ValueListenableBuilder<T> class
Null safety
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.
flutter create
app.
class MyHomePage extends StatefulWidget {
const MyHomePage({super.key, required this.title});
final String title;
@override
State<MyHomePage> 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>[
const Text('You have pushed the button this many times:'),
ValueListenableBuilder<int>(
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: const Icon(Icons.plus_one),
onPressed: () => _counter.value += 1,
),
);
}
}
See also:
- AnimatedBuilder, which also triggers rebuilds from a Listenable without passing back a specific value from a ValueListenable.
- NotificationListener, which lets you rebuild based on Notification coming from its descendant widgets rather than a ValueListenable that you have a direct reference to.
- StreamBuilder, where a builder can depend on a Stream rather than a ValueListenable for more advanced use cases.
- Inheritance
-
- Object
- DiagnosticableTree
- Widget
- StatefulWidget
- ValueListenableBuilder
Constructors
-
ValueListenableBuilder({Key? key, 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
- hashCode → int
- The hash code for this object.
- key → Key?
-
Controls how one widget replaces another widget in the tree.
finalinherited
- runtimeType → Type
-
A representation of the runtime type of the object.
read-onlyinherited
-
valueListenable
→ ValueListenable<
T> -
The ValueListenable whose value you depend on in order to build.
final
Methods
-
createElement(
) → StatefulElement -
Creates a StatefulElement to manage this widget's location in the tree.
inherited
-
createState(
) → State< StatefulWidget> -
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.protected">@protectedinherited -
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.info}) → String -
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 ==(
Object other) → bool -
The equality operator.
nonVirtual">@nonVirtualinherited