Builder class Null safety

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

This widget is a simple inline alternative to defining a StatelessWidget subclass. For example a widget defined and used like this:

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

Center(child: Foo())

Could equally well be defined and used like this, without defining a new widget class:

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

The difference between either of the previous examples and simply 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: 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: Text('hasAppBar'),
          ),
        );
      },
    ),
  );
}

See also:

Inheritance

Constructors

Builder({Key? key, required WidgetBuilder builder})
Creates a widget that delegates its build to a callback. [...]
const

Properties

builder WidgetBuilder
Called to obtain the child widget. [...]
final
hashCode int
The hash code for this object. [...]
@nonVirtual, 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

build(BuildContext context) Widget
Describes the part of the user interface represented by this widget. [...]
override
createElement() StatelessElement
Creates a StatelessElement 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.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, inherited