Question

I have a question about overriding auto-generated accessor methods. The following would not work (I believe) because each getter references the other getter. Is there a rule that accessor methods should not use other accessor methods, or do you just have to watch out for these situations individually?

-(UIImage *) image{

    if(image == nil){
        if(self.data == nil){
            [self performSelectorInBackground: @selector(loadImage) withObject: nil]
        }else{
            self.image = [UIImage imageWithData: self.data];
        }
    }

    return image;
}

-(NSData *) data {

    if(data == nil){
        if(self.image == nil){
            [self performSelectorInBackground: @selector(loadData) withObject: nil]
        }else{
            self.data = UIImageJPEGRepresentation(self.image, 0.85);
        }
    }

    return data;
}

I have to emphasize that the image use presented here is an example, and thoughts concerning what to do in this particular example are less important than in the general case.

Answer 1

First, don’t be too smart for your own good. If you want to get over some bottleneck, first measure and make sure it’s really there. I believe that both UIImage and NSData do some internal lazy loading, so that your code might be essentially useless. Second, even if you really wanted to do something like that by hand, try splitting the caching code into a separate class, so that you don’t pollute the code of the main class.

There is no rule about accessors (at least no that I know of), because people don’t do much lazy loading in accessors. Sometimes I get myself caught by an endless loop caused by lazy [UIViewController loadView] in combination with [UIViewController view], but that’s about it.