Flow class Null safety

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.

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.

To create a local project with this code sample, run:
flutter create --sample=widgets.Flow.1 mysample

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(const FlowApp());

class FlowApp extends StatelessWidget {
  const FlowApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: const Text('Flow Example'),
        ),
        body: const FlowMenu(),
      ),
    );
  }
}

class FlowMenu extends StatefulWidget {
  const FlowMenu({Key? key}) : super(key: key);

  @override
  _FlowMenuState createState() => _FlowMenuState();
}

class _FlowMenuState extends State<FlowMenu> with SingleTickerProviderStateMixin {
  late 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: const 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 Flow(
      delegate: FlowMenuDelegate(menuAnimation: menuAnimation),
      children: menuItems.map<Widget>((IconData icon) => flowMenuItem(icon)).toList(),
    );
  }
}

class FlowMenuDelegate extends FlowDelegate {
  FlowMenuDelegate({required 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>[], Clip clipBehavior}): Creates a flow layout. [...]
Flow.unwrapped({Key? key, required FlowDelegate delegate, List<Widget> children = const <Widget>[], Clip clipBehavior}): Creates a flow layout. [...]

Properties

children → List<Widget>: The widgets below this widget in the tree. [...]
final, inherited
clipBehavior → Clip: The content will be clipped (or not) according to this option. [...]
final
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: 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

widgets library

Flow class Null safety

Constructors

Properties

Methods

Operators