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:
- StatefulBuilder, A stateful utility widget whose build method uses its builder callback to create the widget's child.
- 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.
- 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
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">@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