XCode6 生成prefix.pch文件

        XCode6里, 新建工程默认是没有pch文件的，苹果取消pch文件这一点肯定有它的道理,刚開始非常多人可能不适应,假设我们想使用pch文件，须要手动加入，加入过程例如以下:(依然直接上图)

      @ 取消的原因:

      Stackoverflow上有一个人对此的解释是这种：
       I suspect because of modules, which remove the need for the  #import <Cocoa/Cocoa.h>.
       As to where to put code that you would put in a prefix header, there is no code you should put in a prefix header. Put your imports into the files that need them. Put your definitions into their own files. Put your macros...nowhere. Stop writing macros unless there is no other way (such as when you need __FILE__). If you do need macros, put them in a header and include it.
       The prefix header was necessary for things that are huge and used by nearly everything in the whole system (like Foundation.h). If you have something that huge and ubiquitous, you should rethink your architecture. Prefix headers make code reuse hard, and introduce subtle build problems if any of the files listed can change. Avoid them until you have a serious build time problem that you can demonstrate is dramatically improved with a prefix header.
       In that case you can create one and pass it into clang, but it‘s incredibly rare that it‘s a good idea.

      @ 还能够參考:XcodePrecompilePrefixHeader浅析

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