For loop in swift seems to randomly choose increment - ios

Question

I have a for loop that executes animations and then removes them on completion, and I am trying to call another method upon completion. Here is my code:
func animateMatch(completion: () -> ()) {
var movingShape = level.shapeAtColumn(pointsContainingShapeArray[0].Column, row: pointsContainingShapeArray[0].Row)
let destinationShape = level.shapeAtColumn(pointsContainingShapeArray[0].Column, row: pointsContainingShapeArray[0].Row)
for everyShape in 1..<pointsContainingShapeArray.count {
movingShape = level.shapeAtColumn(pointsContainingShapeArray[everyShape].Column, row: pointsContainingShapeArray[everyShape].Row)
let Duration: NSTimeInterval = 1
let moveShapes = SKAction.moveTo((destinationShape?.sprite!.position)!,duration: Duration)
moveShapes.timingMode = .EaseOut
movingShape?.sprite?.runAction(moveShapes, completion: {
movingShape?.sprite?.removeFromParent()
print("Removed shape \(everyShape)")
if everyShape == pointsContainingShapeArray.count {
completion()
}
})
}
}
Basically the idea is that every shape in the array after the first position moves to the position of the first one and then removes it from the scene. This works fine, but my completion was getting called at random times every time. So finally I added the print statement in there. Here was the output:
Removed shape 3
Removed shape 1
Removed shape 4
Removed shape 2
At this point I got so frustrated I decided it was time for stack overflow. I can't possibly figure out what's going on here. Thanks for any help!
Here is the code that calls this method:
func handleSwipe(move: Move) {
view.userInteractionEnabled = false
level.performMove(move)
scene.animateMove(move) {
self.view.userInteractionEnabled = true
if hasMatch {
self.view.userInteractionEnabled = false
self.scene.animateMatch() {
self.scene.addNewSpritesForShapes(pointsContainingShapeArray)
hasMatch = false
self.view!.userInteractionEnabled = true
}
}
}
}

Answer 1

The issue isn't the for loop, it's the asynchronous nature of the run action.
When you call moveShapes action, it runs asynchronously before calling the completion back on the original thread. This can happen in any order. You can see this yourself by calling your print synchronously:
for everyShape in 1..<pointsContainingShapeArray.count {
print("Synchronous loop: \(everyShape)")
}
I think you'd be better off using a dispatch_group_t for your final completion:
func animateMatch(completion: () -> ()) {
var movingShape = level.shapeAtColumn(pointsContainingShapeArray[0].Column, row: pointsContainingShapeArray[0].Row)
let destinationShape = level.shapeAtColumn(pointsContainingShapeArray[0].Column, row: pointsContainingShapeArray[0].Row)
// HERE
let group = dispatch_group_create()
dispatch_group_notify(group, dispatch_get_main_queue(), completion)
//
for everyShape in 1..<pointsContainingShapeArray.count {
// HERE
dispatch_group_enter(group)
//
movingShape = level.shapeAtColumn(pointsContainingShapeArray[everyShape].Column, row: pointsContainingShapeArray[everyShape].Row)
let Duration: NSTimeInterval = 1
let moveShapes = SKAction.moveTo((destinationShape?.sprite!.position)!,duration: Duration)
moveShapes.timingMode = .EaseOut
movingShape?.sprite?.runAction(moveShapes, completion: {
movingShape?.sprite?.removeFromParent()
print("Removed shape \(everyShape)")
// HERE
dispatch_group_leave(group)
//
})
}
}