Swift AVMIDIPlayer

MidIPlayer

/// soundbanks are either dls or sf2. see http://www.sf2midi.com/
var soundbank:NSURL!
var mp:AVMIDIPlayer!

func playMIDIFile() {

        // Load a SoundFont or DLS file.
        self.soundbank = NSBundle.mainBundle().URLForResource("GeneralUser GS MuseScore v1.442", withExtension: "sf2")
        
        // a standard MIDI file.
        var contents:NSURL = NSBundle.mainBundle().URLForResource("ntbldmtn", withExtension: "mid")
        
        var error:NSError?
        self.mp = AVMIDIPlayer(contentsOfURL: contents, soundBankURL: soundbank, error: &error)
        if self.mp == nil {
            println("nil midi player")
        }
        if let e = error {
            println("Error \(e.localizedDescription)")
        }
        self.mp.prepareToPlay()
        
        self.mp.play(nil)
        
        // there is a crash when you use a completion 
        //self.mp.play({
        //    println("midi done")
        //})
        
  
        // or
//        var completion:AVMIDIPlayerCompletionHandler = {println("done")}
//        mp.play(completion)
        
    }

func toggleMIDIPlayer() {
        if mp.playing {
            mp.stop()
        } else {
            self.mp.play(nil)
        }
}
    
func stopMIDIPLayer() {
        if mp.playing {
            mp.stop()
        }
}

This doesn't seem to work using Swift 3. println no longer works, and self flags errors. I've tried modifying the code, and got rid of all the errors, but the midi file only plays for a micro-second. It seems some way of "holding on" to the process is needed.
NB: if soundbank is Nil, then it uses the default DLSMusicDevice bank. I haven't found a way of specifying it explicitly.