CGRect not changing during animation - iphone

Question

I'm animating an UIView and want to check if its frame intersects with another UIView's frame. This is how I "spawn" one of the UIViews:
- (void) spawnOncomer
{
oncomer1 = [[Oncomer alloc] initWithType:#"car"];
[self.view addSubview:oncomer1];
//make the oncomer race across the screen
[UIView beginAnimations:nil context:NULL]; {
[UIView setAnimationCurve:UIViewAnimationCurveEaseInOut];
[UIView setAnimationDuration:3.0];
[UIView setAnimationDelegate:self];
CGRect f = oncomer1.frame;
f.origin.y = self.view.frame.size.height+oncomer1.frame.size.height;
oncomer1.frame = f;
[UIView setAnimationDidStopSelector:#selector(decountCar)];
}
[UIView commitAnimations];
}
So far so good. Now I want to check if this UIView and my other UIView collide by doing this:
- (void) checkCollision {
bool collision = CGRectIntersectsRect(car.frame, oncomer1.frame);
if (collision) {
NSLog(#"BOOOOOOOM");
} else {
NSLog(#"Oncomer: %#", NSStringFromCGRect(oncomer1.frame));
}
}
However, they never collide. Although I see oncomer1 moving across the screen, loggin oncomer1.frame never changes: it keeps outputting Oncomer: {{50, 520}, {30, 60}}(which are the post-animation values).
Does anyone know why this is?
P.s. Both methods are called directly or indirectly with a NSTimer and are thus performed in the background

Answer 1

UIView geometry updates apply immediately to their CALayer, even in an animation block. To get a version of a layer with animations applied, you can use -[CALayer presentationLayer], like this (warning - untested code):
- (void) checkCollision {
CGRect oncomerFrame = oncomer1.layer.presentationLayer.frame;
bool collision = CGRectIntersectsRect(car.frame, oncomerFrame);
if (collision) {
NSLog(#"BOOOOOOOM");
} else {
NSLog(#"Oncomer: %#", NSStringFromCGRect(oncomerFrame));
}
}

Answer 2

From your code:
CGRect f = oncomer1.frame;
f.origin.y = self.view.frame.size.height+oncomer1.frame.size.height;
oncomer1.frame = f;
A logical explanation of the frame never changing is that you are only changing the y of the frame, and you are always setting it to the same value determined by two heights.