How to wait for all group actions? - ios

Question

I can't wait for group of SKAction because completion handler executes immediately. Here is basic example:
let a1 = SKAction.runAction(SKAction.fadeAlphaTo(0.5, duration: duration), onChildWithName: child1.name!)
let a2 = SKAction.runAction(SKAction.fadeAlphaTo(1, duration: duration), onChildWithName: child2.name!)
runAction(SKAction.group([a1, a2]), completion: { () -> Void in
// do something
})
Nodes child1 and child2 are children of scene. When I run app on iPhone 5s (iOS 8.4) simulator I see that completion block doesn't wait while group actions finished and starts immediately.
So my two questions:
Is it s bug? If no, where is in SKAction documentation I can find explanation of this behaviour.
How to do this right? I know I can use counter and start each action a1 and a2 with completion block and just check when counter became 0 but it looks like a boilerplate code for me.

Answer 1

According to Apple's documentation, SKAction.runAction returns an action of instantaneous duration (that is 0 duration).
Thus in your code both SKActions, a1 and a2, have instantaneous duration. This is why the completion block is called immediately.
From Apple Documentation
SKAction.runAction : This action has an instantaneous duration,
although the action executed on the child may have a duration of its
own.
https://developer.apple.com/library/ios/documentation/SpriteKit/Reference/SKAction_Ref/#//apple_ref/occ/clm/SKAction/runAction:onChildWithName:
One way to set the completion handler would be to set it to one of the child nodes, since both actions have the same duration.
let a1 = SKAction.fadeAlphaTo(0.5, duration: 1);
let a2 = SKAction.fadeAlphaTo(1, duration: 1);
child1.runAction(a1)
child2.runAction(a2) { () -> Void in
//completion code
}