ValueListenableBuilder<T> class

A widget whose content stays synced with a ValueListenable.

Given a ValueListenable<T> and a builder which builds widgets from concrete values of T, this class will automatically register itself as a listener of the ValueListenable and call the builder with updated values when the value changes.

Performance optimizations

If your builder function contains a subtree that does not depend on the value of the ValueListenable, it's more efficient to build that subtree once instead of rebuilding it on every animation tick.

If you pass the pre-built subtree as the child parameter, the ValueListenableBuilder will pass it back to your builder function so that you can incorporate it into your build.

Using this pre-built child is entirely optional, but can improve performance significantly in some cases and is therefore a good practice.

This sample shows how you could use a ValueListenableBuilder instead of setting state on the whole Scaffold in the default flutter create app.
class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);
  final String title;

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

class _MyHomePageState extends State<MyHomePage> {
  final ValueNotifier<int> _counter = ValueNotifier<int>(0);
  final Widget goodJob = const Text('Good job!');
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title)
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text('You have pushed the button this many times:'),
            ValueListenableBuilder(
              builder: (BuildContext context, int value, Widget child) {
                // This builder will only get called when the _counter
                // is updated.
                return Row(
                  mainAxisAlignment: MainAxisAlignment.spaceEvenly,
                  children: <Widget>[
                    Text('$value'),
                    child,
                  ],
                );
              },
              valueListenable: _counter,
              // The child parameter is most helpful if the child is
              // expensive to build and does not depend on the value from
              // the notifier.
              child: goodJob,
            )
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        child: Icon(Icons.plus_one),
        onPressed: () => _counter.value += 1,
      ),
    );
  }
}

See also:

Inheritance

Constructors

ValueListenableBuilder({@required ValueListenable<T> valueListenable, @required ValueWidgetBuilder<T> builder, Widget child })
Creates a ValueListenableBuilder. [...]
const

Properties

builder ValueWidgetBuilder<T>
A ValueWidgetBuilder which builds a widget depending on the valueListenable's value. [...]
final
child Widget
A valueListenable-independent widget which is passed back to the builder. [...]
final
valueListenable ValueListenable<T>
The ValueListenable whose value you depend on in order to build. [...]
final
hashCode int
The hash code for this object. [...]
read-only, inherited
key Key
Controls how one widget replaces another widget in the tree. [...]
final, inherited
runtimeType Type
A representation of the runtime type of the object.
read-only, inherited

Methods

createState() State<StatefulWidget>
Creates the mutable state for this widget at a given location in the tree. [...]
override
createElement() StatefulElement
Creates a StatefulElement to manage this widget's location in the tree. [...]
inherited
debugDescribeChildren() List<DiagnosticsNode>
Returns a list of DiagnosticsNode objects describing this node's children. [...]
@protected, inherited
debugFillProperties(DiagnosticPropertiesBuilder properties) → void
Add additional properties associated with the node. [...]
inherited
noSuchMethod(Invocation invocation) → dynamic
Invoked when a non-existent 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.debug }) String
Returns a string representation of this object.
inherited
toStringDeep({String prefixLineOne: '', String prefixOtherLines, DiagnosticLevel minLevel: DiagnosticLevel.debug }) 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 ==(dynamic other) bool
The equality operator. [...]
inherited