Objective C: EXC_BAD_ACCESS when generating a UIImage

Question

I have a view that generates an image based on a series of layers. I have images for the background, for the thumbnail, and finally for an overlay. Together, it makes one cohesive display.

It seems to work a dream, except for when it doesn't. For seemingly no reason, I get an EXC_BAD_ACCESS on the specified line below after it's generated somewhere between 8 and 20 images. I've run it through the memory leak tool and allocation tool, and it's not eating up tons of memory and it's not leaking. I'm totally stumped.

Here's the relevant code:

- (UIImage *)addLayer:(UIImage *)layer toImage:(UIImage *)background atPoint:(CGPoint)point {
  CGSize size = CGSizeMake(240, 240);
  UIGraphicsBeginImageContext(size);

  [background drawAtPoint:CGPointMake(0, 0)];  // <--- error here
  [layer drawAtPoint:point];

  UIImage* result = UIGraphicsGetImageFromCurrentImageContext();
  UIGraphicsEndImageContext();

  return result;
}


// Build the layered image -- thingPage onto thingBackground,
// then the screenshot on that, then the thingTop on top of it all.
// thingBackground, thingPage and thingTop are all preloaded UIImages.
-(UIImage *)getImageForThing:(Thing *)t {
  [self loadImageCacheIfNecessary];

  if (!t.screenshot) {
    return [UIImage imageNamed:@"NoPreview.png"];
  } else {
    UIImage *screenshot = t.screenshot;
    UIImage *currentImage = [self addLayer:thingPage toImage:thingBackground atPoint:CGPointMake(0, 0)];
    currentImage = [self addLayer:screenshot toImage:currentImage atPoint:CGPointMake(39, 59)];
    currentImage = [self addLayer:thingTop toImage:currentImage atPoint:CGPointMake(0, 1)]; 

    return currentImage;
  }
}

I can't find anywhere that this is going wrong, and I've been tearing my hair out for a couple of hours on this. It's the final known bug in the system, so you can imagine how antsy I am to fix it! :-)

Thanks in advance for any help.

Answer 1

EXC_BAD_ACCESS is almost always due to accessing an object that has already been released. In your code this seems to be t.screenshot. Check creation (and retaining if it is an instance variable) of the object returned by Thing's screenshot property.

Answer 2

As it turns out, the error wasn't in the code I posted, it was in my caching of the thingBackground, thingPage and thingTop images. I wasn't retaining them. Here's the missing code, fixed:

-(void)loadImageCacheIfNecessary {
  if (!folderBackground) {
    thingBackground = [[UIImage imageNamed:@"ThingBack.png"] retain];
  }
  if (!folderPage) {
    thingPage = [[UIImage imageNamed:@"ThingPage.png"] retain];
  }
  if (!folderTop) {
    thingTop = [[UIImage imageNamed:@"ThingTop.png"] retain];
  }
}

I will admit I'm still not comfortable with the whole retain/release/autorelease stuff in Objective C. Hopefully it'll sink in one day soon. :-)