它是一个抽象类，为载入URL的data的一些特定协议提供基础的结构。要实现它里面的函数就必须继承它。

    而NSURLProtocol其中有个成员就是NSURLProtocolClient的一个实例。因为NSURLProtocol是由一系列的回调函数构成的（注册函数除外),而要对URL的data进行各种操作时就到了调用NSURLProtocolClient实例的时候了，这就实现了一个钩子，去操作URL data。

NSURLProtocol有以下一系列的回调方法：

- (id)initWithRequest:(NSURLRequest *)request cachedResponse:(NSCachedURLResponse *)cachedResponse client:(id )client;

+ (BOOL)canInitWithRequest:(NSURLRequest *)request;

+ (NSURLRequest *)canonicalRequestForRequest:(NSURLRequest *)request;

- (void)startLoading;

- (void)stopLoading;

   其中canInitWithRequest是询问是否处理该请求的回调，如果不处理则后面所有函数都不会再调用。startLoading和stopLoading是分别对于loading开始从网页上抓取数据，从网页上抓取完数据的回调。其中startLoading称为我们可以重点利用的函数。

  NSURLProtocolClient主要有以下方法：

- (void)URLProtocol:(NSURLProtocol *)protocol wasRedirectedToRequest:(NSURLRequest *)request redirectResponse:(NSURLResponse *)redirectResponse;

- (void)URLProtocol:(NSURLProtocol *)protocol cachedResponseIsValid:(NSCachedURLResponse *)cachedResponse;

- (void)URLProtocol:(NSURLProtocol *)protocol didReceiveResponse:(NSURLResponse *)response cacheStoragePolicy:(NSURLCacheStoragePolicy)policy;

- (void)URLProtocol:(NSURLProtocol *)protocol didLoadData:(NSData *)data;

- (void)URLProtocolDidFinishLoading:(NSURLProtocol *)protocol;

- (void)URLProtocol:(NSURLProtocol *)protocol didFailWithError:(NSError *)error;

- (void)URLProtocol:(NSURLProtocol *)protocol didReceiveAuthenticationChallenge:(NSURLAuthenticationChallenge *)challenge;

- (void)URLProtocol:(NSURLProtocol *)protocol didCancelAuthenticationChallenge:(NSURLAuthenticationChallenge *)challenge;