iOS14 适配 UITableViewHeaderFooterView 设置透明背景无效

今天，迫不及待把升级到Xcode12 和手机系统更新到iOS14 ,结果跑了项目，发现在我的UITableView 列表中，一个UITableViewHeaderFooterView 背景设置为通明之后，有一个毛玻璃颜色。
情景： 页面是有一个backView 和 UITableView 组成，backView放在底部，是有CAGradientLayer 画了一个半圆，而UITableView 的HeaderView 需要设置通明色，来显示背景半圆效果。如图：

WechatIMG7.png

然而，升级到iOS14之后成了这样子

WechatIMG11.jpeg

然后使用debug view hierarchy 看出来HeaderView 层级有所变化，

WechatIMG8.jpeg

old : ProbabilityCollegeHeaderView -> _UISystemBackgroundView -> _UITableViewHeaderFooterContentView
new : ProbabilityCollegeHeaderView -> _UISystemBackgroundView -> UIView -> _UITableViewHeaderFooterContentView

在系统View 嵌套里面多了一个UIView，并且Api方法里面没有找到这个属性

@available(iOS 6.0, *)
open class UITableViewHeaderFooterView : UIView {

    public init(reuseIdentifier: String?)

    public init?(coder: NSCoder)

    /// Requests the view update its configuration for its current state. This method is called automatically
    /// when the view's `configurationState` may have changed, as well as in other circumstances where an
    /// update may be required. Multiple requests may be coalesced into a single update at the appropriate time.
    @available(iOS 14.0, *)
    open func setNeedsUpdateConfiguration()

    /// When YES, the header/footer will automatically call -updatedConfigurationForState: on its `contentConfiguration` when the header/footer's
    /// configuration state changes, and apply the updated configuration back to the header/footer. The default value is YES.
    @available(iOS 14.0, *)
    open var automaticallyUpdatesContentConfiguration: Bool
    open var contentView: UIView { get }

    // These properties will always return nil when a non-nil `contentConfiguration` is set.
    @available(iOS, introduced: 6.0, deprecated: 100000, message: "Use UIListContentConfiguration instead, this property will be deprecated in a future release.")
    open var textLabel: UILabel? { get }

    @available(iOS, introduced: 6.0, deprecated: 100000, message: "Use UIListContentConfiguration instead, this property will be deprecated in a future release.")
    open var detailTextLabel: UILabel? { get } // only supported for headers in grouped style

    /// When YES, the header/footer will automatically call -updatedConfigurationForState: on its `backgroundConfiguration` when the header/footer's
    /// configuration state changes, and apply the updated configuration back to the header/footer. The default value is YES.
    @available(iOS 14.0, *)
    open var automaticallyUpdatesBackgroundConfiguration: Bool

    // This property is always nil when a non-nil `backgroundConfiguration` is set.
    open var backgroundView: UIView?
    open var reuseIdentifier: String? { get }
    open func prepareForReuse() // if the view is reusable (has a reuse identifier), this is called just before the view is returned from the table view method dequeueReusableHeaderFooterViewWithIdentifier:.  If you override, you MUST call super.
}

却发现iOS14增加了backgroundConfiguration 配置方法。
backgroundConfiguration: 背景配置为您提供了一种轻巧的方式来为视图创建背景。 使用背景配置，可以获得各种不同视图状态的系统默认背景样式。 您可以将后台配置直接应用于UICollectionView和UITableView中的单元格，头部View和 底部View。
所以把UITableViewHeaderFooterView 中的backgroundConfiguration 设置为通明配置，没有默认样式就行了。

        if #available(iOS 14.0, *) {
            self.backgroundConfiguration = UIBackgroundConfiguration.clear()
        } else {
            // Fallback on earlier versions
        }