Flutter StatefulWidget - State class inheritance?

Question

The point of this question, is how to handle state changes, preferably automatically by the original ancestor. It seems to me that it is not possible to keep extending these classes beyond the first child with a StatefulWidget as an ancestor (example 1)?

The only way seems to me, to use mixins. However this has the drawback, that you have to manage state changes manually (example 2).

Am I overlooking something ?

// Example 1 - not working as BarState dose not inherit from Bar

class Foo extends StatefulWidget {
  final String stringFoo;

  Foo({Key key, this.stringFoo}) : super(key: key);

  @override
  FooState createState() => new FooState();
}

class FooState extends State<Foo> {
  bool updating = false;

  Future<void> _update() async {
    // await something(stringFoo)
    setState(() {
      updating = false;
    });
  }

  void update() {
    setState(() {
      updating = true;
    });
    _update();
  }

  // Dummy build, always override
  @override
  Widget build(BuildContext context) {
    return null;
  }

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

class Bar extends Foo {
  final String stringBar;

  Bar({Key key, String stringFoo, this.stringBar}) : super(key: key, stringFoo: stringFoo);

  @override
  BarState createState() => new BarState();
}

class BarState extends FooState {

  @override
  Widget build(BuildContext context) {
    if(updating) {
      return Text('Im updating: ' + widget.stringFoo);
    } else {
      return Text('All done!' + widget.stringBar);
    }
  }

}

// Example 2 - Works but state changes are handled manually

class Foo {
  String stringFoo;
  bool updating = false;

  Widget getFoo() {
    return new Text(stringFoo);
  }

  Future<void> _update() async {
    // await something
    // stringFoo = await result
    updating = false;
    stateChanged();
  }

  void update() {
    updating = true;
    stateChanged();
    _update();
  }

  void stateChanged() {}
}

class Bar extends StatefulWidget {
  final stringBar;

  Bar({Key key, this.stringBar}) : super(key: key);

  @override
  BarState createState() => new BarState();
}

class BarState extends State<Bar> with Foo {
  bool dummy = true;

  Widget getBar() {
    return new Text(widget.stringBar);
  }

  @override
  void stateChanged() {
    setState(() {
      if(dummy) {
        dummy = false;
      } else {
        dummy = true;
      }
    });
  }

  @override
  Widget build(BuildContext context) {
    if(updating) {
      return getBar();
    } else {
      return getFoo();
    }
  }

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

Answer 1

This can be done using templates.

I found a valid case for it so I'll be using it. Here's an example...

The base classes are:

• class BasePage extends StatefulWidget
• class BasePageState<T extends BasePage> extends State<T>

Note that classes you want to be inherited must be public, therefore the names of the classes are BasePage and BasePageState instead of _BasePage and _BasePageState.

Also note that BasePageState is declared as a template, so that the state classes which inherit from them can pass their own widgets there.

The classes which inherit are:

• class MyHomePage extends BasePage
• class _MyHomePageState extends BasePageState<MyHomePage>
• class MyHomePage2 extends BasePage
• class _MyHomePageState2 extends BasePageState<MyHomePage2>

The counter variable and the function incrementCounter() are defined in the BasePageState class, the function increments counter by 1.

In _MyHomePageState2 class I override the incrementCounter() function and make it increase the counter by 2.

I'm still testing it, so don't know if I'll come accross some pitfalls, but so far it seems to be working.

Here are the dart files...

main.dart:

import 'package:flutter/material.dart';
import 'package:svipopusti/main2.dart';
import 'base_page.dart';

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

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: 'HomePage'),
    );
  }
}

class MyHomePage extends BasePage {
  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();
}

class _MyHomePageState extends BasePageState<MyHomePage> {

  @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),
      ),
      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 a 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>[
            FlatButton(
              child: Text("Open main2"),
              onPressed: () => Navigator.push(context, MaterialPageRoute(builder: (context) => MyHomePage2(title: "Homepage 2"))),
            ),
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$counter',
              style: Theme.of(context).textTheme.headline4,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}

main2.dart:

import 'package:flutter/material.dart';
import 'base_page.dart';


class MyHomePage2 extends BasePage {
  MyHomePage2({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
  _MyHomePageState2 createState() => _MyHomePageState2();
}

class _MyHomePageState2 extends BasePageState<MyHomePage2> {

  @override
  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 += 2;
    });
  }

  @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),
      ),
      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 a 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>[
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$counter',
              style: Theme.of(context).textTheme.headline4,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}

base_page.dart:

import 'package:flutter/material.dart';

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

  @override
  BasePageState createState() => BasePageState();
}

class BasePageState<T extends BasePage> extends State<T> {
  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) {
    return Container();
  }
}

Answer 2

Don't. You should never extend a widget. This is anti-pattern. Instead, as stated by flutter documentation :

You create a layout by composing widgets to build more complex widgets.

An example would be :

class Foo extends StatefulWidget {
  final Widget child;

  Foo({this.child});

  @override
  _FooState createState() => new _FooState();
}

class _FooState extends State<Foo> {
  @override
  Widget build(BuildContext context) {
    return new Container(
      child: widget.child
    );
  }
}



class Bar extends StatefulWidget {
  final Widget child;

  Bar({this.child});

  @override
  _BarState createState() => new _BarState();
}

class _BarState extends State<Bar> {
  @override
  Widget build(BuildContext context) {
    return new Foo(
      child: widget.child
    );
  }
}

In this case, Bar has no mixin or inheritance. It just wrap it's child inside a Foo.

Answer 3

I do not think inheritance or mixins (generally extending) a widget is an anti-pattern (even though the Flutter team apparently thinks so).

I generally like the inheritance answer, however annoyingly the superclass' attributes are lost. So if the class has field title, then widget.title will not work with the proposed answer.

I thus prefer extending with a mixin. I give an example here, or just see the gist

// Flutter example with common navigation bar in app
import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}


class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  // Root of application
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      // Define routing information
      initialRoute: '/home',
      routes: {
        '/home': (context) => const Home(),
        '/page': (context) => const Page(),
      },
    );
  }
}


mixin BottomNavigationMixin<T extends StatefulWidget> on State<T>{
  final List<String> _pages = [
    '/home',
    '/page',
  ];

  Widget bottomNavigationBar(BuildContext context, _selectedIndex) {
    return BottomNavigationBar(
      items: const <BottomNavigationBarItem>[
        BottomNavigationBarItem(
          icon: Icon(Icons.home),
          label: 'Home',
        ),
        BottomNavigationBarItem(
          icon: Icon(Icons.analytics_rounded),
          label: 'Page',
        ),
      ],
      currentIndex: _selectedIndex,
      onTap: (index) {
        if (index != _selectedIndex) {
          setState(() {
            _selectedIndex = index;
            Navigator.pushNamed(context, _pages[index]);
          });
        }
      },
    );
  }
}


class Home extends StatefulWidget {
  const Home({Key? key}) : super(key: key);

  @override
  _HomeState createState() => _HomeState();
}

class _HomeState extends State<Home> with BottomNavigationMixin<Home>{
  final int _selectedIndex = 0;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text("home")),
      bottomNavigationBar: bottomNavigationBar(context, _selectedIndex),
    );
  }
}


class Page extends StatefulWidget {
  const Page({Key? key}) : super(key: key);

  @override
  _PageState createState() => _PageState();
}

class _PageState extends State<Page> with BottomNavigationMixin<Page>{
  final int _selectedIndex = 1;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text("page")),
      bottomNavigationBar: bottomNavigationBar(context, _selectedIndex),
    );
  }
}
```

Answer 4

Solution:

// Foo
class Foo extends StatefulWidget {

  // Constructor
  Foo({Key key, this.title}) : super(key: key);

  final String title;

  @override
  FooState createState() => new FooState();
}

// FooState
class FooState<T extends Foo> extends State<T> {
  // Override build
  @override
  Widget build(BuildContext context) {
    return Container();
  }
}

// Bar
class Bar extends Foo {

  // Constructor
  Bar({Key key, String title}) : super(key: key, title: title);

  @override
  BarState createState() => new BarState();
}

// BarState
class BarState extends FooState<Bar> {

}