- (IBAction)downLoadImage:(id)sender {    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{        NSString* url=@"http://pic2.nipic.com/20090413/60867_143033029_2.jpg";        NSData* data=[[NSData alloc]initWithContentsOfURL:[NSURL URLWithString:url]];        UIImage* image=[[UIImage alloc]initWithData:data];        if (image!=nil) {            dispatch_async(dispatch_get_main_queue(), ^{                self.iv.image=image;            });        }        else        {            NSLog(@"下载出错！");        }    });}

运行结果