ConstrainedBox class Null safety

A widget that imposes additional constraints on its child.

For example, if you wanted child to have a minimum height of 50.0 logical pixels, you could use const BoxConstraints(minHeight: 50.0) as the constraints.

This snippet makes the child widget (a Card with some Text) fill the parent, by applying BoxConstraints.expand constraints:
  constraints: const BoxConstraints.expand(),
  child: const Card(child: Text('Hello World!')),

The same behavior can be obtained using the SizedBox.expand widget.

See also:

  • BoxConstraints, the class that describes constraints.
  • UnconstrainedBox, a container that tries to let its child draw without constraints.
  • SizedBox, which lets you specify tight constraints by explicitly specifying the height or width.
  • FractionallySizedBox, which sizes its child based on a fraction of its own size and positions the child according to an Alignment value.
  • AspectRatio, a widget that attempts to fit within the parent's constraints while also sizing its child to match a given aspect ratio.
  • The catalog of layout widgets.


ConstrainedBox({Key? key, required BoxConstraints constraints, Widget? child})
Creates a widget that imposes additional constraints on its child.


child Widget?
The widget below this widget in the tree.
final, inherited
constraints BoxConstraints
The additional constraints to impose on the child.
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() SingleChildRenderObjectElement
RenderObjectWidgets always inflate to a RenderObjectElement subclass.
createRenderObject(BuildContext context) RenderConstrainedBox
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 RenderConstrainedBox 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.


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