Lag in SKNode runAction:completion: in recursive method

Question

Please consider the following recursion:

- (void)addFlashActionToLampWithLampIndex:(int)index {
    LampNode *lamp = (LampNode *)self.children[index];
    int nextLampIndex = index + 1;
    if (nextLampIndex > self.children.count - 1) {
        nextLampIndex = 0;
    }

    SKAction *flash = [SKAction runBlock:^{
        NSLog(@"2");
    }];
//    SKAction *wait = [SKAction waitForDuration:0.015f];

    NSLog(@"1");
    [self runAction:flash completion:^{
        [self addFlashActionToLampWithLampIndex:nextLampIndex];
    }];
}

Output:

2014-03-27 20:51:21.104 SpinNWin[15811:60b] 1
2014-03-27 20:51:21.110 SpinNWin[15811:60b] 2
2014-03-27 20:51:21.110 SpinNWin[15811:60b] 1
2014-03-27 20:51:21.143 SpinNWin[15811:60b] 2
2014-03-27 20:51:21.144 SpinNWin[15811:60b] 1
2014-03-27 20:51:21.177 SpinNWin[15811:60b] 2
2014-03-27 20:51:21.177 SpinNWin[15811:60b] 1
2014-03-27 20:51:21.210 SpinNWin[15811:60b] 2
2014-03-27 20:51:21.210 SpinNWin[15811:60b] 1
2014-03-27 20:51:21.243 SpinNWin[15811:60b] 2

I'm expecting an execution of that method to be instant, without any delays or lags. However, as you can notice, there is a lag between NSLog(@"1"); and NSLog(@"2"); for about 0.03 sec.

Does anyone has ideas why this is happening and how to avoid this?

Answer 1

Because actions need time to progress. Actions are evaluated after the SKScene's update: method runs:

(Source: Sprite Kit Programming Guide)

Therefore if you do runAction: the action will not run until after the scene's update method. Depending on whether you run the action somewhere within the update: message chain, or in didEvaluateActions or didSimulatePhysics Sprite Kit may render one frame without having yet run the action.