ios - How to cancel UIViews block-based animation?

Question

i currently focus the following problem:

i start an animation, where 2 objects-attributes are triggered.

the code is:

    [UIView animateWithDuration:0.3 animations:^{
        greyscaleImage.alpha     = 1;
        activityIndicator.alpha  = 1;
    } completion:^(BOOL f){
        if(f)
        {
            [activityIndicator startAnimating];
        }
    }];

which works fine.

the only problem i discovered is, that i have a 0.3 seconds change to crash the app when the view which holds this activityIndicator and greyscaleImage is deallocated.

To make it more clear please imagine a ViewController, its view presented via default iOS-modal-View ways. Now trigger that animation, which takes 2 minutes. before reaching that 2 minutes, you find that animation is quite boring and you want to dismiss that view. now, that the view, activityIndicator and greyscaleImage are released, the animation o/c cannot know what to do.

so i wonder, what to do here + why the debugger points to

  } completion:^(BOOL f){

instead of e.g. [activityIndicator ...

is there a way, to allow user to dismiss the view before the 2 minutes are over?

Best Regards

Answer 1

If you start a new animation that takes 0.0 seconds and goes to the state you want to go to, it will cancel the old one and start the new (instant) 'animation'.

Example for when you want to stop a moving view by going to the place it already is at:

[UIView animateWithDuration:0.0
                      delay:0.0
                    options:UIViewAnimationOptionBeginFromCurrentState
                 animations:^{movingView.frame = ((CALayer *)movingView.layer.presentationLayer).frame;}
                 completion:^(BOOL finished){}
 ];

options:UIViewAnimationOptionBeginFromCurrentState is important. Not calling it will let your animation start at the end state of the previous animation. In movement, it would warp to the end location before warping to the place you want it to stop at. Even though your cancel-'animation' is instant, the jumping back and forth may be visible.

Note: The animation time doesn't have to be 0.0 seconds, any animation will cancel the old one. Not entirely sure about different types of animations though. For example, I don't know if changing a frame would stop a fade.