flutter - 如何从无状态父窗口小部件调用有状态子窗口小部件的状态函数？

Question

我最近开始使用this tutorial学习 flutter ，并构建了一个简单的应用程序，该应用程序具有带 float 按钮的无状态窗口小部件，该 float 窗口提示用户输入文本，然后从单独的类中调用Row类型的子窗口小部件内的函数，现在的样子:
子窗口小部件状态内的方法如下:

void getPetNameList() async{
    final prefs = await SharedPreferences.getInstance();

    List<String> petNameList = List<String>();
    if(prefs.containsKey("pets")){
      petNameList = prefs.getStringList("pets");
    }
    setState(() {
     petNames = petNameList; 
    });
  }

如您所见，我将用户输入存储在SharedPreferences中，然后从子状态中获取它，我设法将其公开为:
child 状态

class PetList extends StatefulWidget {
  static PetListState petListState;
  PetList(PetListState state)
  {
    petListState = state;
  }
  @override
  PetListState createState() => petListState;
  PetListState getPetListState(){
    return petListState;
  }
}

在父窗口小部件内，我创建状态并将其设置为窗口小部件，如下所示:

PetListState petListState = PetListState();
PetList petList = PetList(petListState);

然后像这样最终调用它:

petList.getPetListState().getPetNameList();

这是制作简单的将项目添加到列表的简单应用程序的正确方法吗？我通过反复试验获得了这个“hacky”解决方案，我读到不应该公开实际状态，但是我应该如何让子小部件运行Build()方法来更新它的外观？在里面是 parent 吗？
非常感谢您以正确的方式实现此功能的任何见解。

Answer 1

如果要在StatefulWidget小部件内调用函数
您需要使用GlobalKey保留YourFormState并使用key.currentState在YourFormState中调用函数
下面的演示是appbar操作在StatefulWidget表单内调用一个函数，因此
appbar操作和表单提交按钮可以使用相同的功能，snackbar也可以正常工作

程式码片段

final key = new GlobalKey<MyCustomFormState>();
...
appBar: AppBar(          
          title: Text(widget.title),
          actions: <Widget>[
            // action button
            IconButton(
              icon: Icon(Icons.access_alarm),
              onPressed: () {
                key.currentState.validateform();
              },
            ),
          ]),
...
children: <Widget>[
        MyCustomForm(key: key),

... 

class MyCustomForm extends StatefulWidget {
  MyCustomForm({ Key key }) : super(key: key);

完整的代码

import 'package:flutter/material.dart';

void main() => runApp(MyApp());
final key = new GlobalKey<MyCustomFormState>();

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        // This is the theme of your application.
        //
        // Try running your application with "flutter run". You'll see the
        // application has a blue toolbar. Then, without quitting the app, try
        // changing the primarySwatch below to Colors.green and then invoke
        // "hot reload" (press "r" in the console where you ran "flutter run",
        // or simply save your changes to "hot reload" in a Flutter IDE).
        // Notice that the counter didn't reset back to zero; the application
        // is not restarted.
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  // This widget is the home page of your application. It is stateful, meaning
  // that it has a State object (defined below) that contains fields that affect
  // how it looks.

  // This class is the configuration for the state. It holds the values (in this
  // case the title) provided by the parent (in this case the App widget) and
  // used by the build method of the State. Fields in a Widget subclass are
  // always marked "final".

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}
var myCustomForm =  MyCustomForm();

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _incrementCounter() {
    setState(() {
      // This call to setState tells the Flutter framework that something has
      // changed in this State, which causes it to rerun the build method below
      // so that the display can reflect the updated values. If we changed
      // _counter without calling setState(), then the build method would not be
      // called again, and so nothing would appear to happen.
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    // This method is rerun every time setState is called, for instance as done
    // by the _incrementCounter method above.
    //
    // The Flutter framework has been optimized to make rerunning build methods
    // fast, so that you can just rebuild anything that needs updating rather
    // than having to individually change instances of widgets.
    return Scaffold(
      appBar: AppBar(
        // Here we take the value from the MyHomePage object that was created by
        // the App.build method, and use it to set our appbar title.
        title: Text(widget.title),
          actions: <Widget>[
      // action button
      IconButton(
      icon: Icon(Icons.access_alarm),
      onPressed: () {
        key.currentState.validateform();
      },
    ),
      ]),
      body: Center(
        // Center is a layout widget. It takes a single child and positions it
        // in the middle of the parent.
        child: Column(
          // Column is also layout widget. It takes a list of children and
          // arranges them vertically. By default, it sizes itself to fit its
          // children horizontally, and tries to be as tall as its parent.
          //
          // Invoke "debug painting" (press "p" in the console, choose the
          // "Toggle Debug Paint" action from the Flutter Inspector in Android
          // Studio, or the "Toggle Debug Paint" command in Visual Studio Code)
          // to see the wireframe for each widget.
          //
          // Column has various properties to control how it sizes itself and
          // how it positions its children. Here we use mainAxisAlignment to
          // center the children vertically; the main axis here is the vertical
          // axis because Columns are vertical (the cross axis would be
          // horizontal).
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            MyCustomForm(key: key),
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.display1,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}

class MyCustomForm extends StatefulWidget {
  MyCustomForm({ Key key }) : super(key: key);

  @override
  MyCustomFormState createState() {
    return MyCustomFormState();
  }

}

// Create a corresponding State class.
// This class holds data related to the form.
class MyCustomFormState extends State<MyCustomForm> {
  // Create a global key that uniquely identifies the Form widget
  // and allows validation of the form.
  //
  // Note: This is a GlobalKey<FormState>,
  // not a GlobalKey<MyCustomFormState>.
  final _formKey = GlobalKey<FormState>();

  @override
  Widget build(BuildContext context) {
    // Build a Form widget using the _formKey created above.
    return Form(
      key: _formKey,
      child: Column(
        crossAxisAlignment: CrossAxisAlignment.start,
        children: <Widget>[
          TextFormField(
            validator: (value) {
              if (value.isEmpty) {
                return 'Please enter some text';
              }
              return null;
            },
          ),
          Padding(
            padding: const EdgeInsets.symmetric(vertical: 16.0),
            child: RaisedButton(
              onPressed: () {
                // Validate returns true if the form is valid, or false
                // otherwise.
                validateform();
              },
              child: Text('Submit'),
            ),
          ),
        ],
      ),
    );
  }

  void validateform() {
    // Validate returns true if the form is valid, or false
    // otherwise.
    if (_formKey.currentState.validate()) {
      // If the form is valid, display a Snackbar.
      Scaffold.of(context)
          .showSnackBar(SnackBar(content: Text('Processing Data')));
    }
  }
}

运行演示