Asynchronous downloaded images with caching in iOS

import <objc/runtime>
 
static char URL_KEY;
 
@implementation UIImageView(Network)
 
@dynamic imageURL;
 
- (void) loadImageFromURL:(NSURL*)url placeholderImage:(UIImage*)placeholder cachingKey:(NSString*)key {
 self.imageURL = url;
 self.image = placeholder;
 dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0ul);
 dispatch_async(queue, ^{
  NSData *data = [NSData dataWithContentsOfURL:url];
 
  UIImage *imageFromData = [UIImage imageWithData:data];
 
  [FTWCache setObject:UIImagePNGRepresentation(imageFromData) forKey:key];
  UIImage *imageToSet = imageFromData;
  if (imageToSet) {
   if ([self.imageURL.absoluteString isEqualToString:url.absoluteString]) {
    dispatch_async(dispatch_get_main_queue(), ^{
     self.image = imageFromData;
    });
   }
  }
  self.imageURL = nil;
 });
}
 
- (void) setImageURL:(NSURL *)newImageURL {
 objc_setAssociatedObject(self, &amp;URL_KEY, newImageURL, OBJC_ASSOCIATION_COPY);
}
 
- (NSURL*) imageURL {
 return objc_getAssociatedObject(self, &amp;URL_KEY);
}
 
@end