LookupBoundary class

A lookup boundary controls what entities are visible to descendants of the boundary via the static lookup methods provided by the boundary.

The static lookup methods of the boundary mirror the lookup methods by the same name exposed on BuildContext and they can be used as direct replacements. Unlike the methods on BuildContext, these methods do not find any ancestor entities of the closest LookupBoundary surrounding the provided BuildContext. The root of the tree is an implicit lookup boundary.

MyWidget(
  child: LookupBoundary(
    child: Builder(
      builder: (BuildContext context) {
        MyWidget? widget = LookupBoundary.findAncestorWidgetOfExactType<MyWidget>(context);
        return Text('$widget'); // "null"
      },
    ),
  ),
)

A LookupBoundary only affects the behavior of the static lookup methods defined on the boundary. It does not affect the behavior of the lookup methods defined on BuildContext.

A LookupBoundary is rarely instantiated directly. They are inserted at locations of the widget tree where the render tree diverges from the element tree, which is rather uncommon. Such anomalies are created by RenderObjectElements that don't attach their RenderObject to the closest ancestor RenderObjectElement, e.g. because they bootstrap a separate stand-alone render tree. This behavior breaks the assumption some widgets have about the structure of the render tree: These widgets may try to reach out to an ancestor widget, assuming that their associated RenderObjects are also ancestors, which due to the anomaly may not be the case. At the point where the divergence in the two trees is introduced, a LookupBoundary can be used to hide that ancestor from the querying widget. The ViewAnchor, for example, wraps its ViewAnchor.view child in a LookupBoundary because the RenderObject produced by that widget subtree is not attached to the render tree that the ViewAnchor itself belongs to.

As an example, Material.of relies on lookup boundaries to hide the Material widget from certain descendant button widget. Buttons reach out to their Material ancestor to draw ink splashes on its associated render object. This only produces the desired effect if the button render object is a descendant of the Material render object. If the element tree and the render tree are not in sync due to anomalies described above, this may not be the case. To avoid incorrect visuals, the Material relies on lookup boundaries to hide itself from descendants in subtrees with such anomalies. Those subtrees are expected to introduce their own Material widget that buttons there can utilize without crossing a lookup boundary.