举例说明NSURLSessionDataDelegate实现方法中，有关block块的说明

pragma mark - NSURLSessionDataDelegate

• (void)URLSession:(NSURLSession *)session
  dataTask:(NSURLSessionDataTask *)dataTask
  didReceiveResponse:(NSURLResponse *)response
  completionHandler:(void (^)(NSURLSessionResponseDisposition disposition))completionHandler
  {
  NSURLSessionResponseDisposition disposition = NSURLSessionResponseAllow;

  if (self.dataTaskDidReceiveResponse) {
  disposition = self.dataTaskDidReceiveResponse(session, dataTask, response);
  }

  if (completionHandler) {
  completionHandler(disposition);
  }
  }
  ==============================================

[[AFAppDotNetAPIClient sharedClient] GET:@"v2/book/1220562" parameters:nil progress:nil success:^(NSURLSessionDataTask * __unused task, id JSON) {
NSArray *postsFromResponse = [JSON valueForKeyPath:@"data"];
NSMutableArray *mutablePosts = [NSMutableArray arrayWithCapacity:[postsFromResponse count]];
for (NSDictionary *attributes in postsFromResponse) {
Post *post = [[Post alloc] initWithAttributes:attributes];
[mutablePosts addObject:post];
}

    if (block) {
        block([NSArray arrayWithArray:mutablePosts], nil);
    }
} failure:^(NSURLSessionDataTask *__unused task, NSError *error) {
    if (block) {
        block([NSArray array], error);
    }
}];

第一个completionHandler回调块，一半平常大家使用block回调，block里面的参数基本都是回调给我们需要处理的内容，但是这个completionHandler回调，里面没有参数，说明什么这个completionHandler回调块，需要我们向他传递参数，回调给系统使用