[ios] KVO Key-Value Observing (咪即係 改 value -> dispatchEvent 器)

http://blog.riaproject.com/objective-c/2147/%E4%BA%86%E8%A7%A3-objective-c-%E4%B8%8A%E7%9A%84-kvokey-value-observing-%E6%A9%9F%E5%88%B6.html