GDC异步获取数据例子

- (void)processImageDataWithBlock:(void (^)(NSData *imageData))processImage 
//声明函数processImageDataWithBlock 接收一个(void (^)(NSData *imageData))processImage 块参数， 该块参数接收一个NSData*参数。实质上这个块参数是异步回调函数
{
NSString *url = self.imageURL;
dispatch_queue_t callerQueue = dispatch_get_current_queue();//获取当前的主UI线程队列，所有对UI的操作都要针对UI线程队列进行
dispatch_queue_t downloadQueue = dispatch_queue_create("Photo Downloader", NULL);//创建一个异步线程队列
dispatch_async(downloadQueue, ^{ //异步执行下载图片任务
    NSData *imageData = *insert code that fetches photo from server*;
    dispatch_async(callerQueue, ^{//下载数据线程返回后，通过传入的块数据更新UI
        processImage(imageData);
    });
});
dispatch_release(downloadQueue);
}

//主UI调用更新UI
- (void)viewWillAppear:(BOOL)animated
{
[spinner startAnimating];
[self.photo processImageDataWithBlock:^(NSData *imageData) {//此为回调函数定义
    if (self.view.window) {
        UIImage *image = [UIImage imageWithData:imageData];
        imageView.image = image;
        imageView.frame = CGRectMake(0, 0, image.size.width, image.size.height);
        scrollView.contentSize = image.size;
        [spinner stopAnimating];
    }
}];
}