A strategy that is used by this draggable to get the anchor offset when it is dragged.
The anchor offset refers to the distance between the users' fingers and the feedback widget when this draggable is dragged.
This property's value is a function that implements DragAnchorStrategy. There are two built-in functions that can be used:
childDragAnchorStrategy, which displays the feedback anchored at the position of the original child.
pointerDragAnchorStrategy, which displays the feedback anchored at the position of the touch that started the drag.
Defaults to childDragAnchorStrategy.
final DragAnchorStrategy dragAnchorStrategy;