解决一个问题：
当我添加一个IBout，
报了如下错误
NSUnknownKeyException' … setValue:forUndefinedKey:]: …not key value coding compliant
一般情况下是
Look in your storyboard or xib (whichever you're using) for the ViewControllerWelcome object. The most likely source of this error is that something has a connection to an action or outlet called done that doesn't exist in the code for that class.
查看你得storyboard or xib 。。最大的可能是有一个connection to an action or outlet 在相应地类中没有对应的代码。