findIndexByKey method Null safety
- Key key
Find index of child element with associated key.
This will be called during
performRebuild in SliverMultiBoxAdaptorElement
to check if a child has moved to a different position. It should return the
index of the child element with associated key, null if not found.
If not provided, a child widget may not map to its existing RenderObject when the order of children returned from the children builder changes. This may result in state-loss.
int? findIndexByKey(Key key) => null;