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.
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Such bugs will manifest either as out-of-date geometry (taps going to different widgets than might be expected by the currently-specified FlowDelegates), or exceptions (e.g. if the last time the Flow was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:
- Wrap, which provides the layout model that some other frameworks call "flow", and is otherwise unrelated to Flow.
- 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.
- Inheritance
Constructors
-
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.
const
Properties
-
children
→ List<
Widget> -
The widgets below this widget in the tree.
finalinherited
- 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.
no setterinherited
- key → Key?
-
Controls how one widget replaces another widget in the tree.
finalinherited
- runtimeType → Type
-
A representation of the runtime type of the object.
no setterinherited
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.
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.
inherited
-
noSuchMethod(
Invocation invocation) → dynamic -
Invoked when a nonexistent 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, int wrapWidth = 65}) → 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.
inherited