Crash after animateWithDuration because of bad access

Question

Please see the below code. It shows animation effect with image.

UIImageView *imgView = [[UIImageView alloc] initWithImage:img];  // ref count = 1
int offX = imgView.frame.size.width / 6;
int offY = (imgView.frame.size.height - 44) / 6;
imgView.frame = CGRectMake(offX, offY, imgView.frame.size.width - offX * 2, 
                           (imgView.frame.size.height - 44) - offY * 2);
[overlayView addSubview:imgView];  // ref count = 2
[imgView release];  // ref count = 1
[UIView animateWithDuration:1.0 animations:^{
    [imgView setCenter:CGPointMake(TRASH_X, TRASH_Y)];
    [imgView setTransform:CGAffineTransformMakeScale(0.1, 0.1)];
    [imgView setAlpha:0.5];
} completion:^(BOOL finished) {
    [imgView removeFromSuperview]; // ref count = 0? destroy imgView object?
}];

From the above code, if I remove [imgView release] there is no crash. but, the above code makes crash from the completion routine. Why? As I think reference count changed as like the comment. Then there should be no crash.

Could you please explain why the completion routine makes crash?

see also http://stackoverflow.com/questions/5957826/using-completion-with-animatewithduration-causes-exc-bad-access — tribalvibes, Feb 13 '12 at 00:00

score 2 · Accepted Answer · answered Apr 29 '11 at 18:02

OK. I solved my problem. In fact, problem was not in the code I showed. There was no problem in this code.

The reason of crash is related with 'img' object which is used as initial argument of UIImageView. Actually, I accidentally released it one more time from the other place.

I think when UIImageView object deallocated it try to release it's Image object. But, it's already deallocate so, crash happend. It just looks like [imgView removeFromSuperview] makes crash.

Sorry for fuss.

score 1 · Answer 2 · answered Apr 29 '11 at 16:06

1

Here is my two cents: Once you release imgView (//ref count = 1) that variable no longer contains a reference to your object. If you plan on using it further ahead, like I see you are doing with the animation, you should release it afterwards. Hope this helps.

answered Apr 29 '11 at 16:06

sunshineDev

323
3
10

Sorry, but this is not the right assumption. after add view to it's superview it's retained by superview. So, there are no problem to release it and use it until call removeFromSuperview. – Wonil Apr 29 '11 at 18:04

Moshe · Answer 3 · 2011-04-29T16:28:49.767

Although the image view is still referenced by the view which you've added it to, you don't have any references to it. Once you call [imgView release], before the animate block, you've lost all of your references to the image view. The rule of thumb is that every alloc, copy, and retain requires a matching release or autorelease. Although the overlayView internally retains a reference to the view, you did not retain it anywhere, so as soon as you release it, it's gone.

I suggest autoreleasing the UIImageView instead. (Or, you may be able to put the release in the completion handler.)

Sample Code:

UIImageView *imgView = [[[UIImageView alloc] initWithImage:img] autorelease];  
int offX = imgView.frame.size.width / 6;
int offY = (imgView.frame.size.height - 44) / 6;
imgView.frame = CGRectMake(offX, offY, imgView.frame.size.width - offX * 2, 
                           (imgView.frame.size.height - 44) - offY * 2);
[overlayView addSubview:imgView]; 
[UIView animateWithDuration:1.0 animations:^{
    [imgView setCenter:CGPointMake(TRASH_X, TRASH_Y)];
    [imgView setTransform:CGAffineTransformMakeScale(0.1, 0.1)];
    [imgView setAlpha:0.5];
} completion:^(BOOL finished) {
    [imgView removeFromSuperview]; 
}];

EDIT:

You can try changing [imageView removeFromSuperview]; to:

if(imageView){
  [imageView removeFromSuperview];
}

Thanks. But, I already tried this way and result is same. crashed. — Wonil, Apr 29 '11 at 16:25
Can you use breakpoints to pinpoint exactly where the crash occurs? — Moshe, Apr 29 '11 at 16:27
Moshe. even if imageView is nil. There should be no problem send message to it. As you know, send message to nil just do nothing from Objective C. Please see my own answer. BTW - Thanks for your help. — Wonil, Apr 29 '11 at 18:07
If you have BAD ACCESS problem even if you check "if (imageView)" you will get crush. If *imageView* was released by the moment of completion any attempt to read its value will give BAD ACCESS. — malex, Jan 09 '14 at 06:57

Crash after animateWithDuration because of bad access

3 Answers3