当我们在查看头文件信息时，一不小心会把头文件修改了，之后运行程序就会报下面的错误：
fatal error: file '/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator7.1.sdk/System/Library/Frameworks/Foundation.framework/Headers/NSArray.h' has been modified since the precompiled header '/Users/ligang/Library/Developer/Xcode/DerivedData/ModuleCache/1O5WP9283VGUA/Foundation.pcm'
was built
note: after modifying system headers, please delete the module cache at '/Users/ligang/Library/Developer/Xcode/DerivedData/ModuleCache/1O5WP9283VGUA'
1 error generated.
解决办法：
前往 /Users/ligang/Library/Developer/Xcode/DerivedData/ModuleCache/1O5WP9283VGUA这个文件夹，删除其文件夹中的内容，然后运行app，没问题了，注意不是删除 2NEVAP7X943D2 文件夹，而是其中的内容。然后再 clean下项目就可以了（product -> clean）
