The velocity the pointer was moving along the primary axis when it stopped contacting the screen, in logical pixels per second.
If the GestureTapDragEndCallback is for a one-dimensional drag (e.g., a horizontal or vertical drag), then this value contains the component of velocity along the primary axis (e.g., horizontal or vertical, respectively). Otherwise, if the GestureTapDragEndCallback 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? primaryVelocity;