Crash after animateWithDuration because of bad access

Please see the below 开发者_开发知识库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?

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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.

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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.

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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];
}