Question

I’m trying to understand Automatic Reference Counting, as I come from a high-level programming language (Python) and I’m working on a project which use this feature of Objective-C. I often get problems with ARC deallocating objects which I need later, but now I got a concrete example for which I hope I’ll get an explanation.

- (void) animateGun:(UIImageView *)gun withFilmStrip:(UIImage *)filmstrip{
  NSMutableArray *frames = [[NSMutableArray alloc] init];
  NSInteger framesno = filmstrip.size.width / gun_width;
  for (int x=0; x<framesno; x++){
    CGImageRef cFrame = CGImageCreateWithImageInRect(filmstrip.CGImage, CGRectMake(x * gun_width, 0, gun_width, gun_height));
    [frames addObject:[UIImage imageWithCGImage:cFrame]];
    CGImageRelease(cFrame);
  }
  gun.image = [frames objectAtIndex:0];
  gun.animationImages = frames;
  gun.animationDuration = .8;
  gun.animationRepeatCount = 1;
  [gun startAnimating];
  dispatch_after(dispatch_time(DISPATCH_TIME_NOW,(arc4random() % 300)/100 * NSEC_PER_SEC), dispatch_get_current_queue(),^{
    [self animateGun:leftGun withFilmStrip:[self getFilmStripForAction:gunShoot andTeam:nil withWeapon:nil]];
  });
}

The idea behind this snippet of code is simple: I have a (UIImageView*)gun which I animate with the images stored in (NSMutableArray *)frames, at random times. (UIImage *)filmstrip is just an image which contains all the frames which will be used on animation. The first iteration of animation works, but the problems appears on the second iteration, where I get -[UIImage _isResizable]: message sent to deallocated instance ... or -[UIImage _contentStretchInPixels]: message sent to deallocated instance ... or -[NSArrayI release]: message sent to deallocated instance .... This happens at

gun.animationImages = frames;

but I don’t understand why. I’m not requesting a fix for my issue, but just to help me understand what’s happening here. Thanks.

Answer 1

ARC is a mechanism that removes the need to manually retain/release objects. Here’s a nice site that explains how this works: http://longweekendmobile.com/2011/09/07/objc-automatic-reference-counting-in-xcode-explained/

Try changing “leftGun” for “gun”. I think that’s probably the one that gets deallocated at some point, if you’re using it through an ivar. Otherwise, leftGun simply isn’t in the scope.

Here’s what it should look like:

In your .h file:

@property (nonatomic, strong) IBOutlet UIImageView *leftGun;

In your .m file:

  dispatch_after(dispatch_time(DISPATCH_TIME_NOW,(arc4random() % 300)/100 * NSEC_PER_SEC), dispatch_get_current_queue(),^{
    [self animateGun:gun withFilmStrip:[self getFilmStripForAction:gunShoot andTeam:nil withWeapon:nil]];
  });

Also, not quite sure where “gunShoot” is coming from. Is that supposed to be an enum?

EDIT

Added an example of how the leftGun property should be defined. The reason behind using a property over an ivar is for memory management purposes. If you want to release or destroy an object that is a property, simply set it to nil and the property will take care of releasing the object if it has to.