onTouchMove method
Called when a touch-move gesture occurs after tap-and-hold.
Touch-move mode is triggered by a tap immediately followed by another pointer down in the same area, then a move.
Parameters
startPos: Drag start location.endPos: Drag end location.
Implementation
@override
void onTouchMove(final Point<int> startPos, final Point<int> endPos) {
_touchMoveCallback?.call(startPos, endPos);
}