在老师创建的项目中可以正常运行，但是自己创建的项目下就出这个问题，大伙给看一看，是哪里的错误。
fatal error: file '/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.9.sdk/System/Library/Frameworks/Foundation.framework/Headers/NSString.h' has been modified since the precompiled header '/Users/dqw/Library/Developer/Xcode/DerivedData/ModuleCache/2RM2UR8OL7GQA/Foundation.pcm' was built

顶一个，有谁知道给解答一下啊，很捉急的。