Help! Can't not access the variable linearVelocity of TwistPublisher.cs from another script in Unity

Hi all, My OS is: Windows 10 My Unity Version is: Unity 2018.4 My ROS Distribution is: Kinetic My ROS # Version is:1.4 TwistPublisher.cs is just a sample script from ROSsharp unity package which help you read the velocity of a unity gameobject. The content of TwistPublisher.cs is shown below:

 using UnityEngine;
 
 namespace RosSharp.RosBridgeClient
 {
     public class TwistPublisher : Publisher<Messages.Geometry.Twist>
     {
         public Transform PublishedTransform;
 
         private Messages.Geometry.Twist message;
         private float previousRealTime;        
         private Vector3 previousPosition = Vector3.zero;
         private Quaternion previousRotation = Quaternion.identity;
 
         protected override void Start()
         {
             base.Start();
             InitializeMessage();
         }
 
         private void FixedUpdate()
         {
             UpdateMessage();
         }
 
         private void InitializeMessage()
         {
             message = new Messages.Geometry.Twist();
             message.linear = new Messages.Geometry.Vector3();
             message.angular = new Messages.Geometry.Vector3();
         }
         private void UpdateMessage()
         {
             float deltaTime = Time.realtimeSinceStartup - previousRealTime;
 
             Vector3 linearVelocity = (PublishedTransform.position - previousPosition)/deltaTime;
             Vector3 angularVelocity = (PublishedTransform.rotation.eulerAngles - previousRotation.eulerAngles)/deltaTime;
                 
             message.linear = GetGeometryVector3(linearVelocity.Unity2Ros()); ;
             message.angular = GetGeometryVector3(- angularVelocity.Unity2Ros());
 
             previousRealTime = Time.realtimeSinceStartup;
             previousPosition = PublishedTransform.position;
             previousRotation = PublishedTransform.rotation;
 
             Publish(message);
         }
 
         private static Messages.Geometry.Vector3 GetGeometryVector3(Vector3 vector3)
         {
             Messages.Geometry.Vector3 geometryVector3 = new Messages.Geometry.Vector3();
             geometryVector3.x = vector3.x;
             geometryVector3.y = vector3.y;
             geometryVector3.z = vector3.z;
             return geometryVector3;
         }
     }
 }

I am trying to read the variable linearVelocity of TwistPublisher.cs from another script which is attached on a normal 3D gameobject in Unity. As you know, linearVelocity of TwistPublisher.cs can represent the velocity of a related gameobject in unity(In my case, the related gameobject is a cube), I simply want to access this linearVelocity from another script which I named CsvWriteScript. Please take a look at my CsvWriteScript:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using System.Text;
 using System.IO;
 using System;
 using RosSharp.RosBridgeClient;
 
 public class CsvWriteScript : MonoBehaviour
 {
     GameObject obj;
     TwistPublisher twistpublisherScript;
 
     void Start()
     {  
         obj = GameObject.FindGameObjectWithTag("RosConnector");
         twistpublisherScript = obj.GetComponent<TwistPublisher>();
     }
 
     void Update()
     {
         UnityEngine.Debug.Log("the velocity of this cube:" + twistpublisherScript.linearVelocity);
     }
 }

In the TwistPublisher.cs I used Debug.Log("Vlinear" + linearVelocity); to print the velocity

 void UpdateMessage()
         {
             float deltaTime = Time.realtimeSinceStartup - previousRealTime;
             Vector3 linearVelocity = (PublishedTransform.position - previousPosition) / deltaTime;
             Vector3 angularVelocity = (PublishedTransform.rotation.eulerAngles - previousRotation.eulerAngles)/deltaTime; 
             //Steven
             Debug.Log("Vlinear" + linearVelocity);

Then I moved the cube in unity by dragging it with the mouse, it turned out the print from Debug.Log("Vlinear" + linearVelocity); was updating but the print from UnityEngine.Debug.Log("the velocity of this cube:" + twistpublisherScript.linearVelocity); was always(0, 0 , 0)

I am sure the link between the TwistPublisher.cs and my CsvWriteScript is built. Any chance you know why the twistpublisherScript.linearVelocity does't update when the linearVelocity from your TwistPublisher.cs is changing?

Thanks a million for your help.

To find out the problems, I write my own script to read the velocity of the cube object to replace the TwistPublisher.cs. The script is shown below

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class ObjVelocityReader : MonoBehaviour
 {
     Vector3 PrevPos;
     Vector3 NewPos;
     Vector3 PrevRot;
     Vector3 NewRot;
 
     public Vector3 ObjVelocity;
     public Vector3 ObjRotation;
     // Use this for initialization
     void Start()
     {
         PrevPos = transform.position;
         PrevRot = transform.eulerAngles;
     }
 
     // Update is called once per frame
     void FixedUpdate()
     {
         transform.rotation = Quaternion.identity;
 
         NewPos = transform.position;  // each frame track the new position
         ObjVelocity = (NewPos - PrevPos) / Time.fixedDeltaTime;
         PrevPos = NewPos;  // update position for next frame calculation
 
         NewRot = transform.eulerAngles;  // each frame track the new rotation
         ObjRotation = (NewRot - PrevRot) / Time.fixedDeltaTime;
         PrevRot = NewRot;  // update position for next frame calculation
     }
 }
       

And then I used my CsvWriteScript(shown below)to access the velocity variable from my ObjVelocityReader（it is a replacement to your TwistPublisher.cs here), it was a success. I could access the velocity from my CsvWriteScript successfully. So I really couldn't figure out how I should modify the TwistPublisher.cs to fix the problem. Can you please do me a favor? Really appreciated.

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using System.Text;
 using System.IO;
 using System;
 using RosSharp.RosBridgeClient;
 
 public class CsvWriteScript : MonoBehaviour
 {
     GameObject obj;
     ObjVelocityReader velocityScript;
 
     // Start is called before the first frame update
     void Start()
     {
         //obj = GameObject.FindGameObjectWithTag("RosConnector");
         //twistpublisherScript = obj.GetComponent<TwistPublisher>();
         obj = GameObject.Find("Sphere");
         velocityScript = obj.GetComponent<ObjVelocityReader>();        
     }
     void Update()
     {
         UnityEngine.Debug.Log("V:" + velocityScript. ObjVelocity);
     }
 }