NSURLConnection的代理Protocol定义有三类：NSURLConnectionDelegate、NSURLConnectionDataDelegate和NSURLConnectionDownloadDelegate。

• NSURLConnectionDelegate：全部类型NSURLConnection的基础代理方法，都是Optional的方法，主要是涉及SSL/TSL加密的相关接口。

@protocol NSURLConnectionDelegate <NSObject>

@optional

//请求错误（失败）的时候调用（请求超时\断网\没有网\。一般指client错误）

- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error;

- (BOOL)connectionShouldUseCredentialStorage:(NSURLConnection *)connection;

- (void)connection:(NSURLConnection *)connection willSendRequestForAuthenticationChallenge:(NSURLAuthenticationChallenge *)challenge;

- (BOOL)connection:(NSURLConnection *)connection canAuthenticateAgainstProtectionSpace:(NSURLProtectionSpace *)protectionSpace NS_DEPRECATED(10_6, 10_10, 3_0, 8_0, "Use -connection:willSendRequestForAuthenticationChallenge: instead.");

- (void)connection:(NSURLConnection *)connection didReceiveAuthenticationChallenge:(NSURLAuthenticationChallenge *)challenge NS_DEPRECATED(10_2, 10_10, 2_0, 8_0, "Use -connection:willSendRequestForAuthenticationChallenge: instead.");

- (void)connection:(NSURLConnection *)connection didCancelAuthenticationChallenge:(NSURLAuthenticationChallenge *)challenge NS_DEPRECATED(10_2, 10_10, 2_0, 8_0, "Use -connection:willSendRequestForAuthenticationChallenge: instead.");

@end

• NSURLConnectionDataDelegate：用于将网络请求的数据存放到内存中(以NSData的形式)的代理方法。

  全部方法都是Optional的。

@protocol NSURLConnectionDataDelegate <NSURLConnectionDelegate>

@optional

- (NSURLRequest *)connection:(NSURLConnection *)connection willSendRequest:(NSURLRequest *)request redirectResponse:(NSURLResponse *)response;

//当接收到server的响应（连通了server）时会调用

- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response;

//当接收到server的数据时会调用（可能会被调用多次。每次仅仅传递部分数据）

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data;

- (NSInputStream *)connection:(NSURLConnection *)connection needNewBodyStream:(NSURLRequest *)request;

- (void)connection:(NSURLConnection *)connection   didSendBodyData:(NSInteger)bytesWritten

                                                 totalBytesWritten:(NSInteger)totalBytesWritten

                                         totalBytesExpectedToWrite:(NSInteger)totalBytesExpectedToWrite;

- (NSCachedURLResponse *)connection:(NSURLConnection *)connection willCacheResponse:(NSCachedURLResponse *)cachedResponse;

//当server的数据载入完成时就会调用

- (void)connectionDidFinishLoading:(NSURLConnection *)connection;

@end

• NSURLConnectionDownloadDelegate：用于将网络资源直接保存到文件里的代理方法。除了connectionDidFinishDownloading:destinationURL:都是Optional的方法。connectionDidFinishDownloading:destinationURL回调能够告知你下载的网络数据终于存放的文件位置，正常都是在iPhone应用沙盒的/tmp文件夹下。

@protocol NSURLConnectionDownloadDelegate <NSURLConnectionDelegate>

@optional

- (void)connection:(NSURLConnection *)connection didWriteData:(long long)bytesWritten totalBytesWritten:(long long)totalBytesWritten expectedTotalBytes:(long long) expectedTotalBytes;

- (void)connectionDidResumeDownloading:(NSURLConnection *)connection totalBytesWritten:(long long)totalBytesWritten expectedTotalBytes:(long long) expectedTotalBytes;

@required

- (void)connectionDidFinishDownloading:(NSURLConnection *)connection destinationURL:(NSURL *) destinationURL;

@end

• 由于生成NSURLConnectin对象传入delegate參数时类型就是id。而不是传统id<*Delegate>形式。那么怎样确定当前代理实现的是什么类型的NSURLConnectionDelegate代理呢？方法也非常诡异。假设你的代理实现了connectionDidFinishDownloading:destinationURL:，那么就表示你要实现的是NSURLConnectionDownloadDelegate，NSURLConnectionDataDelegate中的connection:DidReceiveData就不会得到回调，即使你实现了它。道理非常easy。这两类代理一个是用于将下载数据保存到文件上，还有一个是保存到内存中，仅仅能两者居其一。

• 故事还没有结束 ，假设你实现了connectionDidFinishDownloading:destinationURL而且想通过回到得到的destinationURL读取保存数据的文件时。令人吃惊的发现这个文件竟然不存在，由于这类NSURLConnectionDataDelegate回调是用于Newsstand类型的App开发的，用于将杂志等信息保存到本地文件。

  实在想不通为什么仅仅有Newsstand类型App才干用这组接口，非常多开发人员早已发了bug报告给Apple，Apple也已经确认，可是从iOS5到了iOS7。这个“bug”还是没有被修复。