How to update volume of cells in PeriodicFlowEnigne

Dear all,

I'd like to use PeriodicFlowEngine to get Volume of Voronoi cell.I run MWE and I get Volume of Voronoi cell successfully in the first time. But the volume is not updated as the pack changes. How to update volume of cells in PeriodicFlowEngine?

Thanks for your any suggestions.

#####MWE#####

from yade import pack,qt
O.periodic=True

O.cell.hSize=Matrix3(0.2, 0, 0,
    0 ,0.2, 0,
    0, 0, 0.2)

sp=pack.SpherePack()
radius=5e-3
num=sp.makeCloud(Vector3().Zero,O.cell.refSize,radius,.2,5000,periodic=True) # min,max,radius,rRelFuzz,spheresInCell,periodic
O.bodies.append([sphere(s[0],s[1]) for s in sp])

O.engines=[
 ForceResetter(),
 InsertionSortCollider([Bo1_Sphere_Aabb()],verletDist=.05*radius),
 InteractionLoop(
  [Ig2_Sphere_Sphere_ScGeom()],
  [Ip2_FrictMat_FrictMat_FrictPhys()],
  [Law2_ScGeom_FrictPhys_CundallStrack()]
 ),
 PeriTriaxController(dynCell=True,mass=0.2,maxUnbalanced=0.01,relStressTol=0.02,goal=(-1e4,-1e4,0),stressMask=3,globUpdate=5,maxStrainRate=(1.,1.,1.),label='triax'),
    NewtonIntegrator(damping=.2),
    PeriodicFlowEngine(dead=1,useSolver=3,label="flow"),
    PyRunner(command='test()',iterPeriod=1000)
]
O.dt=PWaveTimeStep()
O.run();
qt.View()

def test():
    volume=0
    flow.compTessVolumes()
    for b in O.bodies:
        volume += flow.volume(b.id)
    print (volume)