Flow class
A widget that sizes and positions children efficiently, according to the logic in a FlowDelegate.
Flow layouts are optimized for repositioning children using transformation matrices.
The flow container is sized independently from the children by the FlowDelegate.getSize function of the delegate. The children are then sized independently given the constraints from the FlowDelegate.getConstraintsForChild function.
Rather than positioning the children during layout, the children are positioned using transformation matrices during the paint phase using the matrices from the FlowDelegate.paintChildren function. The children can be repositioned efficiently by simply repainting the flow, which happens without the children being laid out again (contrast this with a Stack, which does the sizing and positioning together during layout).
The most efficient way to trigger a repaint of the flow is to supply an animation to the constructor of the FlowDelegate. The flow will listen to this animation and repaint whenever the animation ticks, avoiding both the build and layout phases of the pipeline.
See also:
- Wrap, which provides the layout model that some other frameworks call "flow", and is otherwise unrelated to Flow.
- FlowDelegate, which controls the visual presentation of the children.
- Stack, which arranges children relative to the edges of the container.
- CustomSingleChildLayout, which uses a delegate to control the layout of a single child.
- CustomMultiChildLayout, which uses a delegate to position multiple children.
- The catalog of layout widgets.
flutter create --sample=widgets.Flow.1 mysample
import 'package:flutter/material.dart';
void main() => runApp(FlowApp());
class FlowApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return MaterialApp(
home: Scaffold(
appBar: AppBar(
title: const Text('Flow Example'),
),
body: FlowMenu(),
),
);
}
}
class FlowMenu extends StatefulWidget {
@override
_FlowMenuState createState() => _FlowMenuState();
}
class _FlowMenuState extends State<FlowMenu> with SingleTickerProviderStateMixin {
AnimationController menuAnimation;
IconData lastTapped = Icons.notifications;
final List<IconData> menuItems = <IconData>[
Icons.home,
Icons.new_releases,
Icons.notifications,
Icons.settings,
Icons.menu,
];
void _updateMenu(IconData icon) {
if (icon != Icons.menu)
setState(() => lastTapped = icon);
}
@override
void initState() {
super.initState();
menuAnimation = AnimationController(
duration: const Duration(milliseconds: 250),
vsync: this,
);
}
Widget flowMenuItem(IconData icon) {
final double buttonDiameter = MediaQuery.of(context).size.width / menuItems.length;
return Padding(
padding: const EdgeInsets.symmetric(vertical: 8.0),
child: RawMaterialButton(
fillColor: lastTapped == icon ? Colors.amber[700] : Colors.blue,
splashColor: Colors.amber[100],
shape: CircleBorder(),
constraints: BoxConstraints.tight(Size(buttonDiameter, buttonDiameter)),
onPressed: () {
_updateMenu(icon);
menuAnimation.status == AnimationStatus.completed
? menuAnimation.reverse()
: menuAnimation.forward();
},
child: Icon(
icon,
color: Colors.white,
size: 45.0,
),
),
);
}
@override
Widget build(BuildContext context) {
return Container(
child: Flow(
delegate: FlowMenuDelegate(menuAnimation: menuAnimation),
children: menuItems.map<Widget>((IconData icon) => flowMenuItem(icon)).toList(),
),
);
}
}
class FlowMenuDelegate extends FlowDelegate {
FlowMenuDelegate({this.menuAnimation}) : super(repaint: menuAnimation);
final Animation<double> menuAnimation;
@override
bool shouldRepaint(FlowMenuDelegate oldDelegate) {
return menuAnimation != oldDelegate.menuAnimation;
}
@override
void paintChildren(FlowPaintingContext context) {
double dx = 0.0;
for (int i = 0; i < context.childCount; ++i) {
dx = context.getChildSize(i).width * i;
context.paintChild(
i,
transform: Matrix4.translationValues(
dx * menuAnimation.value,
0,
0,
),
);
}
}
}
- Inheritance
Constructors
-
Flow({Key key,
@required FlowDelegate delegate, List< Widget> children: const <Widget>[]}) - Creates a flow layout. [...]
-
Flow.unwrapped({Key key,
@required FlowDelegate delegate, List< Widget> children: const <Widget>[]}) - Creates a flow layout. [...]
Properties
-
children
→ List<
Widget> -
The widgets below this widget in the tree. [...]
final, inherited
- delegate → FlowDelegate
-
The delegate that controls the transformation matrices of the children.
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
- runtimeType → Type
-
A representation of the runtime type of the object.
read-only, inherited
Methods
-
createElement(
) → MultiChildRenderObjectElement -
RenderObjectWidgets always inflate to a RenderObjectElement subclass.
inherited
-
createRenderObject(
BuildContext context ) → RenderFlow -
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. [...]
inherited
-
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 -
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
-
updateRenderObject(
BuildContext context, covariant RenderFlow 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