通过appearance统一设置UITabBarItem的文字属性

// 通过appearance统一设置UITabBarItem的文字属性
    NSMutableDictionary *attrs = [NSMutableDictionary dictionary];
    attrs[NSFontAttributeName] = [UIFont systemFontOfSize:12];
    attrs[NSForegroundColorAttributeName] = [UIColor grayColor];
    
    NSMutableDictionary *selectedAttrs = [NSMutableDictionary dictionary];
    selectedAttrs[NSFontAttributeName] = [UIFont systemFontOfSize:12];
    selectedAttrs[NSForegroundColorAttributeName] = [UIColor darkGrayColor];
    
    // 拿到UITabBarItem的appearance
    UITabBarItem *item = [UITabBarItem appearance];
    [item setTitleTextAttributes:attrs forState:UIControlStateNormal];
    [item setTitleTextAttributes:selectedAttrs forState:UIControlStateSelected];