Builder class

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

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

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

Center(child: Foo())

Could equally well be defined and used like this, without defining a new component 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 component, is the extra BuildContext element that the additional component adds. This is particularly noticeable when the tree contains an inherited component that is referred to by a method like Scaffold.of, which visits the child component's BuildContext ancestors.

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

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

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

Component 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:

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

Inheritance

• Object
• Component
• StatelessComponent
• Builder

Constructors

Builder({Key? key, required ComponentBuilder builder})

Creates a component that delegates its build to a callback.

const

Builder.single({Key? key, required SingleComponentBuilder builder})

Creates a component that delegates its build to a callback that returns a single child component.

Properties

builder → ComponentBuilder

Called to obtain the child component.

final

hashCode → int

The hash code for this object.

no setter, inherited

key → Key?

Controls how one component replaces another component in the tree.

final, inherited

runtimeType → Type

A representation of the runtime type of the object.

no setter, inherited

Methods

build(BuildContext context) → Iterable<Component>

Describes the part of the user interface represented by this component.

override

createElement() → Element

Creates a StatelessElement to manage this component's location in the tree.

inherited

noSuchMethod(Invocation invocation) → dynamic

Invoked when a nonexistent method or property is accessed.

inherited

toString() → String

A string representation of this object.

inherited

Operators

operator ==(Object other) → bool

The equality operator.

inherited