LayoutBuilder class Null safety

Builds a widget tree that can depend on the parent widget's size.

Similar to the Builder widget except that the framework calls the builder function at layout time and provides the parent widget's constraints. This is useful when the parent constrains the child's size and doesn't depend on the child's intrinsic size. The LayoutBuilder's final size will match its child's size.

The builder function is called in the following situations:

  • The first time the widget is laid out.
  • When the parent widget passes different layout constraints.
  • When the parent widget updates this widget.
  • When the dependencies that the builder function subscribes to change.

The builder function is not called during layout if the parent passes the same constraints repeatedly.

If the child should be smaller than the parent, consider wrapping the child in an Align widget. If the child might want to be bigger, consider wrapping it in a SingleChildScrollView or OverflowBox.

This example uses a LayoutBuilder to build a different widget depending on the available width. Resize the DartPad window to see LayoutBuilder in action!
To create a local project with this code sample, run:
flutter create --sample=widgets.LayoutBuilder.1 mysample

See also:



LayoutBuilder({Key? key, required LayoutWidgetBuilder builder})
Creates a widget that defers its building until layout. [...]


builder LayoutWidgetBuilder
Called at layout time to construct the widget tree. [...]
read-only, override
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


createElement() → _LayoutBuilderElement<BoxConstraints>
RenderObjectWidgets always inflate to a RenderObjectElement subclass.
createRenderObject(BuildContext context) → _RenderLayoutBuilder
Creates an instance of the RenderObject class that this RenderObjectWidget represents, using the configuration described by this RenderObjectWidget. [...]
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. [...]
didUnmountRenderObject(covariant RenderObject renderObject) → void
A render object previously associated with this widget has been removed from the tree. The given RenderObject will be of the same type as returned by this object's createRenderObject.
@protected, inherited
noSuchMethod(Invocation invocation) → dynamic
Invoked when a non-existent method or property is accessed. [...]
toDiagnosticsNode({String? name, DiagnosticsTreeStyle? style}) DiagnosticsNode
Returns a debug representation of the object that is used by debugging tools and by DiagnosticsNode.toStringDeep. [...]
toString({DiagnosticLevel minLevel:}) String
A string representation of this object. [...]
toStringDeep({String prefixLineOne: '', String? prefixOtherLines, DiagnosticLevel minLevel: DiagnosticLevel.debug}) String
Returns a string representation of this node and its descendants. [...]
toStringShallow({String joiner: ', ', DiagnosticLevel minLevel: DiagnosticLevel.debug}) String
Returns a one-line detailed description of the object. [...]
toStringShort() String
A short, textual description of this widget.
updateRenderObject(BuildContext context, covariant RenderObject renderObject) → void
Copies the configuration described by this RenderObjectWidget to the given RenderObject, which will be of the same type as returned by this object's createRenderObject. [...]
@protected, inherited


operator ==(Object other) bool
The equality operator. [...]
@nonVirtual, inherited