ios - UIView animations with autoreverse

Question

I have a problem with the setting UIViewAnimationOptionAutoReverse. Here is my code.

CALayer *aniLayer = act.viewToChange.layer;
[UIView animateWithDuration:2.0 delay:1.0 options:(UIViewAnimationCurveLinear | UIViewAnimationOptionAutoreverse) animations:^{
                    viewToAnimate.frame = GCRectMake(1,1,100,100);
                    [aniLayer setValue:[NSNumber numberWithFloat:degreesToRadians(34)] forKeyPath:@"transform.rotation"];
                } completion:nil ];

The problem is that, after the animation has reversed, the view jumps back to the frame set in the animation block. I want the view to grow and "ungrow" and stop at its original position.

Is there a solution without programming two consecutive animations?

Answer 1

You have three options.

When you use the -[UIView animateWithDuration:…] methods, the changes you make in the animations block are applied immediately to the views in question. However, there is also an implicit CAAnimation applied to the view that animates from the old value to the new value. When a CAAnimation is active on a view, it changes the displayed view, but does not change the actual properties of the view.

For example, if you do this:

NSLog(@"old center: %@", NSStringFromCGPoint(someView.center));
[UIView animateWithDuration:2.0 animations: ^{ someView.center = newPoint; }];
NSLog(@"new center: %@", NSStringFromCGPoint(someView.center));

you will see that 'old center' and 'new center' are different; new center will immediately reflect the values of newPoint. However, the CAAnimation that was implicitly created will cause the view to still be displayed at the old center and smoothly move its way to the new center. When the animation finishes, it is removed from the view and you switch back to just seeing the actual model values.

When you pass UIViewAnimationOptionAutoreverse, it affects the implicitly created CAAnimation, but does NOT affect the actual change you're making to the values. That is, if our example above had the UIViewAnimationOptionAutoreverse defined, then the implicitly created CAAnimation would animate from oldCenter to newCenter and back. The animation would then be removed, and we'd switch back to seeing the values we set… which is still at the new position.

As I said, there are three ways to deal with this. The first is to add a completion block on the animation to reverse it, like so:

First Option

CGPoint oldCenter = someView.center;
[UIView animateWithDuration:2.0
                 animations: ^{ someView.center = newPoint; }
                 completion: 
   ^(BOOL finished) {
       [UIView animateWithDuration:2.0
                        animations:^{ someView.center = oldCenter; }];
   }];

Second Option

The second option is to autoreverse the animation like you're doing, and set the view back to its original position in a completion block:

CGPoint oldCenter = someView.center;
[UIView animateWithDuration:2.0
                      delay:0
                    options: UIViewAnimationOptionAutoreverse
                 animations: ^{ someView.center = newPoint; }
                 completion: ^(BOOL finished) { someView.center = oldCenter; }];

However, this may cause flickering between the time that the animation autoreverse completes and when the completion block runs, so it's probably not your best choice.

Third Option

The last option is to simply create a CAAnimation directly. When you don't actually want to change the final value of the property you're changing, this is often simpler.

#import <QuartzCore/QuartzCore.h>

CABasicAnimation *animation = [CABasicAnimation animationWithKeyPath:@"position"];
animation.autoreverses = YES;
animation.repeatCount = 1; // Play it just once, and then reverse it
animation.toValue = [NSValue valueWithCGPoint:newPoint];
[someView.layer addAnimation:animation forKey:nil];

Note that the CAAnimation way of doing things never changes the actual values of the view; it just masks the actual values with an animation. The view still thinks it's in the original location. (This means, for example, that if your view responds to touch events, it will still be watching for those touch events to happen at the original location. The animation only changes the way the view draws; nothing else.

The CAAnimation way also requires that you add it to the view's underlying CALayer. If this scares you, feel free to use the -[UIView animateWithDuration:…] methods instead. There's additional functionality available by using CAAnimation, but if it's something you're not familiar with, chaining UIView animations or resetting it in the completion block is perfectly acceptable. In fact, that's one of the main purposes of the completion block.

So, there you go. Three different ways to reverse an animation and keep the original value. Enjoy!