1. override
void visitAncestorElements(bool visitor(Element element))

Walks the ancestor chain, starting with the parent of this build context's widget, invoking the argument for each ancestor. The callback is given a reference to the ancestor widget's corresponding Element object. The walk stops when it reaches the root widget or when the callback returns false. The callback must not return null.

This is useful for inspecting the widget tree.

Calling this method is relatively expensive (O(N) in the depth of the tree).

Source 

@override
void visitAncestorElements(bool visitor(Element element)) {
  Element ancestor = _parent;
  while (ancestor != null && visitor(ancestor))
    ancestor = ancestor._parent;
}