error："_OBJC_CLASS_$_XXX", referenced from: objc-class-ref in XXX.o

Undefined symbols for architecture x86_64:
“OBJC_CLASS$_TodayViewController”, referenced from:
objc-class-ref in ViewController.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

这里的错误是因为你的在building phases中没有引用相关的头文件，因此，只需要在building phase中添加对应的.m文件就可以了。

或者是缺少相应地框架：
在target-》build phase-》Link binary with libraries中加入相应的框架即可。