detach method Null safety
override
Mark this node as detached.
Typically called only from the parent's detach, and by the owner to mark the root of a tree as detached.
Subclasses with children should override this method to first call their inherited detach method, and then detach all their children.
Implementations of this method should end with a call to the inherited
method, as in super.detach().
Implementation
@override
void detach() {
_offset.removeListener(markNeedsLayout);
super.detach();
}