- (void)test
{
    [[[NSOperationQueue alloc] init] addOperationWithBlock:^{
        // 图片的网络路径
       NSURL *url = [NSURL URLWithString:@"http://img.pconline.com.cn/images"];
        // 加载图片
        NSData *data = [NSData dataWithContentsOfURL:url];
        
        // 生成图片
        UIImage *image = [UIImage imageWithData:data];
        
        // 回到主线程
        [[NSOperationQueue mainQueue] addOperationWithBlock:^{
            self.imageView.image = image;
        }];
    }];
}

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        // 图片的网络路径
        NSURL *url = [NSURL URLWithString:@"http://img.pconline.com.cn/images/photoblog/9/9/8/1/9981681/200910/11/1255259355826.jpg"];

        // 加载图片
        NSData *data = [NSData dataWithContentsOfURL:url];

        // 生成图片
        UIImage *image = [UIImage imageWithData:data];

        // 回到主线程
        dispatch_async(dispatch_get_main_queue(), ^{
            self.imageView.image = image;
        });
    });
}

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    [self performSelectorInBackground:@selector(download3) withObject:nil];
- (void)download3
{
    // 图片的网络路径
    NSURL *url = [NSURL URLWithString:@"http://img.pconline.com.cn/images/photoblog/9/9/8/1/9981681/200910/11/1255259355826.jpg"];
    
    // 加载图片
    NSData *data = [NSData dataWithContentsOfURL:url];
    
    // 生成图片
    UIImage *image = [UIImage imageWithData:data];
    
    // 回到主线程，显示图片
    [self.imageView performSelector:@selector(setImage:) onThread:[NSThread mainThread] withObject:image waitUntilDone:NO];
//    [self.imageView performSelectorOnMainThread:@selector(setImage:) withObject:image waitUntilDone:NO];
//    [self performSelectorOnMainThread:@selector(showImage:) withObject:image waitUntilDone:YES];
}
}