FollowObjectPositionUnity.cs

using System;
using System.Collections;
using System.Collections.Generic;
using UnityEngine;

/// <summary>
/// Ensures that the position of this object relative to ObjectToFollow
/// remains the same, in the frame of reference of their first common ancestor
/// </summary>
public class MatchPosition : MonoBehaviour
{
    public Transform ObjectToFollow;
    private Vector3 commonAncestorOffset;
    private Transform commonAncestor;

    // Use this for initialization
    void Start()
    {
        commonAncestor = FindCommonAncestor(transform, ObjectToFollow);
        // We want the offset me -> follow IN COMMON ANCESTOR SPACE
        commonAncestorOffset = commonAncestor.InverseTransformDirection(transform.position - ObjectToFollow.position);
        Debug.Log(string.Format("Common ancestor is {0} offset is {1}", commonAncestor.name, commonAncestorOffset));
    }
    
    /// <summary>
    /// Returns the common ancestor between A and B, or null if no common ancestor is found
    /// </summary>
    /// <param name="a"></param>
    /// <param name="b"></param>
    /// <returns></returns>
    private Transform FindCommonAncestor(Transform a, Transform b)
    {
        List<Transform> aAncestors = new List<Transform>();
        Transform aCurrent = a;
        while(aCurrent != null && aCurrent != a.root)
        {
            aAncestors.Add(aCurrent);
            aCurrent = aCurrent.parent;
        }
        Transform bCurrent = b;
        while(bCurrent != null && bCurrent != b.root)
        {
            if (aAncestors.Contains(bCurrent))
            {
                return bCurrent;
            }
            bCurrent = bCurrent.parent;
        }
        return null;
    }

    // Update is called once per frame
    void Update()
    {
        transform.position = ObjectToFollow.position + commonAncestor.transform.TransformDirection(commonAncestorOffset);
    }
}