GCD_demo

原帖：http://blog.youkuaiyun.com/favormm/article/details/6453260

@interface UIImageView (DispatchLoad)  
- (void) setImageFromUrl:(NSString*)urlString;  
- (void) setImageFromUrl:(NSString*)urlString   
              completion:(void (^)(void))completion;  
@end

#import "UIImageView+DispatchLoad.h"  
@implementation UIImageView (DispatchLoad)  
- (void) setImageFromUrl:(NSString*)urlString {  
    [self setImageFromUrl:urlString completion:NULL];  
}  
- (void) setImageFromUrl:(NSString*)urlString   
              completion:(void (^)(void))completion {  

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{  

        NSLog(@"Starting: %@", urlString);  
        UIImage *avatarImage = nil;   
        NSURL *url = [NSURL URLWithString:urlString];  
        NSData *responseData = [NSData dataWithContentsOfURL:url];  
        avatarImage = [UIImage imageWithData:responseData];  
        NSLog(@"Finishing: %@", urlString);  

        if (avatarImage) {  
            dispatch_async(dispatch_get_main_queue(), ^{  
                self.image = avatarImage;  
            });  
            dispatch_async(dispatch_get_main_queue(), completion);  
        }  
        else {  
            NSLog(@"-- impossible download: %@", urlString);  
        }  
    });     
}  
@end