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