UIViewControllerTransitioningDelegate dismiss animation with NSLayoutConstraint issue - ios

Question

1) I have a task to present and dismiss modal UIViewController with custom animation.
2) Custom animation is to change alpha and move one child element
3) I created FadeInAnimationController and FadeOutAnimationController classes to implement UIViewControllerAnimatedTransitioning like this:
- (void)animateTransition:(id<UIViewControllerContextTransitioning>)transitionContext
{
// obtain state from the context
CIToViewController *toViewController = (CIToViewController *)[transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];
// obtain the container view
UIView *containerView = [transitionContext containerView];
// set the intial state
toViewController.view.alpha = 0.0f;
toViewController.elementBottomPosition.constant -= 20.0f;
[toViewController.view layoutIfNeeded];
// add the view
[containerView addSubview:toViewController.view];
// animate
[UIView animateWithDuration:[self transitionDuration:transitionContext]
animations:^{
toViewController.view.alpha = 1.0f;
toViewController.elementBottomPosition.constant += 20.0f;
[toViewController.view layoutIfNeeded];
}
completion:^(BOOL finished) {
[transitionContext completeTransition:YES];
}];
}
4) elementBottomPosition is NSLayoutConstraint and it works fine for Present animation
5) PROBLEM:
For Dismiss animation NSLayoutConstraint doesn't work, so I had to do the same thing using Frame and it worked. It is not very good with AutoLayout and iOS7, but since I need to dismiss this view I don't care for its autolayout.
So the question is why NSLayoutConstraint approach doesn't work??? I logged constraints in animateTransition:
NSLog(#"constraints %#", fromViewController.view.constraints);
And they are still present.

Answer 1

Do not set auto layout constants in your animation block.
toViewController.elementBottomPosition.constant -= 20.0f;
[self.view layoutIfNeeded];
toViewController.elementBottomPosition.constant += 20.0f;
//Animation block here ^{
[self.view layoutIfNeeded];
}