Collide Once then don't do it again - swift

Question

How do I make my SKnodes collide only once? I have an particle system that is appearing when they collide but if they collide again a bunch appears. I don't want that. Just want one then thats it. How would I go about pulling this effect off?
if bodyA.categoryBitMask == 1 && bodyB.categoryBitMask == 3 || bodyA.categoryBitMask == 3 && bodyB.categoryBitMask == 1 {
print("END GAME")
if let dieexplostionNode = SKEmitterNode(fileNamed: "Explosion.sks"){
dieexplostionNode.targetNode = self
dieexplostionNode.position = player.position
// add 3 actions
let wait = SKAction.wait(forDuration: 1.1)
let addExplosion = SKAction.run {
self.player.addChild(dieexplostionNode)
self.player.alpha = 0
}
let removePlayer = SKAction.run {
self.player.removeFromParent()
self.player.removeAllActions()
dieexplostionNode.removeFromParent()
}
// put them in a sequence.
let seq = SKAction.sequence([addExplosion,wait,removePlayer])
// run the sequence
self.run(seq)
}

Answer 1

Your issue is multiple contacts can happen, so you need to place a check in your code to tell your system you already evaluate a contact and do not want to do any other evaluation. My preferred method is to reserve bit 31 on the category bit mask flag, and if that is set, do not evaulate.
guard bodyA.categoryBitMask < 1 << 31 && bodyB.categoryBitMask < 1 << 31 else {return}
if bodyA.categoryBitMask == 1 && bodyB.categoryBitMask == 3 || bodyA.categoryBitMask == 3 && bodyB.categoryBitMask == 1 {
print("END GAME")
player.categoryBitMask += 1<<31
if let dieexplostionNode = SKEmitterNode(fileNamed: "Explosion.sks"){
dieexplostionNode.targetNode = self
dieexplostionNode.position = player.position
// add 3 actions
let wait = SKAction.wait(forDuration: 1.1)
let addExplosion = SKAction.run {
self.player.addChild(dieexplostionNode)
self.player.alpha = 0
}
let removePlayer = SKAction.run {
self.player.removeFromParent()
self.player.removeAllActions()
dieexplostionNode.removeFromParent()
}
// put them in a sequence.
let seq = SKAction.sequence([addExplosion,wait,removePlayer])
// run the sequence
self.run(seq)
}