Animations not working properly - ios

Question

I present a view controller modally in my application. I'd like for the user to be able to "flick" the view away with a gesture. I wrote the code below for that:
- (void)handlePan:(UIPanGestureRecognizer *)recognizer {
CGFloat elasticThreshold = 100;
CGFloat dismissThreshold = 200;
CGPoint translation = [recognizer translationInView:self.view];
CGFloat newY = 0;
CGFloat translationFactor = 0.5;
if (recognizer.state == UIGestureRecognizerStateEnded) {
if (translation.y < dismissThreshold) {
newY = 0;
}
} else {
if (translation.y > elasticThreshold) {
CGFloat frictionLength = translation.y - elasticThreshold;
CGFloat frictionTranslation = 30 * atan(frictionLength/120) + frictionLength/10;
newY = frictionTranslation + (elasticThreshold * translationFactor);
} else {
newY = translation.y*translationFactor;
}
}
if (translation.y > dismissThreshold) {
[UIView animateKeyframesWithDuration:0.5 delay:0.0 options:0 animations:^{
[UIView addKeyframeWithRelativeStartTime:0.0 relativeDuration:0.5 animations:^{
self.overlay.effect = nil;
self.collectionView.transform = CGAffineTransformMakeTranslation(0, self.view.frame.size.height);
}];
[UIView addKeyframeWithRelativeStartTime:0.1 relativeDuration:0.1 animations:^{
self.pageControl.frame = CGRectMake((self.view.frame.size.width-200)/2, self.view.frame.size.height, 200, 20);
}];
} completion:^(BOOL finished) {
if (finished) {
[self dismissViewControllerAnimated:YES completion:nil];
}
}];
} else {
self.collectionView.transform = CGAffineTransformMakeTranslation(0, newY);
self.pageControl.transform = CGAffineTransformMakeTranslation(0, (newY+self.collectionView.frame.size.height)-20);
}
}
This is hooked up to a UIGestureRecognizer:
self.pan = [[UIPanGestureRecognizer alloc] initWithTarget:self action:#selector(handlePan:)];
self.pan.delegate = self;
self.pan.maximumNumberOfTouches = 1;
[self.view addGestureRecognizer:self.pan];
However, what happens is that the completion block executes immediately. So you'll see the view move down (because of dismissViewControllerAnimated) and at the same time see the overlay.effect go away. However, what I would like is for my animations to happen and then for the view controller to dismiss itself silently.
Any ideas what's going wrong here?

Answer 1

This occurs because you are nesting UIView animation blocks. You should use dispatch_after for this:
double delayInSeconds = 0.5;
dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, delayInSeconds * NSEC_PER_SEC);
dispatch_after(popTime, dispatch_get_main_queue(), ^(void){
[self dismissViewControllerAnimated:YES completion:nil];
});

Answer 2

A UIView keyframe (or standard) animation will execute its completion block immediately if there is no work to be done. It won't wait for the duration of the animation if there is nothing to animate.
It seems that in your case when you are kicking off the dismiss animation, the views are already in their final state, hence there is no animation and the completion block is run immediately.