicking "ok" on the alarm interruption, the audio play continued. Unlike previously noted, the solution does NOT require an interruption handler (which @Leo Dabus has since removed).

However if you are using an interruption handler, .play() must NOT be invoked within handleInterruption() as doing so does NOT guarantee play to resume & seems to prevent audioPlayerEndInterruption() to be called (see docs). Instead .play() must be invoked within audioPlayerEndInterruption() (any of its 3 versions) to guarantee resumption.

Furthermore, AVAudioSession must be give option .MixWithOthers noted by @Simon Newstead if you want your app to resume play after interruption when your app is in the background. It seems that if a user wants the app to continue playing when it goes into the background, it is logical to assume the user also wants the app to resume playing after an interruption while the app is in the background. Indeed that is the behaviour exhibited by the Apple Music app.