Flutter ExpansionPanelList - expansionCallback 不工作

Question

我通过使用 Provider 包而不是使 Widget 成为 statefulWidget 来实现 ExpansionPanelList。但是，expansionCallback 永远无法正常工作。如果我点击屏幕上的第一个折叠列表，它永远不会展开。具体来说，data[0].isExpanded 在 SummitView.dart 中始终为 false。控制台没有报错。

这是我的 SummitView.dart:

import 'package:delta_y/modules/summits/model/Item.dart';
import 'package:delta_y/modules/summits/model/ItemsList.dart';
import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
import 'package:provider/provider.dart';

class SummitView extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text("Summit/Milestone"),
        ),
        body: ChangeNotifierProvider(
          create: (context) => ItemsList(),
          child: Summit(),
        ));
  }
}

class Summit extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final itemModel = Provider.of<ItemsList>(context);
    // List<Item> _data = generateItems(8);
    itemModel.generateItems(8);

    return Stack(children: <Widget>[
      SingleChildScrollView(
        child: Container(
          child: ExpansionPanelList(
            expansionCallback: (int index, bool isExpanded) {
              itemModel.setExpanded(index, isExpanded);
              print("INDEX : $index ... $isExpanded");
            },
            children: itemModel.data.map<ExpansionPanel>((Item item) {
              bool isData0Expanded = itemModel.data[0].isExpanded;
              print('**********');
              print(isData0Expanded);
              print('**********');

              return ExpansionPanel(
                headerBuilder: (BuildContext context, bool isExpanded) {
                  return ListTile(
                    title: Text(item.headerValue),
                  );
                },
                body: ListTile(
                    title: Text(item.expandedValue),
                    subtitle:
                    Text('To delete this panel, tap the trash can icon'),
                    trailing: Icon(Icons.delete),
                    onTap: () {
                      itemModel.deleteItem(item);
                    }),
                isExpanded: item.isExpanded,
              );
            }).toList(),
          ),
        ),
      ),
      Align(
          alignment: Alignment.bottomCenter,
          child: FractionallySizedBox(
            child: ElevatedButton(
              style: ElevatedButton.styleFrom(
                primary: Colors.white,
                onPrimary: Colors.black,
              ),
              onPressed: () {
                Navigator.pop(context);
              },
              child: Text('Create Summit'),
            ),
            widthFactor: 0.9,
          )),
    ]);
  }
}

这是 ItemsList.dart:

import 'package:flutter/cupertino.dart';
import 'Item.dart';

class ItemsList extends ChangeNotifier{
  List<Item> _data;

  List<Item> get data => _data;

  generateItems(int numberOfItems) {
    _data = List.generate(numberOfItems, (int index) {
      return Item(
        headerValue: 'Panel $index',
        expandedValue: 'This is item number $index',
      );
    });
  }

  setExpanded(int index, bool isExpanded) {
    _data[index].isExpanded = !isExpanded;
    notifyListeners();
  }

  deleteItem(Item item) {
    _data.removeWhere((currentItem) => item == currentItem);
    notifyListeners();
  }
}

这是 Item.dart:

import 'package:flutter/material.dart';

class Item{
  Item({
    @required this.expandedValue,
    @required this.headerValue,
    this.isExpanded = false,
  });

  String expandedValue;
  String headerValue;
  bool isExpanded;
}

Answer 1

问题是由这一行引起的:

class Summit extends StatelessWidget {
  @override
  Widget build(BuildContext context) {

    // ... other lines

    itemModel.generateItems(8); <---- 

在 build() 中调用方法时，每次屏幕从 Provider 接收到事件时都会调用该方法。在这种情况下，您的项目列表每次都会生成，这使得 isExpanded 始终为 false。

要解决此问题，最好添加一些条件以防止重置项目列表，或者在创建 ItemsList 时生成项目，例如:

body: ChangeNotifierProvider(
    create: (context) => ItemsList()..generateItems(8),
    child: Summit(),

结果: