https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/URLLoadingSystem/URLLoadingSystem.html#//apple_ref/doc/uid/10000165-BCICJDHA
The URL loading system includes classes that load URLs along with a number of important helper classes that work with those URL loading classes to modify their behavior. The major helper classes fall into five categories: protocol support, authentication and credentials, cookie storage, configuration management, and cache management.
原文：http://www.cnblogs.com/feng9exe/p/7203407.html
