Flutter ExpansionPanel 可展开的收缩控件

文档：https://api.flutter.dev/flutter/material/ExpansionPanel-class.html

demo：

import 'package:flutter/material.dart';

class ExpansionPanelItem {
  final String headerText;
  final Widget body;
  bool isExpanded;

  ExpansionPanelItem({
    this.headerText,
    this.body,
    this.isExpanded,
  });
}

class ExpansionPanelDemo extends StatefulWidget {
  @override
  _ExpansionPanelDemoState createState() => _ExpansionPanelDemoState();
}

class _ExpansionPanelDemoState extends State<ExpansionPanelDemo> {
  List<ExpansionPanelItem> _expansionPanelItems;

  @override
  void initState() {
    super.initState();

    _expansionPanelItems = <ExpansionPanelItem>[
      ExpansionPanelItem(
        headerText: 'Panel A',
        body: Container(
          padding: EdgeInsets.all(16.0),
          width: double.infinity,
          child: Text('Content for Panel A.'),
        ),
        isExpanded: false,
      ),
      ExpansionPanelItem(
        headerText: 'Panel B',
        body: Container(
          padding: EdgeInsets.all(16.0),
          width: double.infinity,
          child: Text('Content for Panel B.'),
        ),
        isExpanded: false,
      ),
      ExpansionPanelItem(
        headerText: 'Panel C',
        body: Container(
          padding: EdgeInsets.all(16.0),
          width: double.infinity,
          child: Text('Content for Panel C.'),
        ),
        isExpanded: false,
      ),
    ];
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('ExpansionPanelDemo'),
        elevation: 0.0,
      ),
      body: Container(
        padding: EdgeInsets.all(16.0),
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            ExpansionPanelList(
              expansionCallback: (int panelIndex, bool isExpanded) {
                setState(() {
                  _expansionPanelItems[panelIndex].isExpanded = !isExpanded;
                });
              },
              children: _expansionPanelItems.map(
                (ExpansionPanelItem item) {
                  return ExpansionPanel(
                    isExpanded: item.isExpanded,
                    body: item.body,
                    headerBuilder: (BuildContext context, bool isExpanded) {
                      return Container(
                        padding: EdgeInsets.all(16.0),
                        child: Text(
                          item.headerText,
                          style: Theme.of(context).textTheme.title,
                        ),
                      );
                    },
                  );
                }
              ).toList(),
            ),
          ],
        ),
      ),
    );
  }
}

效果：