在使用Xcode時，有時會出現一下錯誤提示：
fatal error: file ‘/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator8.1.sdk/System/Library/Frameworks/Foundation.framework/Headers/NSObjCRuntime.h’ has been modified since the precompiled header ‘/Users/imac/Library/Developer/Xcode/DerivedData/ModuleCache/1M29HZNHW3I5B/UIKit-2M4VYLEDI34V0.pcm’ was built

解決辦法是：刪除以上標記的’/Users/imac/Library/Developer/Xcode/DerivedData/ModuleCache/1M29HZNHW3I5B/UIKit-2M4VYLEDI34V0.pcm檔案。

刪除之後好有可能出現fatal error: could not build module ‘UIKit’錯誤提示， 辦法是刪除/Users/imac/Library/Developer/Xcode/DerivedData/ModuleCache資料夾，重新編譯、執行方可。

注意：imac下的Library（資源庫）一般都隱藏啦，
進入終端，輸入chflags nohidden ~/Library/就可以了。