动画进入后台后使App崩溃

Question

After looking for some feedback on how to clean up animations once they've finished I implemented the following;

- (void)onAnimationComplete:(NSString *)animationID finished:(NSNumber *)finished context:(void *)context {
    UIImageView *AnimationImageEnd = context;
    [AnimationImageEnd removeFromSuperview];
    [AnimationImageEnd release];
}

and to set them off (I've removed some of the frames and rotation calcs);

UIImageView * AnimationImageStart = [[UIImageView alloc] initWithImage: AnimationImage];
[self addSubview:AnimationImageStart];
[UIView beginAnimations:nil context:AnimationImageStart];
[UIView setAnimationDuration:iSpeed];
AnimationImageStart.transform = transform;
[UIView setAnimationDidStopSelector:@selector(onAnimationComplete:finished:context:)];
[UIView setAnimationDelegate:self];
[UIView commitAnimations];

This seems to be working fine and cleaning up all used memory once the animation completes.

Since implementing this approach I have now got issues with re-opening the App once it goes into the background if an animation had started before hand. The App is now crashing with the following error;

*** -[UIImage _isDecompressing]: message sent to deallocated instance 0x21083ad0

I've had a tinker and it appears to be all down to the release used. If I remove the [AnimationImageEnd release]; the crashes stop (but it obviously starts some memory leaks off..).

Is there something simple I am misunderstanding about the Animations and the App entering / re-opening from the background?

My guess is that iOS is automatically cleaning up any animations in progress once it enters the background and automatically fires the release to each. If this is the case, should I just simply expect them to have been cleaned up and hold a flag within the applicationDidBecomeActive to bypass the release?

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Edit 1 I've read that the finished flag on onAnimationComplete will let me know if the animation completed it's full animation or not. Is it enough to just trust that if it did not finish it has already been cleaned up (removed from superview and released)?

---

Edit 2

As per David's response below I have updated this to a block;

[UIView animateWithDuration:5.0f
                          delay:0.0f
                        options:0
                     animations:^{

                         imgStart.frame = CGRectMake( 0,  0, 10.0f, 10.0f);

                     }
                 completion:^(BOOL finished){

                     [imgStart removeFromSuperview];

                     //This still crashes the App as soon as the App returns from being in the background
                     [imgStart release];

                 }];

The same behaviour as detailed above is still present. As per Edit 1, is it enough to just assume that whenever the finished BOOL is NO, it has already been cleaned up (removed from superview + released)?

---

Any information is greatly appreciated.

Ralph

Answer 1

First, please don't use an initial capital letter for an object - reserve those for classes (it makes reading code much harder for most of us).

Generally it's bad practice to release objects in callbacks. Use a dispatch block to the main queue to release the object or a performSelectorOnMainthread:, and see if that helps.

EDIT: didn't mean to imply you needed a complete block. Reading the docs again shows no indication of retain change if the animation does not complete. Since you have a release in your code, you are not using ARC.

Non-ARC: so imgStart is alloc/inited, no autorelease, so its retain count is 1. You add it to the subview, its now two. You (or the system) removes it from the subview, its now 1. The final release makes it 0 (I'm ignoring the retain/release by the block itself). I just cannot see how the system could release it, as its not really anywhere it can be seen (its not an ivar, so viewDidUnload cannot release it). Right now this is a mystery.

Suppose you converted to ARC: imgStart is released at the start of the animation, two things retain it, the subview array and the block. When its removed from the subview array (by whoever), retain count goes down by 1, and when the block completes it will release it as well, causing the retain count to go to 0 so it will be released.

If nothing above rings a bell or helps, you can at least try and find when/where that object is getting dealloced. Create a UIImageView subclass in your .m file:

@implementation MyIV : UIIMageView
@end
@interface MyIV

- (void)dealloc
{
  [super dealloc];
  NSLog(@"MYIV DEALLOC!!!!");    
}
@end

Put a breakpoint on the log message, and you can see exactly where its getting dealloc'd. I've found this so helpful in the past I created an ARC class to do this even for class clusters on github, called Tracker .