onMove method
Called when a single-pointer move (drag) gesture occurs.
A move event is emitted for drag gestures formed by pointer down → move → pointer up.
Parameters
startPos: Drag start location.endPos: Drag end location.
Implementation
@override
void onMove(final Point<num> startPoint, final Point<num> endPoint) {
_moveCallback?.call(startPoint, endPoint);
}