onDragCancel method
Override this to handle if a drag/pan gesture is cancelled that was
previously started on 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.
This event is not that common, it can happen for example when the user is interrupted by a system-modal dialog in the middle of the drag.
Implementation
bool onDragCancel() {
return true;
}