Question

I’m trying to implement multistage animation using UIViewAnimationOptionBeginFromCurrentState to allow the user to cancel the animation at will. The animation is a view that continually and cyclically animates between two sizes. When the user touches the view to cancel the animation, I want the view to quickly revert back to its original, small size, whether it was growing or shrinking at the time.

I’m implementing the multistaging aspect by having two separate animations, one for growing the view and one for shrinking it. Each calls the other routine in its completion block, thus cycling forever unless the abort flag has been set.

I get the expected behaviour if abort is called during the grow-the-view animation: the animation quickly and immediately returns the view to its original, small size and stops. Good!

However, if abort is called during the shrink-the-view animation cycle, the view continues to shrink at the same speed (and then stops as expected), as if the UIViewAnimationOptionBeginFromCurrentState option was never invoked.

Code will hopefully make this clearer and hopefully somebody can see what I can’t.

- (void)stopAnimating {
    abort = YES;

    [UIView animateWithDuration:.2 // some small interval
                          delay:0
                        options:UIViewAnimationOptionBeginFromCurrentState
                     animations:^{ self.frame = minRect;}
                     completion:^(BOOL done){}
     ];
}

- (void)animateSmall {
    [UIView animateWithDuration:4
                          delay:0
                        options:UIViewAnimationOptionAllowUserInteraction
                     animations:^{self.frame = minRect;}
                     completion:^(BOOL done){if (!abort)[self animateBig];}
     ];
}

- (void)animateBig {
    [UIView animateWithDuration:4
                          delay:0
                        options:UIViewAnimationOptionAllowUserInteraction

                     animations:^{self.frame = maxRect;}
                     completion:^(BOOL done){if (!abort)[self animateSmall];}
     ];
}

Answer 1

Just a guess here, because your code looks exactly as I would have done it. But I think what’s going on is that the abort animation is setting the same attribute to the same value as the animation it’s interrupting, and this gets treated as equivalent and not in need of change (even though the duration changes).

A test of this theory – and a fix to the problem – would be to make your oscillating minRect just a little bit different in size than your steady state minRect.

Hope this works. Good luck.