Open Radar

 
Summary:
Attempting to use the `prepareToPlay(completionHandler:)` method on `MPMusicPlayerController.systemMusicPlayer()` fails because the callback is never invoked.

Steps to Reproduce:
Invoke the `prepareToPlay(completionHandler:)` method on `MPMusicPlayerController.systemMusicPlayer()`
Sample code in the context of a view controller:

override func viewDidLoad() {
    super.viewDidLoad()
    MPMediaLibrary.requestAuthorization { (status) in
        switch status {
        case .authorized:
            let musicPlayerController: MPMusicPlayerController = MPMusicPlayerController.systemMusicPlayer()
            musicPlayerController.setQueue(with: MPMediaQuery.songs())
            NSLog("About to call prepareToPlay")
            musicPlayerController.prepareToPlay(completionHandler: { (error) in
                guard error == nil else {
                    let alert = UIAlertController(title: "Error", message: "Error preparing to play. \(error!.localizedDescription)", preferredStyle: .alert)
                    alert.addAction(UIAlertAction(title: "OK", style: .default, handler: nil))
                    self.present(alert, animated: true, completion: nil)
                    return
                }
                NSLog("Ready to play")
                musicPlayerController.play()
            })
            
        case .denied, .notDetermined, .restricted:
            let alert = UIAlertController(title: "Access Denied", message: "You need to allow access to the media library.", preferredStyle: .alert)
            alert.addAction(UIAlertAction(title: "Settings", style: .default, handler: { (action) in
                UIApplication.shared.open(URL(string:UIApplicationOpenSettingsURLString)!)
            }))
            self.present(alert, animated: true, completion: nil)
        }
    }
}

Expected Results:

Console logs:
`About to call prepareToPlay`
`Ready to play`

Music plays.

Actual Results:

Console logs:
`About to call prepareToPlay`

No music plays and no error is displayed.

Regression:

Tested with iOS 10.1.1 on iPhone 6 and iPhone SE

Notes: