在xcode 5 下，为了更易于开发，增加了modules和 auto-linking 这两个新特性。用 @import 来增加框架 到项目中比用 #import会更有效. 我们来看看为什么：

"Modules for system frameworks speed build time and PRovide an alternate means to import APIs from the SDK instead of using the C preprocessor. Modules provide many of the build-time improvements of precompiled headers with less maintenance or need for optimization. They are designed for easy adoption with little or no source changes. Beyond build-time improvements, modules provide a cleaner API model that enables many great features in the tools, such as Auto Linking."

Modules and auto-linking 默认情况下是enabled的。 如果是旧的项目，你可以通过设置"Language - Modules." 来设置Enable Modules 和Link Frameworks Automatically 为Yes。