从有状态小部件更新无状态小部件答案

【问题标题】：Updating a Stateless Widget from a Stateful Widget从有状态小部件更新无状态小部件
【发布时间】：2021-06-11 08:42:49
【问题描述】：

Main.dart：

import 'package:flutter/material.dart';
import 'package:mypackage/widgets/home_widget.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: "My App",
      theme: ThemeData(primarySwatch: Colors.blueGrey),
      home: Home(),
    );
  }
}

Home.dart：

import 'package:flutter/material.dart';
import 'package:mypackage/widgets/secondary_page.dart';
import 'package:mypackage/widgets/home_view.dart';
import 'package:mypackage/widgets/settings_page.dart';

class Home extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    return _HomeState();
  }
}

class _HomeState extends State<Home> {
  int currentPageIndex = 0;
  final List<Widget> pageWidgets = [HomeView(), SecondaryPage(), SettingsPage()];

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text("My App"),
          centerTitle: true,
        ),
        body: pageWidgets[currentPageIndex],
        bottomNavigationBar: BottomNavigationBar(
          onTap: onTabTapped,
          currentIndex: currentPageIndex,
          items: [
            BottomNavigationBarItem(
                icon: new Icon(Icons.home), label: "Tab One"),
            BottomNavigationBarItem(
                icon: new Icon(Icons.account_balance), label: "Tab Two"),
            BottomNavigationBarItem(
                icon: new Icon(Icons.settings), label: "Tab Three"),
          ],
        ),
        floatingActionButton: FloatingActionButton(onPressed: () {
          // Update data from here
          HomeView().updateDisplay();
        }));
  }

  // Changes the index when the tapped page has loaded
  void onTabTapped(int index) {
    setState(() {
      currentPageIndex = index;
    });
  }
}

HomeView.dart：

import 'package:flutter/material.dart';
import 'package:mypackage/models/mydisplay.dart';
import 'package:mypackage/network/mydata.dart';

class HomeView extends StatelessWidget {
  final List<MyDisplay> dataList = [
    new MyDisplay("Sample Display",
        "SampleFile", 0),
  ];

  Future<void> updateDisplay() async {
    MyData myData = new MyData();

    dataList.forEach((d) async {
      d.changePercentage = await myData
          .getDataPercentage("assets/data/" + d.fileName + ".xml");

      print(d.dataName +
          " change percentage: " +
          d.changePercentage.toString() +
          "%");
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: new ListView.builder(
          itemCount: dataList.length,
          itemBuilder: (BuildContext context, int index) =>
              buildCard(context, index)),
    );
  }

  Widget buildCard(BuildContext context, int index) {
    return Container(
        child: Card(
            color: Colors.teal.shade200,
            child: Padding(
                padding: const EdgeInsets.all(25),
                child: Column(children: <Widget>[
                  Text(dataList[index].dataName,
                      style: new TextStyle(
                          fontSize: 20, fontWeight: FontWeight.bold)),
                  Text(dataList[index].changePercentage.toString() + "%")
                ]))));
  }
}

当我从 floatingActionButton 调用 updateDisplay() 方法时，它按预期工作，数据被打印到控制台，但是它似乎并没有更新实际的 UI，卡片的内容，它们只是保持在他们的初始设定值。

澄清一下：如何从有状态小部件更新列表视图内（无状态小部件内）卡片的内容？

很抱歉，如果这是一个重复的问题，我已经查看并继续寻找问题的答案，但我根本无法让它工作。

【问题讨论】：

标签： flutter dart flutter-state

【解决方案1】：

虽然我理解您希望实现的目标，但我相信这将与“Flutter 方式”背道而驰。您可以使用一个键来实现您想要的结果，但我相信将状态提升到小部件树会更容易和更惯用。您可以在文档here 中了解它。

因此，在您的情况下，您可以将其更改为：

class Home extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    return _HomeState();
  }
}

class _HomeState extends State<Home> {
  int currentPageIndex = 0;
  // Added this line
  List<MyDisplay> dataList = [
    new MyDisplay("Sample Display",
        "SampleFile", 0),
  ];

  final List<Widget> pageWidgets; 

  // moved the update function from HomeView to here
  Future<void> updateDisplay() async {
    MyData myData = new MyData();

    dataList.forEach((d) async {
      d.changePercentage = await myData
          .getDataPercentage("assets/data/" + d.fileName + ".xml");

      print(d.dataName +
          " change percentage: " +
          d.changePercentage.toString() +
          "%");
    });
    setState((){});
  }

  @override
  Widget build(BuildContext context) {
    pageWidgets = [HomeView(dataList: dataList), SecondaryPage(), SettingsPage()]

    return Scaffold(
        appBar: AppBar(
          title: Text("My App"),
          centerTitle: true,
        ),
        body: pageWidgets[currentPageIndex],
        bottomNavigationBar: BottomNavigationBar(
          onTap: onTabTapped,
          currentIndex: currentPageIndex,
          items: [
            BottomNavigationBarItem(
                icon: new Icon(Icons.home), label: "Tab One"),
            BottomNavigationBarItem(
                icon: new Icon(Icons.account_balance), label: "Tab Two"),
            BottomNavigationBarItem(
                icon: new Icon(Icons.settings), label: "Tab Three"),
          ],
        ),
        floatingActionButton: FloatingActionButton(onPressed: () {
          updateDisplay();
        }));
  }

  // Changes the index when the tapped page has loaded
  void onTabTapped(int index) {
    setState(() {
      currentPageIndex = index;
    });
  }
}

然后为您的 HomeView 小部件：

class HomeView extends StatelessWidget {
  HomeView({this.dataList});

  final List<MyDisplay> dataList;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: new ListView.builder(
          itemCount: dataList.length,
          itemBuilder: (BuildContext context, int index) =>
              buildCard(context, index)),
    );
  }

  Widget buildCard(BuildContext context, int index) {
    return Container(
        child: Card(
            color: Colors.teal.shade200,
            child: Padding(
                padding: const EdgeInsets.all(25),
                child: Column(children: <Widget>[
                  Text(dataList[index].dataName,
                      style: new TextStyle(
                          fontSize: 20, fontWeight: FontWeight.bold)),
                  Text(dataList[index].changePercentage.toString() + "%")
                ],
              ),
            ),
          ),
        );
  }
}

我没有测试这是否有效。我可能会晚一点。

【讨论】：

【解决方案2】：

您需要使用有状态小部件，因为您无法更新无状态小部件。检查this 链接以获取更多信息。下一个问题，调用它，可以使用globalKeys 修复。唯一奇怪的是您必须将 Home 有状态小部件和 HomeView 有状态小部件保存在同一个 dart 文件中。 .试试这个代码让它工作。

首页视图

import 'package:flutter/material.dart';
import 'package:mypackage/widgets/secondary_page.dart';
import 'package:mypackage/widgets/settings_page.dart';
import 'package:mypackage/models/mydisplay.dart';
import 'package:mypackage/network/mydata.dart';

class Home extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    return _HomeState();
  }
}

class _HomeState extends State<Home> {
  int currentPageIndex = 0;
  final List<Widget> pageWidgets = [HomeView(), SecondaryPage(), SettingsPage()];
  GlobalKey<_HomeViewState> _myKey = GlobalKey();// We declare a key here
  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text("My App"),
          centerTitle: true,
        ),
        body: pageWidgets[currentPageIndex],
        bottomNavigationBar: BottomNavigationBar(
          onTap: onTabTapped,
          currentIndex: currentPageIndex,
          items: [
            BottomNavigationBarItem(
                icon: new Icon(Icons.home), label: "Tab One"),
            BottomNavigationBarItem(
                icon: new Icon(Icons.account_balance), label: "Tab Two"),
            BottomNavigationBarItem(
                icon: new Icon(Icons.settings), label: "Tab Three"),
          ],
        ),
        floatingActionButton: FloatingActionButton(onPressed: () {
          // Update data from here
          _myKey.currentState.updateDisplay();//This is how we call the function
        }));
  }

  // Changes the index when the tapped page has loaded
  void onTabTapped(int index) {
    setState(() {
      currentPageIndex = index;
    });
  }
}

class HomeView extends StatefulWidget {
  Function updateDisplay;
  HomeView({Key key}): super(key: key);//This key is what we use
  @override
  _HomeViewState createState() => _HomeViewState();
}

class _HomeViewState extends State<HomeView> {
  final List<MyDisplay> dataList = [
    new MyDisplay("Sample Display",
        "SampleFile", 0),
  ];

@override
  void initState(){
    super.initState();
    widget.updateDisplay = () async {
      MyData myData = new MyData();

      dataList.forEach((d) async {
        d.changePercentage = await myData
            .getDataPercentage("assets/data/" + d.fileName + ".xml");
        print(d.dataName +
            " change percentage: " +
            d.changePercentage.toString() +
            "%");
      });
      setState((){});//This line rebuilds the scaffold
    };
  }
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: new ListView.builder(
          itemCount: dataList.length,
          itemBuilder: (BuildContext context, int index) =>
              buildCard(context, index)),
    );
  }

  Widget buildCard(BuildContext context, int index) {
    return Container(
        child: Card(
            color: Colors.teal.shade200,
            child: Padding(
                padding: const EdgeInsets.all(25),
                child: Column(children: <Widget>[
                  Text(dataList[index].dataName,
                      style: new TextStyle(
                          fontSize: 20, fontWeight: FontWeight.bold)),
                  Text(dataList[index].changePercentage.toString() + "%")
                ]))));
  }
}

我们使用 setState((){}) 来重建小部件。只要记住一个想法。无论您在哪里使用 updateDisplay 方法，都不要让它在重建后循环运行。例如。您在孩子内部的未来构建器中使用它。这样它会不断重建

编辑

我们向 HomeView 添加了一个密钥，现在我们可以使用该密钥，例如 _myKey.currentState.yourFunction。希望我有所帮助

【讨论】：

将 HomeView 转换为有状态小部件后，我无法再从 home_widget.dart 调用 updateDisplay()
哦。让我想办法解决这个问题。我会添加另一条评论，让你知道我什么时候完成了
@AdamBrodin 我已经做到了。请检查它是否有效。我正在添加解释
它不起作用，说“方法调用在 null 上调用”
哦。我认为 initState() 在这样调用时不会运行。还有另一种使用键的方法，但它很复杂，所以我会在弄清楚时发布它

【解决方案3】：

您需要将您的HomeView 转换为StatefulWidget。

之后你需要将updateDisplay修改成这样：

Future<void> updateDisplay() async {
    MyData myData = new MyData();

    dataList.forEach((d) async {
      d.changePercentage = await myData
          .getDataPercentage("assets/data/" + d.fileName + ".xml");

      print(d.dataName +
          " change percentage: " +
          d.changePercentage.toString() +
          "%");
    });
    
    setState(() {});
  }

通过调用setState (() {});，您可以调用StatefulWidget 中的构建方法。

编辑

像这样更改您的HomeView：

class HomeView extends StatefulWidget {
  final List<MyDisplay> dataList = [
    new MyDisplay("Sample Display",
        "SampleFile", 0),
  ];

  Future<void> updateDisplay() async {
    MyData myData = new MyData();

    dataList.forEach((d) async {
      d.changePercentage = await myData
          .getDataPercentage("assets/data/" + d.fileName + ".xml");

      print(d.dataName +
          " change percentage: " +
          d.changePercentage.toString() +
          "%");
    });
  }
  @override
  _HomeViewState createState() => _HomeViewState();
}

class _HomeViewState extends State<HomeView> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: new ListView.builder(
          itemCount: widget.dataList.length,
          itemBuilder: (BuildContext context, int index) =>
              buildCard(context, index)),
    );
  }

  Widget buildCard(BuildContext context, int index) {
    return Container(
        child: Card(
            color: Colors.teal.shade200,
            child: Padding(
                padding: const EdgeInsets.all(25),
                child: Column(children: <Widget>[
                  Text(widget.dataList[index].dataName,
                      style: new TextStyle(
                          fontSize: 20, fontWeight: FontWeight.bold)),
                  Text(widget.dataList[index].changePercentage.toString() + "%")
                ]))));
  }
}

你的Home 是这样的：

class Home extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    return _HomeState();
  }
}

class _HomeState extends State<Home> {
  int currentPageIndex = 0;
  HomeView _homeView = HomeView();
  List<Widget> pageWidgets = [_homeView, SecondaryPage(), SettingsPage()];

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text("My App"),
          centerTitle: true,
        ),
        body: pageWidgets[currentPageIndex],
        bottomNavigationBar: BottomNavigationBar(
          onTap: onTabTapped,
          currentIndex: currentPageIndex,
          items: [
            BottomNavigationBarItem(
                icon: new Icon(Icons.home), label: "Tab One"),
            BottomNavigationBarItem(
                icon: new Icon(Icons.account_balance), label: "Tab Two"),
            BottomNavigationBarItem(
                icon: new Icon(Icons.settings), label: "Tab Three"),
          ],
        ),
        floatingActionButton: FloatingActionButton(onPressed: () {
          // Update data from here
          _homeView.updateDisplay();
        }));
  }

  // Changes the index when the tapped page has loaded
  void onTabTapped(int index) {
    setState(() {
      currentPageIndex = index;
    });
  }
}

【讨论】：

将 HomeView 转换为有状态小部件后，我无法再从 home_widget.dart 调用 updateDisplay()
嗯，我认为你需要静态方法，检查我的编辑！
静态不起作用。我们需要使用小部件。就像我在回答中使用的那样
@SiddharthAgrawal 这是真的。我写了一个新的解决方案！
那是我的解决方案。你不能复制别人的解决方案。顺便说一句，根据亚当布罗丁的说法，它不起作用。我有一个使用密钥的新解决方案

【解决方案4】：

如下更新你的 home.dart。

import 'package:flutter/material.dart';
import 'package:mypackage/widgets/secondary_page.dart';
import 'package:mypackage/widgets/home_view.dart';
import 'package:mypackage/widgets/settings_page.dart';

class Home extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    return _HomeState();
  }
}

class _HomeState extends State<Home> {
  int currentPageIndex = 0;
  Widget _homeView = HomeView();
  final List<Widget> pageWidgets = [_homeView, SecondaryPage(), SettingsPage()];

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text("My App"),
          centerTitle: true,
        ),
        body: pageWidgets[currentPageIndex],
        bottomNavigationBar: BottomNavigationBar(
          onTap: onTabTapped,
          currentIndex: currentPageIndex,
          items: [
            BottomNavigationBarItem(
                icon: new Icon(Icons.home), label: "Tab One"),
            BottomNavigationBarItem(
                icon: new Icon(Icons.account_balance), label: "Tab Two"),
            BottomNavigationBarItem(
                icon: new Icon(Icons.settings), label: "Tab Three"),
          ],
        ),
        floatingActionButton: FloatingActionButton(onPressed: () {
          // Update data from here
          _homeView.updateDisplay();
        }));
  }

  // Changes the index when the tapped page has loaded
  void onTabTapped(int index) {
    setState(() {
      currentPageIndex = index;
    });
  }
}

home_view.dart如下

import 'package:flutter/material.dart';
import 'package:mypackage/models/mydisplay.dart';
import 'package:mypackage/network/mydata.dart';

class HomeView extends StatelessWidget {
  final List<MyDisplay> dataList = [
    new MyDisplay("Sample Display",
        "SampleFile", 0),
  ];

  Future<void> updateDisplay() async {
    MyData myData = new MyData();

    dataList.forEach((d) async {
      d.changePercentage = await myData
          .getDataPercentage("assets/data/" + d.fileName + ".xml");

      print(d.dataName +
          " change percentage: " +
          d.changePercentage.toString() +
          "%");
    });
     setState((){});
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: new ListView.builder(
          itemCount: dataList.length,
          itemBuilder: (BuildContext context, int index) =>
              buildCard(context, index)),
    );
  }

  Widget buildCard(BuildContext context, int index) {
    return Container(
        child: Card(
            color: Colors.teal.shade200,
            child: Padding(
                padding: const EdgeInsets.all(25),
                child: Column(children: <Widget>[
                  Text(dataList[index].dataName,
                      style: new TextStyle(
                          fontSize: 20, fontWeight: FontWeight.bold)),
                  Text(dataList[index].changePercentage.toString() + "%")
                ]))));
  }
}

【讨论】：

您在 pageWidgets 和 floatingActionButton 中创建了两个 HomeView 实例。

【解决方案5】：

你需要在HomeView().updateDisplay();之后调用setState(() {});

【讨论】：

现在试了，还是不更新界面