InheritedWidget class abstract

Base class for widgets that efficiently propagate information down the tree.

To obtain the nearest instance of a particular type of inherited widget from a build context, use BuildContext.dependOnInheritedWidgetOfExactType.

Inherited widgets, when referenced in this way, will cause the consumer to rebuild when the inherited widget itself changes state.

class FrogColor extends InheritedWidget {
  const FrogColor({
    super.key,
    required this.color,
    required super.child,
  });

  final Color color;

  static FrogColor? maybeOf(BuildContext context) {
    return context.dependOnInheritedWidgetOfExactType<FrogColor>();
  }

  static FrogColor of(BuildContext context) {
    final FrogColor? result = maybeOf(context);
    assert(result != null, 'No FrogColor found in context');
    return result!;
  }

  @override
  bool updateShouldNotify(FrogColor oldWidget) => color != oldWidget.color;
}

Implementing the of and maybeOf methods

The convention is to provide two static methods, of and maybeOf, on the InheritedWidget which call BuildContext.dependOnInheritedWidgetOfExactType. This allows the class to define its own fallback logic in case there isn't a widget in scope.

The of method typically returns a non-nullable instance and asserts if the InheritedWidget isn't found, and the maybeOf method returns a nullable instance, and returns null if the InheritedWidget isn't found. The of method is typically implemented by calling maybeOf internally.

Sometimes, the of and maybeOf methods return some data rather than the inherited widget itself; for example, in this case it could have returned a Color instead of the FrogColor widget.

Occasionally, the inherited widget is an implementation detail of another class, and is therefore private. The of and maybeOf methods in that case are typically implemented on the public class instead. For example, Theme is implemented as a StatelessWidget that builds a private inherited widget; Theme.of looks for that private inherited widget using BuildContext.dependOnInheritedWidgetOfExactType and then returns the ThemeData inside it.

Calling the of or maybeOf methods

When using the of or maybeOf methods, the context must be a descendant of the InheritedWidget, meaning it must be "below" the InheritedWidget in the tree.

// continuing from previous example...
class MyPage extends StatelessWidget {
  const MyPage({super.key});

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: FrogColor(
        color: Colors.green,
        child: Builder(
          builder: (BuildContext innerContext) {
            return Text(
              'Hello Frog',
              style: TextStyle(color: FrogColor.of(innerContext).color),
            );
          },
        ),
      ),
    );
  }
}

// continuing from previous example...

class MyOtherPage extends StatelessWidget {
  const MyOtherPage({super.key});

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: FrogColor(
        color: Colors.green,
        child: Text(
          'Hello Frog',
          style: TextStyle(color: FrogColor.of(context).color),
        ),
      ),
    );
  }
}

See also:

• StatefulWidget and State, for widgets that can build differently several times over their lifetime.
• StatelessWidget, for widgets that always build the same way given a particular configuration and ambient state.
• Widget, for an overview of widgets in general.
• InheritedNotifier, an inherited widget whose value can be a Listenable, and which will notify dependents whenever the value sends notifications.
• InheritedModel, an inherited widget that allows clients to subscribe to changes for subparts of the value.