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:

link
This example uses the Flow widget to create a menu that opens and closes as it is interacted with, shown above. The color of the button in the menu changes to indicate which one has been selected. 
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 []})
Creates a flow layout. [...]
Flow.unwrapped({Key key, @required FlowDelegate delegate, List<Widget> children: const []})
Creates a flow layout. [...]

Properties

delegate → FlowDelegate
The delegate that controls the transformation matrices of the children.
final
children → List<Widget>
The widgets below this widget in the tree. [...]
final, inherited
hashCode → int
The hash code for this object. [...]
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

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