SizedBox class Null safety

A box with a specified size.

If given a child, this widget forces it to have a specific width and/or height. These values will be ignored if this widget's parent does not permit them. For example, this happens if the parent is the screen (forces the child to be the same size as the parent), or another SizedBox (forces its child to have a specific width and/or height). This can be remedied by wrapping the child SizedBox in a widget that does permit it to be any size up to the size of the parent, such as Center or Align.

If either the width or height is null, this widget will try to size itself to match the child's size in that dimension. If the child's size depends on the size of its parent, the height and width must be provided.

If not given a child, SizedBox will try to size itself as close to the specified height and width as possible given the parent's constraints. If height or width is null or unspecified, it will be treated as zero.

The new SizedBox.expand constructor can be used to make a SizedBox that sizes itself to fit the parent. It is equivalent to setting width and height to double.infinity.

This snippet makes the child widget (a Card with some Text) have the exact size 200x300, parental constraints permitting:
SizedBox(
  width: 200.0,
  height: 300.0,
  child: const Card(child: Text('Hello World!')),
)

See also:

Inheritance

Constructors

SizedBox({Key? key, double? width, double? height, Widget? child})
Creates a fixed size box. The width and height parameters can be null to indicate that the size of the box should not be constrained in the corresponding dimension.
const
SizedBox.expand({Key? key, Widget? child})
Creates a box that will become as large as its parent allows.
const
SizedBox.fromSize({Key? key, Widget? child, Size? size})
Creates a box with the specified size.
SizedBox.shrink({Key? key, Widget? child})
Creates a box that will become as small as its parent allows.
const

Properties

child Widget?
The widget below this widget in the tree. [...]
final, inherited
hashCode int
The hash code for this object. [...]
@nonVirtual, read-only, inherited
height double?
If non-null, requires the child to have exactly this height.
final
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
width double?
If non-null, requires the child to have exactly this width.
final

Methods

createElement() SingleChildRenderObjectElement
RenderObjectWidgets always inflate to a RenderObjectElement subclass.
inherited
createRenderObject(BuildContext context) RenderConstrainedBox
Creates an instance of the RenderObject class that this RenderObjectWidget represents, using the configuration described by this RenderObjectWidget. [...]
override
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. [...]
override
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. [...]
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.
override
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. [...]
override

Operators

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