Builder class

A stateless utility widget whose build method uses its builder callback to create the widget's child.

This widget is an inline alternative to defining a StatelessWidget subclass. For example, instead of defining a widget as follows:

class Foo extends StatelessWidget {
  const Foo({super.key});
  @override
  Widget build(BuildContext context) => const Text('foo');
}

...and using it in the usual way:

// continuing from previous example...
const Center(child: Foo())

...one could instead define and use it in a single step, without defining a new widget class:

Center(
  child: Builder(
    builder: (BuildContext context) => const Text('foo'),
  ),
)

The difference between either of the previous examples and creating a child directly without an intervening widget, is the extra BuildContext element that the additional widget adds. This is particularly noticeable when the tree contains an inherited widget that is referred to by a method like Scaffold.of, which visits the child widget's BuildContext ancestors.

In the following example the button's onPressed callback is unable to find the enclosing ScaffoldState with Scaffold.of:

Widget build(BuildContext context) {
  return Scaffold(
    body: Center(
      child: TextButton(
        onPressed: () {
          // Fails because Scaffold.of() doesn't find anything
          // above this widget's context.
          print(Scaffold.of(context).hasAppBar);
        },
        child: const Text('hasAppBar'),
      )
    ),
  );
}

A Builder widget introduces an additional BuildContext element and so the Scaffold.of method succeeds.

Widget build(BuildContext context) {
  return Scaffold(
    body: Builder(
      builder: (BuildContext context) {
        return Center(
          child: TextButton(
            onPressed: () {
              print(Scaffold.of(context).hasAppBar);
            },
            child: const Text('hasAppBar'),
          ),
        );
      },
    ),
  );
}

See also:

• StatefulBuilder, A stateful utility widget whose build method uses its builder callback to create the widget's child.