The amount the pointer has moved along the primary axis in the coordinate space of the event receiver since the previous update.
If the GestureDragUpdateCallback is for a one-dimensional drag (e.g., a horizontal or vertical drag), then this value contains the component of delta along the primary axis (e.g., horizontal or vertical, respectively). Otherwise, if the GestureDragUpdateCallback is for a two-dimensional drag (e.g., a pan), then this value is null.
Defaults to null if not specified in the constructor.
final double? primaryDelta;