StateListener<S> class

A widget that listens to a StateReadable and calls a listener when the state changes.

StateListener is used for performing side effects in response to state changes such as showing dialogs, navigating to another screen, or logging events. It does not rebuild its child when the state changes.

The listener function is called whenever the state changes and the optional listenWhen condition returns true.

{@tool snippet} This example shows how to use StateListener to log state changes and perform navigation:

class CounterWidget extends StatelessWidget {
  final CounterReadable counterReadable;
  final NavigatorState navigator;

  const CounterWidget({
    required this.counterReadable,
    required this.navigator,
    super.key,
  });

  @override
  Widget build(BuildContext context) => StateListener<int>(
    stateReadable: counterReadable,
    listenWhen: (previous, current) => current > 5, // Only listen when counter exceeds 5
    listener: (context, count) {
      // Log the current count
      debugPrint('Counter reached milestone: $count');

      // Navigate to a different screen when count reaches 10
      if (count >= 10) {
        navigator.pushNamed('/milestone-reached');
      }
    },
    child: const SizedBox(), // A simple placeholder widget
  );
}


class CounterReadable implements StateReadable<int> {
  final StreamController<int> _controller = StreamController<int>.broadcast();
  int _count = 0;

  @override
  int get state => _count;

  @override
  Stream<int> get stream => _controller.stream;

  void increment() {
    _count++;
    _controller.add(_count);
  }

  Future<void> dispose() => _controller.close();
}

{@end-tool}

See also:

  • StateBuilder, which rebuilds when the state changes instead of performing side effects.
  • StateConsumer, which combines both rebuilding and side effects.
  • StateSelector, which rebuilds only when a specific part of the state changes.
Inheritance

Constructors

StateListener({required StateReadable<S> stateReadable, required StateWidgetListener<S> listener, required Widget child, StateListenerCondition<S>? listenWhen, Key? key})
Creates a new StateListener.
const

Properties

child → Widget
The widget below this widget in the tree.
final
hashCode → int
The hash code for this object.
no setterinherited
key → Key?
Controls how one widget replaces another widget in the tree.
finalinherited
listener → StateWidgetListener<S>
The function that is called when the state changes.
final
listenWhen → StateListenerCondition<S>?
Optional condition to determine when to call listener.
final
runtimeType → Type
A representation of the runtime type of the object.
no setterinherited
stateReadable → StateReadable<S>
The source of the state.
final

Methods

createElement() → StatefulElement
Creates a StatefulElement to manage this widget's location in the tree.
inherited
createState() → State<StateListener<S>>
Creates the mutable state for this widget at a given location in the tree.
override
debugDescribeChildren() → List<DiagnosticsNode>
Returns a list of DiagnosticsNode objects describing this node's children.
inherited
debugFillProperties(DiagnosticPropertiesBuilder properties) → void
Add additional properties associated with the node.
inherited
noSuchMethod(Invocation invocation) → dynamic
Invoked when a nonexistent method or property is accessed.
inherited
toDiagnosticsNode({String? name, DiagnosticsTreeStyle? style}) → DiagnosticsNode
Returns a debug representation of the object that is used by debugging tools and by DiagnosticsNode.toStringDeep.
inherited
toString({DiagnosticLevel minLevel = DiagnosticLevel.info}) → String
A string representation of this object.
inherited
toStringDeep({String prefixLineOne = '', String? prefixOtherLines, DiagnosticLevel minLevel = DiagnosticLevel.debug, int wrapWidth = 65}) → String
Returns a string representation of this node and its descendants.
inherited
toStringShallow({String joiner = ', ', DiagnosticLevel minLevel = DiagnosticLevel.debug}) → String
Returns a one-line detailed description of the object.
inherited
toStringShort() → String
A short, textual description of this widget.
inherited

Operators

operator ==(Object other) → bool
The equality operator.
inherited