- covariant RenderObject? child
Returns a rect in this object's coordinate system that describes
which SemanticsNodes produced by the
child should be included in the
semantics tree. SemanticsNodes from the
child that are positioned
outside of this rect will be dropped. Child SemanticsNodes that are
positioned inside this rect, but outside of describeApproximatePaintClip
will be included in the tree marked as hidden. Child SemanticsNodes
that are inside of both rect will be included in the tree as regular
This method only returns a non-null value if the semantics clip rect is different from the rect returned by describeApproximatePaintClip. If the semantics clip rect and the paint clip rect are the same, this method returns null.
A viewport would typically implement this method to include semantic nodes in the semantics tree that are currently hidden just before the leading or just after the trailing edge. These nodes have to be included in the semantics tree to implement implicit accessibility scrolling on iOS where the viewport scrolls implicitly when moving the accessibility focus from the last visible node in the viewport to the first hidden one.
- RenderViewportBase.cacheExtent, used by viewports to extend their semantics clip beyond their approximate paint clip.
Rect? describeSemanticsClip(covariant RenderObject? child) => null;