1.Command-Option-Shift-K to clean out the build folder
XCode4.2 finder中找到 /Users/apple/Library/Developer/Xcode （注：Library资源库是隐藏的文件夹）
里面有DerivedData和Snapshots两个文件夹，删除里面文件即可
Remove all its contents because there's a bug where Xcode will run an old version of your project that's in there somewhere.
2.(Xcode 4.2 will show you the Derived Data folder: choose Window > Organizer and switch to the Projects tab. Click the right-arrow to the right of the Derived Data folder name.)
In the simulator, choose iOS Simulator > Reset Content and Settings.