iOS是運行在iPhone、iPod Touch或iPad上的操作系統，之前叫做iPhone OS，iOS與Mac OS X有共同的基礎架構和底層技術。但iOS是根據移動設備的特點而設計的，所以和Mac OS X系統略有區別，比如對多點觸摸和加速感應器的支持。
下面來看看iOS框架簡介：
iOS 的系統架構分為四個層次：核心操作系統層（Core OS layer）、核心服務層（Core Services layer）、媒體層（Media layer）和可觸摸層（Cocoa Touch layer）。下圖展示瞭Mac OS X和iOS系統架構層次的一個對比。
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