Flutter / Dart: StatefulWidget - access class variable inside Widget

Question

I declare a class variable for a StatefulWidget - in the code below it's someString. Is it possible to use this variable in the build(…)-method without declaring it as static?

class MyClass extends StatefulWidget {
  String someString;
  MyClass() {
    this.someString = "foo";
  }
  @override
  _MyClassState createState() => _MyClassState();
}

class _MyClassState extends State<MyClass> {
  _MyClassState();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("someString - how to access it here?!"),
        // title: Text(someString), is not possible, obviously
      ),
    );
  }
}

Thanks in advance for help!

Solution

Attention: MyClass should be immutable.

1. If someString will never change

Keep it inside MyClass but define it as final.

class MyClass extends StatefulWidget {
  final String someString;

  const MyClass({Key key, this.someString = 'foo'}) : super(key: key);

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

Then, inside the State, you can use it as widget.someString:

class _MyClassState extends State<MyClass> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('${widget.someString} is accessed here?!')),
    );
  }
}

2. If someString will change

It should be defined in the state.

class MyClass extends StatefulWidget {
  final String initialValue;

  const MyClass({Key key, this.initialValue = 'foo'}) : super(key: key);

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

class _MyClassState extends State<MyClass> {
  String someString;

  @override
  void initState() {
    someString = widget.initialValue;
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('$someString is accessed here?!')),
      body: Center(
        child: OutlinedButton(
          onPressed: () => setState(() => someString = 'NEW Value'),
          child: Text('Update value'),
        ),
      ),
    );
  }
}

Comments

[Raine Dale Holgado]

you can directly access it like this Text(widget.someString);

[BNetz]

Thanks, Thierry, this is was extremely helpful! If someString will change - how is it possible to pass it to the State via the constructor?

[Thierry]

Is it changing within the MyClass widget or only from ancestors in the Widget Tree?

[BNetz]

In the MyClass widget.

[Thierry]

MyClass(initialValue: 'Azerty')