Play a sound loop while at least one sprite of its class is on stage - swift

Question

I'm working on a game with some monsters of different kinds (one kind = one subclass of SKSpriteNode) crossing the scene.
To each kind is given a specific sound.
One example is I can have at a specific point in time:
5 monsters A
2 monsters B
0 monster C
What I would like at any time, is to loop a sound for each class which is part of the scene (A, B) (and not for each sprite !) , and to stop playing a sound for each class absent from the scene(C).
My first idea was to store a specific sound into each class of monster and to add a SKAction to each sprite that would play loop the sound of its class.
But this would play loop as many sounds as sprites on scene, and this doesn't match with my expectations.
Do you have an idea on the best approach to adopt for this ?
Is there a possibility to implement a kind of observer that would be able to notify when an instance of class is on the scene and when it is not ?
Many thanks for your help !

Answer 1

As #Knight0fDragon said, "SKAction is not designed to play looped music"
Maybe you can do: (I diden't test it, now I'm on windows machine)
Class MonsterA : SKSpriteNode {
static var monsterAInScene = Set<Monster>()
static let audioAction: SKAction = SKAction.playSoundFileNamed("MonsterA-audio") //Change to AVAudio for loop
static let audioNode: SKNode()
init (scene:SKScene) {
//Your init
if let MonsterA.monsterAInScene.count == 0 {
//play audio
scene.addChild(MonsterA.audioNode)
MonsterA.playAudio()
}
MonsterA.monsterAInScene.insert(self)
}
static func playAudio() {
//Play audio code
if MonsterA.audioNode.actionFroKey("audio") != nil {return}
MonsterA.audioNode.runaction(MonsterA.audioAction, forKey "audio")
}
override removeFromParent() {
MonsterA.monsterAInScene.remove(self)
if let MonsterA.monsterAInScene.count == 0 {
//stop audio code
MonsterA.audioNode.removeActionWithKey("audio")
MonsterA.audioNode.removeFromParent()
}
super.removeFromParent()
}
}
#Knight0fDragon and #Alessandro Ornano are more expert than me, they can suggest if is the best way to do.

Answer 2

I have finally adopted SKAudioNode as advised bu KnightOfDragon.
At the beginning of the GameScene class, I have added an observer to check the actual number of monster of each kind (Here with kind A):
var monsterASound = SKAudioNode()
var currentNumberOfMonsterA = 0 {
didSet {
if currentNumberOfMonsterA > 0 {
monsterASound.runAction(SKAction.play())
} else{
monsterASound.runAction(SKAction.stop())
}
}
}
currentNumberOfMonsterA is updated each time I add/remove a monster
//Add a monster A
self.currentNumberOfMonsterA += 1
//Removing a monster A
self.currentNumberOfMonsterA -= 1
//Destroying a monster A
self.currentNumberOfMonsterA -= 1
Then in the didMoveToView, I have added the SKAudioNode to the scene
self.monsterASound = SKAudioNode(URL: NSBundle.mainBundle().URLForResource("monsterA", withExtension: "wav")!)
self.monsterASound.runAction(SKAction.stop())
addChild(helicoSound)