Source of UIView Implicit Animation delay?

Question

I have a block of UIView animation code that looks like this:

[UIView beginAnimations:@"pushView" context:nil];
[UIView setAnimationDelay:0];
[UIView setAnimationDuration:.5];
[UIView setAnimationDelegate:self];
[UIView setAnimationWillStartSelector:@selector(animationWillStart)];

view.frame = CGRectMake(0, 0, 320, 416);
[UIView commitAnimations];

The code basically mimics the animation of a ModalView presentation and is tied to a button on my interface. When the button is pressed, I get a long (.5 sec) delay (on iPod Touch...twice as fast on iPhone 3GS) before the animationWillStart: actually gets called. My app has lots going on besides this, but I've timed various points of my code and the delay definitely occurs at this block. In other words, a timestamp immediately before this code block and a timestamp when animationWillStart: gets called shows a .5 sec difference.

I'm not too experienced with Core Animation and I'm just trying to figure out what the cause of the delay is...Memory use is stable when the animation starts and CoreAnimation FPS seems to be fine in Instruments. The view that gets animated does have upwards of 20 total subviews, but if that were the issue wouldn't it cause choppiness after the animation starts, rather than before? Any ideas?

Answer 1

1. Try it with a single subview or with no subviews at all to make sure the delay is not caused by so many children.
2. Profile the code in Instruments to see where exactly the code lags. You might get down to some internal Core Animation function call that will hint you what’s going on.
3. Try the code without the “lot that’s going on” to make sure you’re not stepping on Core Animation’s toes with your other code.

Or, in short: experiment and measure, because conjectures seldom work when optimizing.

Answer 2

In your pasted block, you specify the selector animationWillStart (no colon), but later in your question, you refer to animationWillStart: (with colon). These selectors are not equivalent, so is it possible that your intended selector is never being called on account of this animation, and is being called 0.5 seconds later on account of some other animation?