解决OBJC_CLASS_$_MBProgressHUD无法引用

Undefined symbols for architecture i386:

"_OBJC_CLASS_$_MBProgressHUD", referenced from:

objc-class-ref in ViewController.o

ld: symbol(s) not found for architecture i386

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中检查是否有对应的.h .m 文件.