NSLayoutConstraint.constant ignoring animation

Question

I\'m creating an autolayout-friendly split view class for one of my applications. Among its various features is that it can collapse panes, and can animate their collapse, much

Answer 1

The completion handler is firing immediately because it thinks there aren't any animations that need to be run. I would check and confirm that the animation you created is still attached to the view. By default CABasicAnimation is set to remove itself upon completion by way of the removedOnCompletion property it inherits from CAAnimation (which by default is set to YES).

you'll want to

anim.removedOnCompletion = NO;

Answer 2

I agree, this is pretty strange, and could well be a bug. I'd definitely report it as such because, to the best of my knowledge, this should work.

I was able to get it to work by using the NSAnimationContext class method +runAnimationGroup:completionHandler: instead of the beginGrouping and endGrouping statements:

[NSAnimationContext runAnimationGroup:^(NSAnimationContext* context){
    [constraint.animator setConstant:self.width];   
} completionHandler:^(void){
    [theView removeConstraint:constraint];
    NSLog(@"completed");
}];

Answer 3

You have to first set the completion handler and only then send the message to the animator proxy. Otherwise, it seems that setting the completion handler after the animation started fires it immediately and the constant is removed before the animation has time to finish. I have just checked this with a piece of simple code:

[NSAnimationContext beginGrouping];
NSAnimationContext.currentContext.duration = animagionDuration;
NSAnimationContext.currentContext.completionHandler = ^{
  [self removeConstraint:collapseConstraint];
};
[collapseConstraint.animator setConstant:expandedHeight];

[NSAnimationContext endGrouping]; This works perfectly, but if you set completion handler after -setConstant:, the animation does not have a chance to run.

Answer 4

I'm just getting to grips with this stuff myself so this may be a naive analysis but:

It seems to me that you are specifying that an animation on the constraints' properties (in your else block) but, then, immediately setting the reference to the constraint to nil (potentially releasing it) before the animation has a chance to run.

I would expect that you would want to set hiddenConstraint to nil from within, or triggered by, the animation completion block.

Note that if, as is likely, I am wrong I would appreciate a word or two about why to help me understand it better :)