SingletonMusicPlayer playing overitself - swift

Question

I want a midi manager that means it can be called from anywhere...a Singleton instance...and handle music requests from different threads.
This is in the Playground
import PlaygroundSupport
import AudioToolbox
class MusicPlayerManager {
static let sharedInstance = MusicPlayerManager()
private init() {}
var musicPlayer : MusicPlayer? = nil
var sequence : MusicSequence? = nil
var track : MusicTrack? = nil
var time = MusicTimeStamp(1.0)
var player: OSStatus? = nil
var musicTrack: OSStatus? = nil
func playNotes(notes: [UInt8]) {
_ = NewMusicSequence(&sequence)
player = NewMusicPlayer(&musicPlayer)
player = MusicPlayerSetSequence(musicPlayer!, sequence)
player = MusicPlayerStart(musicPlayer!)
musicTrack = MusicSequenceNewTrack(sequence!, &track)
for index:Int in 0...6 {
var note = MIDINoteMessage(channel: 0,
note: notes[index],
velocity: 64,
releaseVelocity: 0,
duration: 1.0)
guard let track = track else {fatalError()}
musicTrack = MusicTrackNewMIDINoteEvent(track, time, &note)
time += 1
}
player = MusicPlayerSetSequence(musicPlayer!, sequence)
player = MusicPlayerStart(musicPlayer!)
}
}
var notes: [UInt8] = [71,69,62,72,71,69,67]
DispatchQueue.main.asyncAfter(deadline: .now() + 1) {
MusicPlayerManager.sharedInstance.playNotes(notes: notes)
}
DispatchQueue.global().asyncAfter(deadline: .now() + 1) {
MusicPlayerManager.sharedInstance.playNotes(notes: notes)
}
PlaygroundPage.current.needsIndefiniteExecution = true
Unfortunately the music often doesn't play at all, or plays over itself from each instance.
At worst it should play the first instance of the music; why doesn't it nicely do that?

Answer 1

I think you ran into a form of readers-writers problem here by attempting to simultaneously manipulate instance variables on your singleton from different threads. I don't know what your desired music player behavior is if it's being called from multiple threads at once, but for me one easy improvement would be to give your MusicPlayerManager singleton it's own DispatchQueue, where multiple overlapping requests can be handled sequentially.
You could define one like this:
let concurrentQueue = DispatchQueue(label: "com.stackoverflow.gamma", attributes: .concurrent)
And then wrap the code inside the playNotes function like this:
func playNotes(notes: [UInt8]) {
self.concurrentQueue.async(flags: .barrier) {
...
}
}