MoveObject

using UnityEngine;
using System.Collections;

public class MoveObject : MonoBehaviour
{

    private GameObject _target;

    private Ray _ray;
    private bool isMouseDrag;
    private Vector3 screenPosition;
    private Vector3 offset;


    void Update ()
    {
        moveObject ();
    }

    GameObject ReturnClickedObject(out RaycastHit hit)
    {
        _target = null;

        _ray = Camera.main.ScreenPointToRay (Input.mousePosition);

        if (Physics.Raycast (_ray.origin, _ray.direction * 10, out hit))
        {
            _target = hit.collider.gameObject;
        }
        return _target;
    }

    public void moveObject()
    {
        if (Input.GetMouseButtonDown(0))
        {
            RaycastHit hitInfo;
            _target = ReturnClickedObject(out hitInfo);

            if (_target != null)
            {
                isMouseDrag = true;
                Debug.Log("_target position :" + _target.transform.position);

                //Convert world position to screen position.
                screenPosition = Camera.main.WorldToScreenPoint(_target.transform.position);
                offset = _target.transform.position - Camera.main.ScreenToWorldPoint(new Vector3(Input.mousePosition.x, Input.mousePosition.y, screenPosition.z));
            }
        }

        if (Input.GetMouseButtonUp(0))
        {
            isMouseDrag = false;
        }

        if (isMouseDrag)
        {
            //track mouse position.
            Vector3 currentScreenSpace = new Vector3(Input.mousePosition.x, Input.mousePosition.y, screenPosition.z);

            //convert screen position to world position with offset changes.
            Vector3 currentPosition = Camera.main.ScreenToWorldPoint(currentScreenSpace) + offset;

            //It will update _target gameobject's current postion.
            _target.transform.position = currentPosition;
        }
    }
}