參考文章
https://www.cnblogs.com/yandada/p/6098564.html
https://blog.csdn.net/li15809284891/article/details/54923273
https://blog.csdn.net/u011146511/article/details/64920969
static和extern是用來設(shè)置作用域的
const:設(shè)置變量只可以被讀,不可以被修改
定義整個(gè)項(xiàng)目中的全局變量 .h .m
extern NSString *const home_name;
NSString *const home_name = @"name";
static NSString *const name = @"name";