NarKampen/Assets/Scripts/Drag.cs

using UnityEngine;
using System.Collections;

[RequireComponent(typeof(BoxCollider2D))]

public class Drag : MonoBehaviour {
    public GameObject target;
    public bool isMouseDrag = false;
    private Vector3 screenPosition;
    private Vector3 offset;

    // Update is called once per frame
    void Update()
    {

        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;
        }

    }

    GameObject ReturnClickedObject(out RaycastHit hit)
    {
        GameObject target = null;
        Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
        if (Physics.Raycast(ray.origin, ray.direction * 10, out hit))
        {
            target = hit.collider.gameObject;
        }
        return target;
    }
}