Flutter - Stateful Widget Doesn't Save Counter State When Switching Tabs

Question

I am learning flutter and I am working with tabBars and I am having an issue with saving the state. I have put a small working example of my issue below. Basically, there is a button and a stateful counter. When I click the button, I see the text field update correctly. But, when I switch to a different tab and come back, the text field is back to zero.

I have found if i move the following line outside of _CounterState so its defined at the top level of the file, then, it works correctly. When I switch tabs, the counter stays at the correct count when I switch back

int _counter = 0;

I don't feel like this is the appropriate way to do this and all of the examples I have seen have the variable inside of the class. Can anyone give me any insights? Why would it reset if it is inside the class? Am I supposed to keep it outside the class? Below is the simplified full example.

import 'package:flutter/material.dart';

void main() {
  runApp(new TabBarDemo());
}

class TabBarDemo extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      home: new DefaultTabController(
        length: 3,
        child: new Scaffold(
          appBar: new AppBar(
            bottom: new TabBar(
              tabs: [
                new Tab(icon: new Icon(Icons.directions_car)),
                new Tab(icon: new Icon(Icons.directions_transit)),
                new Tab(icon: new Icon(Icons.directions_bike)),
              ],
            ),
            title: new Text('Tabs Demo'),
          ),
          body: new TabBarView(
            children: [
              new Counter(),
              new Icon(Icons.directions_transit),
              new Icon(Icons.directions_bike),
            ],
          ),
        ),
      ),
    );
  }
}

class Counter extends StatefulWidget {
  @override
  _CounterState createState() => new _CounterState();
}

class _CounterState extends State<Counter> {
  int _counter = 0;

  void _increment() {
    setState(() {
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return new Row(
      children: <Widget>[
        new RaisedButton(
          onPressed: _increment,
          child: new Text('Increment'),
        ),
        new Text('Count: $_counter'),
      ],
    );
  }
}

Below is the example with the counter moved outside of the class

import 'package:flutter/material.dart';

void main() {
  runApp(new TabBarDemo());
}

class TabBarDemo extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      home: new DefaultTabController(
        length: 3,
        child: new Scaffold(
          appBar: new AppBar(
            bottom: new TabBar(
              tabs: [
                new Tab(icon: new Icon(Icons.directions_car)),
                new Tab(icon: new Icon(Icons.directions_transit)),
                new Tab(icon: new Icon(Icons.directions_bike)),
              ],
            ),
            title: new Text('Tabs Demo'),
          ),
          body: new TabBarView(
            children: [
              new Counter(),
              new Icon(Icons.directions_transit),
              new Icon(Icons.directions_bike),
            ],
          ),
        ),
      ),
    );
  }
}

class Counter extends StatefulWidget {
  @override
  _CounterState createState() => new _CounterState();
}

int _counter = 0; //<-- MOVED OUTSIDE THE _CounterState CLASS
class _CounterState extends State<Counter> {


  void _increment() {
    setState(() {
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return new Row(
      children: <Widget>[
        new RaisedButton(
          onPressed: _increment,
          child: new Text('Increment'),
        ),
        new Text('Count: $_counter'),
      ],
    );
  }
}

Answer 1

As _CounterState widget is built everytime you go to the given TabView you'll need to put _counter variable in the state configuration class (Counter).

class Counter extends StatefulWidget {
  int _counter = 0;
  @override
  _CounterState createState() => new _CounterState();
}

class _CounterState extends State<Counter> {
  void _increment() {
    setState(() {
      widget._counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return new Row(
      children: <Widget>[
        new RaisedButton(
          onPressed: _increment,
          child: new Text('Increment'),
        ),
        new Text('Count: ${widget._counter}'),
      ],
    );
  }
}