+ (void)initialize {
    
    UIFont *textFont = [UIFont systemFontOfSize:12];
    NSMutableDictionary *attrs = [NSMutableDictionary dictionary];
    attrs[NSFontAttributeName] = textFont;
    attrs[NSForegroundColorAttributeName] = TextColor;
    
    NSMutableDictionary *selectedAttrs = [NSMutableDictionary dictionary];
    selectedAttrs[NSFontAttributeName] = textFont;
    selectedAttrs[NSForegroundColorAttributeName] = BtnColor;
    
    if (@available(iOS 13.0,*)) {
        UITabBarAppearance *appearance = [UITabBarAppearance new];
        appearance.stackedLayoutAppearance.normal.titleTextAttributes = attrs;
        appearance.stackedLayoutAppearance.selected.titleTextAttributes = selectedAttrs;
        appearance.backgroundImage = [UIImage imageWithColor:[UIColor clearColor]];
        appearance.shadowImage = [UIImage imageWithColor:[UIColor clearColor]];
         // 官方文档写的是 重置背景和阴影为透明
        [appearance configureWithTransparentBackground];
        UITabBar.appearance.standardAppearance = appearance;
        
    }else{
        UITabBarItem *item = [UITabBarItem appearance];
        [item setTitleTextAttributes:attrs forState:UIControlStateNormal];
        [item setTitleTextAttributes:selectedAttrs forState:UIControlStateSelected];
        UITabBar.appearance.backgroundImage = [UIImage new];
        UITabBar.appearance.shadowImage = [UIImage new];
    }
    
}

// 通过颜色生成图片
+ (UIImage *)imageWithColor:(UIColor *)color {
    CGRect rect = CGRectMake(0.0f, 0.0f, 1.0f, 1.0f);
    UIGraphicsBeginImageContext(rect.size);
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextSetFillColorWithColor(context, [color CGColor]);
    CGContextFillRect(context, rect);
    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return image;
}

值得注意的是，在iOS13以上，UITabBar文字背景设置是由UITabBar的UITabBarAppearance属性去设置。