DirectionalFocusTraversalPolicyMixin mixin

A mixin class that provides an implementation for finding a node in a particular direction.

This can be mixed in to other FocusTraversalPolicy implementations that only want to implement new next/previous policies.

Since hysteresis in the navigation order is undesirable, this implementation maintains a stack of previous locations that have been visited on the policy data for the affected FocusScopeNode. If the previous direction was the opposite of the current direction, then the this policy will request focus on the previously focused node. Change to another direction other than the current one or its opposite will clear the stack.

For instance, if the focus moves down, down, down, and then up, up, up, it will follow the same path through the widgets in both directions. However, if it moves down, down, down, left, right, and then up, up, up, it may not follow the same path on the way up as it did on the way down, since changing the axis of motion resets the history.

This class implements an algorithm that considers an infinite band extending along the direction of movement, the width or height (depending on direction) of the currently focused widget, and finds the closest widget in that band along the direction of movement. If nothing is found in that band, then it picks the widget with an edge closest to the band in the perpendicular direction. If two out-of-band widgets are the same distance from the band, then it picks the one closest along the direction of movement.

The goal of this algorithm is to pick a widget that (to the user) doesn't appear to traverse along the wrong axis, as it might if it only sorted widgets by distance along one axis, but also jumps to the next logical widget in a direction without skipping over widgets.

See also:

Superclass Constraints
Mixin Applications


requestFocusCallback TraversalRequestFocusCallback
The callback used to move the focus from one focus node to another when traversing them using a keyboard. By default it requests focus on the next node and ensures the node is visible if it's in a scrollable.
hashCode int
The hash code for this object.
no setterinherited
runtimeType Type
A representation of the runtime type of the object.
no setterinherited


changedScope({FocusNode? node, FocusScopeNode? oldScope}) → void
This is called whenever the given node is re-parented into a new scope, so that the policy has a chance to update or invalidate any cached data that it maintains per scope about the node.
debugFillProperties(DiagnosticPropertiesBuilder properties) → void
Add additional properties associated with the node.
findFirstFocus(FocusNode currentNode, {bool ignoreCurrentFocus = false}) FocusNode?
Returns the node that should receive focus if focus is traversing forwards, and there is no current focus.
findFirstFocusInDirection(FocusNode currentNode, TraversalDirection direction) FocusNode?
Returns the first node in the given direction that should receive focus if there is no current focus in the scope to which the currentNode belongs.
findLastFocus(FocusNode currentNode, {bool ignoreCurrentFocus = false}) FocusNode
Returns the node that should receive focus if focus is traversing backwards, and there is no current focus.
inDirection(FocusNode currentNode, TraversalDirection direction) bool
Focuses the next widget in the given direction in the FocusScope that contains the currentNode.
invalidateScopeData(FocusScopeNode node) → void
Clears the data associated with the given FocusScopeNode for this object.
next(FocusNode currentNode) bool
Focuses the next widget in the focus scope that contains the given currentNode.
noSuchMethod(Invocation invocation) → dynamic
Invoked when a nonexistent method or property is accessed.
previous(FocusNode currentNode) bool
Focuses the previous widget in the focus scope that contains the given currentNode.
sortDescendants(Iterable<FocusNode> descendants, FocusNode currentNode) Iterable<FocusNode>
Sorts the given descendants into focus order.
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 =}) String
A string representation of this object.
toStringShort() String
A brief description of this object, usually just the runtimeType and the hashCode.


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