通过appearance统一设置所有UITabBarItem的文字属性
NSMutableDictionary *attrs = [NSMutableDictionary dictionary];
attrs[NSFontAttributeName] = [UIFont systemFontOfSize:12];
attrs[NSForegroundColorAttributeName] = [UIColor grayColor];
NSMutableDictionary *selectAttrs = [NSMutableDictionary dictionary];
selectAttrs[NSFontAttributeName] = [UIFont systemFontOfSize:12];
selectAttrs[NSForegroundColorAttributeName] = [UIColor darkGrayColor];
UITabBarItem *item = [UITabBarItem appearance];
[item setTitleTextAttributes:attrs forState:UIControlStateNormal];
[item setTitleTextAttributes:selectAttrs forState:UIControlStateSelected];
后面带有UI_APPEARANCE_SELECTOR的方法,都可以通过appearance对象来统一设置
