Objective C享元模式

http://www.devdiv.com/iOS_iPhone-Objective_C--%E4%BA%AB%E5%85%83%E6%A8%A1%E5%BC%8F-thread-18664-1-1.html

《Objective-C编程之道 iOS设计模式解析》（英文名为Pro Objective-C Design Patterns for iOS,[美]Carlo Chung著）http://ky-storm.cn/blog/191.html#26