Assets/Scripts/DragDrop.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.EventSystems;
using UnityEngine.UI;

public class DragDrop : MonoBehaviour, IBeginDragHandler, IEndDragHandler, IDragHandler
{

    private RectTransform rectTransform;
    private CanvasGroup canvasGroup;

    public static GameObject itemBeingDragged;
    Vector3 startPosition;
    Transform startParent;



    private void Awake()
    {

        rectTransform = GetComponent<RectTransform>();
        canvasGroup = GetComponent<CanvasGroup>();

    }


    public void OnBeginDrag(PointerEventData eventData)
    {

        Debug.Log("OnBeginDrag");
        if (canvasGroup)
        {
            canvasGroup.alpha = .6f;
            //So the ray cast will ignore the item itself.
            canvasGroup.blocksRaycasts = false;
            startPosition = transform.position;
            startParent = transform.parent;
            transform.SetParent(transform.root);
            itemBeingDragged = gameObject;
        }

    }

    public void OnDrag(PointerEventData eventData)
    {
        if (canvasGroup)
        {
            //So the item will move with our mouse (at same speed)  and so it will be consistant if the canvas has a different scale (other then 1);
            rectTransform.anchoredPosition += eventData.delta;
        }

    }



    public void OnEndDrag(PointerEventData eventData)
    {
        if (canvasGroup)
        {
            itemBeingDragged = null;

            if (transform.parent == startParent || transform.parent == transform.root)
            {
                transform.position = startPosition;
                transform.SetParent(startParent);

            }

            Debug.Log("OnEndDrag");
            canvasGroup.alpha = 1f;
            canvasGroup.blocksRaycasts = true;
        }
    }



}