PreferredSize class Null safety

A widget with a preferred size.

This widget does not impose any constraints on its child, and it doesn't affect the child's layout in any way. It just advertises a preferred size which can be used by the parent.

Parents like Scaffold use PreferredSizeWidget to require that their children implement that interface. To give a preferred size to an arbitrary widget so that it can be used in a child property of that type, this widget, PreferredSize, can be used.

Widgets like AppBar implement a PreferredSizeWidget, so that this PreferredSize widget is not necessary for them.

This sample shows a custom widget, similar to an AppBar, which uses a PreferredSize widget, with its height set to 80 logical pixels. Changing the PreferredSize can be used to change the height of the custom app bar.
To create a local project with this code sample, run:
flutter create --sample=widgets.PreferredSize.1 mysample

See also:

Inheritance
Implemented types

Constructors

PreferredSize({Key? key, required Widget child, required Size preferredSize})
Creates a widget that has a preferred size that the parent can query.
const

Properties

child Widget
The widget below this widget in the tree.
final
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
preferredSize Size
The size this widget would prefer if it were otherwise unconstrained.
final
runtimeType Type
A representation of the runtime type of the object.
read-only, inherited

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, inherited
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, inherited