- (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 = http://www.mamicode.com/*insert code that fetches photo from server*;>

//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];    }}];}