Objective-C Definedness

Question

This is an agonizingly rookie question, but here I am learning a new language and framework, and I'm trying to answer the question "What is Truth?" as pertains to Obj-C.

I'm trying to lazy-load images across the network. I have a data class called Event that has properties including:

@property (nonatomic, retain) UIImage image;
@property (nonatomic, retain) UIImage thumbnail;

in my AppDelegate, I fetch up a bunch of data about my events (this is an app that shows local arts event listings), and pre-sets each event.image to my default "no-image.png".

Then in the UITableViewController where I view these things, I do:

if (thisEvent.image == NULL) {
    NSLog(@"Going for this item's image");
    UIImage *tempImage = [UIImage imageWithData:[NSData dataWithContentsOfURL:
                  [NSURL URLWithString:
                    [NSString stringWithFormat:
                    @"http://www.mysite.com/content_elements/%@_image_1.jpg",
                              thisEvent.guid]]]];
    thisEvent.image = tempImage;

}

We never get that NSLog call. Testing thisEvent.image for NULLness isn't the thing. I've tried == nil as well, but that also doesn't work.

Answer 1

如果你将image设置为no-image.png，它将不会是nil（Objective-C对于对象值使用nil，你应该使用它而不是NULL，它具有不同的目的，尽管它具有相同的值）。

Answer 2

Lazy loading will look like this :

@property (nonatomic, read-only) UIImage *image;

- (UIImage *)image {
   if (!image) {
       image = [[UIImage imageWithData:[NSData dataWithContentsOfURL:
              [NSURL URLWithString:
                [NSString stringWithFormat:
                @"http://www.mysite.com/content_elements/%@_image_1.jpg",
                          thisEvent.guid]]]] retain];
  }

  return image;
}

And do not forget to release image in dealloc.

Regards,

Answer 3

You really don't want to load images from the web as you build the table cells, your table scrolling will be terribly slow.

See Apple's example LazyTableImages for how to do this and this SO question may help too.