EG:
File.h
UIKIT_EXTERN NSString *const SEARCH_KEY_INDEXPATH;
File.m
NSString * const SEARCH_KEY_INDEXPATH = @"SEARCH_KEY_INDEXPATH";
四种写法：
static const NSString *HSCoder = @"汉斯哈哈哈";
const NSString *HSCoder = @"汉斯哈哈哈";
NSString const *HSCoder = @"汉斯哈哈哈";
NSString * const HSCoder = @"汉斯哈哈哈";
全局常量：不管你定义在任何文件夹，外部都能访问
const NSString *HSCoder = @"汉斯哈哈哈";
局部常量：用static修饰后，不能提供外界访问
static const NSString *HSCoder = @"汉斯哈哈哈";
