@abstract This method registers a protocol class, making it visible
to several other NSURLProtocol class methods.
@discussion When the URL loading system begins to load a request,
each protocol class that has been registered is consulted in turn to
see if it can be initialized with a given request. The first
protocol handler class to provide a YES answer to
<tt>+canInitWithRequest:</tt> "wins" and that protocol
implementation is used to perform the URL load. There is no
guarantee that all registered protocol classes will be consulted.
Hence, it should be noted that registering a class places it first
on the list of classes that will be consulted in calls to
<tt>+canInitWithRequest:</tt>, moving it in front of all classes
that had been registered previously.
<p>A similar design governs the process to create the canonical form
of a request with the <tt>+canonicalRequestForRequest:</tt> class
method.
@param protocolClass the class to register.
@result YES if the protocol was registered successfully, NO if not.
The only way that failure can occur is if the given class is not a
subclass of NSURLProtocol.
原文：http://www.cnblogs.com/feng9exe/p/7203436.html