Consumer class

Build a widget tree while listening to providers.

Consumer can be used to listen to providers inside a StatefulWidget or to rebuild as few widgets as possible when a provider updates.

As an example, consider:

final helloWorldProvider = Provider((_) => 'Hello world');

We can then use Consumer to listen to helloWorldProvider inside a StatefulWidget like so:

class Example extends StatefulWidget {
  @override
  _ExampleState createState() => _ExampleState();
}

class _ExampleState extends State<Example> {
  @override
  Widget build(BuildContext context) {
    return Consumer(
      builder: (context, ref, child) {
        final value = ref.watch(helloWorldProvider);
        return Text(value); // Hello world
      },
    );
  }
}

Note You can watch as many providers inside Consumer as you want to:

Consumer(
  builder: (context, ref, child) {
    final value = ref.watch(someProvider);
    final another = ref.watch(anotherProvider);
    ...
  },
);

Performance optimizations

If your builder function contains a subtree that does not depend on the animation, it is more efficient to build that subtree once instead of rebuilding it on every provider update.

If you pass the pre-built subtree as the child parameter, the Consumer 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 Consumer

final counterProvider = StateProvider((ref) => 0);

class MyHomePage extends ConsumerWidget {
  MyHomePage({Key? key, required this.title}) : super(key: key);
  final String title;

  @override
  Widget build(BuildContext context, WidgetRef ref) {
    return Scaffold(
      appBar: AppBar(
        title: Text(title)
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text('You have pushed the button this many times:'),
            Consumer(
              builder: (BuildContext context, WidgetRef ref, Widget? child) {
                // This builder will only get called when the counterProvider
                // is updated.
                final count = ref.watch(counterProvider);

                return Row(
                  mainAxisAlignment: MainAxisAlignment.spaceEvenly,
                  children: <Widget>[
                    Text('$count'),
                    child!,
                  ],
                );
              },
              // The child parameter is most helpful if the child is
              // expensive to build and does not depend on the value from
              // the notifier.
              child: Text('Good job!'),
            )
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        child: Icon(Icons.plus_one),
        onPressed: () => ref.read(counterProvider.notifier).state++,
      ),
    );
  }
}

See also:

  • ConsumerWidget, a base-class for widgets that wants to listen to providers.
Inheritance
Annotations
  • @sealed

Constructors

Consumer({Key? key, required ConsumerBuilder builder, Widget? child})
Build a widget tree while listening to providers.
const

Properties

hashCode int
The hash code for this object.
no setterinherited
key Key?
Controls how one widget replaces another widget in the tree.
finalinherited
runtimeType Type
A representation of the runtime type of the object.
no setterinherited

Methods

build(BuildContext context, WidgetRef ref) Widget
Describes the part of the user interface represented by this widget.
override
createElement() ConsumerStatefulElement
Creates a StatefulElement to manage this widget's location in the tree.
inherited
createState() → _ConsumerState
Creates the mutable state for this widget at a given location in the tree.
inherited
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}) 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