How to call state functions of a stateful child widget from a stateless parent widget?

Question

I recently began learning flutter with this tutorial and I've built a simple app that has a stateless widget with a floating button that prompts the user for a text input and then calls a function inside a child widget of type Row from a separate class, this is how it looks like right now:

The method inside the child widget's state is the following:

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

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

As you can see I store the user input in SharedPreferences and then get it from inside the child state, which I managed to expose like so:

The child state

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

Inside the parent widget I create the state and set it to the widget like so:

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

And then finally invoke it like so:

petList.getPetListState().getPetNameList();

Is this the correct way to go about making a simple app that adds items to a list? I got to this 'hacky' solution by trial and error, I've read that exposing the actual state should not be done but then how am I supposed to get the child widget to run the Build() method to update how it looks from inside it's parent?

Any insight in how to achieve this functionality in the correct way is greatly appreciated.

Answer 1

If you want to call a function inside a StatefulWidget widget
You need to use GlobalKey to keep YourFormState and call function inside YourFormState with key.currentState
The following demo is appbar action call a function inside a form StatefulWidget, so appbar action and form submit button can use same function and snackbar also work fine

code snippet

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);

full code

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')));
    }
  }
}

running demo