自动计算 `SliverPersistentHeaderDelegate` 的 `maxExtent`

Question

考虑以下示例，该示例旨在在CustomScrollView.

class TitleBar extends StatelessWidget {
  TitleBar(this.text);

  final String text;

  @override
  Widget build(BuildContext context) => Text(
        text,
        style: TextStyle(fontSize: 30),
        maxLines: 3,
        overflow: TextOverflow.ellipsis,
      );
}

class TitleBarDelegate extends SliverPersistentHeaderDelegate {
  final String text;

  TitleBarDelegate(this.text);

  @override
  Widget build(BuildContext context, double shrinkOffset, bool overlapsContent) => TitleBar(text);

  @override
  bool shouldRebuild(TitleBarDelegate oldDelegate) => oldDelegate.text != text;

  @override
  double get maxExtent => ???;

  @override
  double get minExtent => maxExtent; // doesn't shrink
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: Scaffold(
        body: CustomScrollView(
          slivers: [
            SliverPersistentHeader(
              pinned: true,
              delegate: TitleBarDelegate('Potentially very long text'),
            ),
            SliverToBoxAdapter(
              child: Text("Foo " * 1000),
            ),
          ],
        ),
      ),
    );
  }
}

问题是：我如何maxExtent根据实际的TitleBar. 问题是实际TitleBar的大小取决于文本，因此通常可以提前计算。

请注意，它TitleBar可能还有一些比上面示例中更复杂的布局。所以一般的问题是如何“收缩包装”一个SliverPersistentHeaderDelegate.

Answer 1

我想你可能正在寻找_titleBar.preferredSize.height但我不能确定