ExpansionTile不保持状态 [英] ExpansionTile doesn't keep state

问题描述

以下问题：
我有一个 ExpansionTiles 效果很好。我面临的唯一问题是，从视图中滚动出来的扩展ExpansionTile在再次滚动到视图中后将不再扩展。

following problem: I have a list of ExpansionTiles which works very well. The only problem I'm facing is that a expanded ExpansionTile which is scrolled out of view will, after scrolling it into view again, no longer be expanded. This leads to undesired user experience and also a kind of "jumpy" scrolling.

文档指出以下内容：

与ListView之类的滚动小部件一起使用时，必须指定一个唯一键，以使ExpansionTile在滚动到视图中和从视图中退出时能够保存和恢复其展开状态。

When used with scrolling widgets like ListView, a unique key must be specified to enable the ExpansionTile to save and restore its expanded state when it is scrolled in and out of view.

但这不起作用。到目前为止，我还没有找到使之可行的方法。

这是到目前为止的代码：

This doesn't work though. So far I have found no way to make this work.
Here is the code so far:

import 'package:flutter/material.dart';

void main() {
  runApp(new MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'ExpansionTile Test',
      home: new MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => new _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  List<Widget> _getChildren() {
    List<Widget> elements = [];
    for (int i = 0; i < 20; i++) {
      elements.add(new ListChild());
    }
    return elements;
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text('ExpansionTile Test'),
      ),
      body: new ListView(
        children: _getChildren(),
      ),
    );
  }
}

class ListChild extends StatefulWidget {
  @override
  State createState() => new ListChildState();
}

class ListChildState extends State<ListChild> {
  GlobalKey<ListChildState> _key = new GlobalKey();
  @override
  Widget build(BuildContext context) {
    return new ExpansionTile(
      key: _key,
      title: const Text('Test Tile'),
      children: <Widget>[
        const Text('body'),
      ],
    );
  }
}

推荐答案

使用 PageStorageKey 代替 GlobalKey 。

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