StatelessWidget class
A widget that does not require mutable state.
A stateless widget is a widget that describes part of the user interface by building a constellation of other widgets that describe the user interface more concretely. The building process continues recursively until the description of the user interface is fully concrete (e.g., consists entirely of RenderObjectWidgets, which describe concrete RenderObjects).
Stateless widget are useful when the part of the user interface you are describing does not depend on anything other than the configuration information in the object itself and the BuildContext in which the widget is inflated. For compositions that can change dynamically, e.g. due to having an internal clock-driven state, or depending on some system state, consider using StatefulWidget.
Performance considerations
The build method of a stateless widget is typically only called in three situations: the first time the widget is inserted in the tree, when the widget's parent changes its configuration, and when an InheritedWidget it depends on changes.
If a widget's parent will regularly change the widget's configuration, or if it depends on inherited widgets that frequently change, then it is important to optimize the performance of the build method to maintain a fluid rendering performance.
There are several techniques one can use to minimize the impact of rebuilding a stateless widget:
-
Minimize the number of nodes transitively created by the build method and any widgets it creates. For example, instead of an elaborate arrangement of Rows, Columns, Paddings, and SizedBoxes to position a single child in a particularly fancy manner, consider using just an Align or a CustomSingleChildLayout. Instead of an intricate layering of multiple Containers and with Decorations to draw just the right graphical effect, consider a single CustomPaint widget.
-
Use
const
widgets where possible, and provide aconst
constructor for the widget so that users of the widget can also do so. -
Consider refactoring the stateless widget into a stateful widget so that it can use some of the techniques described at StatefulWidget, such as caching common parts of subtrees and using GlobalKeys when changing the tree structure.
-
If the widget is likely to get rebuilt frequently due to the use of InheritedWidgets, consider refactoring the stateless widget into multiple widgets, with the parts of the tree that change being pushed to the leaves. For example instead of building a tree with four widgets, the inner-most widget depending on the Theme, consider factoring out the part of the build function that builds the inner-most widget into its own widget, so that only the inner-most widget needs to be rebuilt when the theme changes.
GreenFrog
:
class GreenFrog extends StatelessWidget {
const GreenFrog({ Key key }) : super(key: key);
@override
Widget build(BuildContext context) {
return Container(color: const Color(0xFF2DBD3A));
}
}
final
property. The next example shows the more generic widget Frog
which can be given a color and a child:
class Frog extends StatelessWidget {
const Frog({
Key key,
this.color: const Color(0xFF2DBD3A),
this.child,
}) : super(key: key);
final Color color;
final Widget child;
@override
Widget build(BuildContext context) {
return Container(color: color, child: child);
}
}
By convention, widget constructors only use named arguments. Named arguments
can be marked as required using @required
. Also by convention, the first
argument is key, and the last argument is child
, children
, or the
equivalent.
See also:
- StatefulWidget and State, for widgets that can build differently several times over their lifetime.
- InheritedWidget, for widgets that introduce ambient state that can be read by descendant widgets.
- Inheritance
- Object
- Diagnosticable
- DiagnosticableTree
- Widget
- StatelessWidget
- Implementers
- AboutDialog
- AboutListTile
- ActionChip
- AlertDialog
- AnimatedIcon
- BackButton
- BackButtonIcon
- Banner
- Builder
- ButtonBar
- Card
- CheckboxListTile
- CheckedModeBanner
- Chip
- ChoiceChip
- CircleAvatar
- CloseButton
- Container
- CupertinoActionSheet
- CupertinoActionSheetAction
- CupertinoAlertDialog
- CupertinoDialog
- CupertinoDialogAction
- CupertinoFullscreenDialogTransition
- CupertinoNavigationBarBackButton
- CupertinoPageScaffold
- CupertinoPageTransition
- CupertinoPopupSurface
- CupertinoTabBar
- DataTable
- DayPicker
- Dialog
- Divider
- Drawer
- DrawerHeader
- DropdownMenuItem
- FilterChip
- FlutterLogo
- GestureDetector
- GridPaper
- GridTile
- GridTileBar
- Icon
- IconButton
- ImageIcon
- InputChip
- KeyedSubtree
- ListTile
- MaterialButton
- ModalBarrier
- NavigationToolbar
- NotificationListener
- OrientationBuilder
- PageStorage
- Placeholder
- PositionedDirectional
- PreferredSize
- RadioListTile
- SafeArea
- ScrollView
- SimpleDialog
- SimpleDialogOption
- SingleChildScrollView
- SliverPersistentHeader
- SliverSafeArea
- SnackBar
- Spacer
- SwitchListTile
- Tab
- TabPageSelector
- TabPageSelectorIndicator
- Text
- Theme
- Title
- TwoLevelList
- TwoLevelListItem
- VerticalDivider
- Visibility
Constructors
- StatelessWidget({Key key })
-
Initializes
key
for subclasses.const
Properties
Methods
-
build(
BuildContext context) → Widget -
Describes the part of the user interface represented by this widget. [...]
@protected
-
createElement(
) → StatelessElement -
Creates a StatelessElement to manage this widget's location in the tree. [...]
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. [...]
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 toStringDeep. [...]
inherited
-
toString(
{DiagnosticLevel minLevel: DiagnosticLevel.debug }) → String -
Returns 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 ==(
dynamic other) → bool -
The equality operator. [...]
inherited