onDragUpdate method
Override this to handle the update of a drag/pan gesture that is within
the boundaries (determined by Component.containsPoint) of the component
that this mixin is used on.
Return true if you want this event to continue to be passed on to
components underneath (lower priority) this component.
Implementation
@override
bool onDragUpdate(DragUpdateInfo info) {
_unscaledDelta.add(info.delta.global);
return false;
}