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 only 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.

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

Hit testing and hidden Flow widgets

The Flow widget recomputes its children's positions (as used by hit testing) during the paint phase rather than during the layout phase.

Widgets like Opacity avoid painting their children when those children would be invisible due to their opacity being zero.

Unfortunately, this means that hiding a Flow widget using an Opacity widget will cause bugs when the user attempts to interact with the hidden region, for example, by tapping it or clicking it.

Such bugs will manifest either as out-of-date geometry (taps going to different widgets than might be expected by the currently-specified FlowDelegates), or exceptions (e.g. if the last time the Flow was painted, a different set of children was specified).

To avoid this, when hiding a Flow widget with an Opacity widget (or AnimatedOpacity or similar), it is wise to also disable hit testing on the widget by using IgnorePointer. This is generally good advice anyway as hit-testing invisible widgets is often confusing for the user.

See also:



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


children List<Widget>
The widgets below this widget in the tree.
clipBehavior Clip
The content will be clipped (or not) according to this option.
delegate FlowDelegate
The delegate that controls the transformation matrices of the children.
hashCode int
The hash code for this object.
no setterinherited
key Key?
Controls how one widget replaces another widget in the tree.
runtimeType Type
A representation of the runtime type of the object.
no setterinherited


createElement() MultiChildRenderObjectElement
RenderObjectWidgets always inflate to a RenderObjectElement subclass.
createRenderObject(BuildContext context) RenderFlow
Creates an instance of the RenderObject class that this RenderObjectWidget represents, using the configuration described by this RenderObjectWidget.
debugDescribeChildren() List<DiagnosticsNode>
Returns a list of DiagnosticsNode objects describing this node's children.
debugFillProperties(DiagnosticPropertiesBuilder properties) → void
Add additional properties associated with the node.
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.
noSuchMethod(Invocation invocation) → dynamic
Invoked when a nonexistent method or property is accessed.
toDiagnosticsNode({String? name, DiagnosticsTreeStyle? style}) DiagnosticsNode
Returns a debug representation of the object that is used by debugging tools and by DiagnosticsNode.toStringDeep.
toString({DiagnosticLevel minLevel = DiagnosticLevel.info}) String
A string representation of this object.
toStringDeep({String prefixLineOne = '', String? prefixOtherLines, DiagnosticLevel minLevel = DiagnosticLevel.debug}) String
Returns a string representation of this node and its descendants.
toStringShallow({String joiner = ', ', DiagnosticLevel minLevel = DiagnosticLevel.debug}) String
Returns a one-line detailed description of the object.
toStringShort() String
A short, textual description of this widget.
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.


operator ==(Object other) bool
The equality operator.