Flutter自定义SliverAppBar不显示flexibleSpace

Question

我是扑扑和飞镖编程的新手。但是最近我遇到了一些奇怪的行为，我无法弄清。每当我在代码中使用默认的SliverAppBar时，就会出现flexibleSpace，如以下代码所示。

SliverAppBar(
  pinned: true,
  expandedHeight: 200.0,
  floating: true,
  snap: true,
  elevation: 0.1,
  shape: Border(
    bottom: BorderSide(width: 1.0, color: Colors.red),
  ),
  title: Text('No title'),
  flexibleSpace: FlexibleSpaceBar(
    background: Image.asset('assets/images/placeholder.png'),
  ),
),

但是当我使用如下所示的自定义SliverAppBar时，flexibleSpace没有显示。

MySliverAppBar(
  pinned: true,
  expandedHeight: 200.0,
  title: Text('No title'),
  flexibleSpace: FlexibleSpaceBar(
    background: Image.asset('assets/images/placeholder.png'),
  ),
),

....

class MySliverAppBar extends SliverAppBar {
  MySliverAppBar({
    Key key,
    Widget leading,
    Widget title,
    bool pinned = false,
    Widget flexibleSpace,
    List<Widget> actions,
    PreferredSizeWidget bottom,
    double expandedHeight,
    bool forceElevated = false,
  }) : super(
          key: key,
          leading: leading,
          title: title,
          elevation: 0.1,
          actions: actions,
          floating: true,
          snap: true,
          pinned: pinned,
          shape: Border(
            bottom: BorderSide(width: 1.0, color: Colors.red),
          ),
          bottom: bottom,
          forceElevated: forceElevated,
          expandedHeight: expandedHeight,
        );
}

我不知道发生了什么。请帮忙。