NarKampen/Assets/Scripts/Draggable.cs

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

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

    public Vector3 startPosition;
    public Transform parent = null;
    public Transform placeholderParent = null;
    GameObject placeholder = null;

    public void OnDrag(PointerEventData eventData) {
        this.transform.position = eventData.position;
        
        if (placeholder.transform.parent != placeholderParent) {
            placeholder.transform.SetParent(placeholderParent);
        }

        int newSiblingIndex = placeholderParent.childCount;

        for (int i = 0; i < placeholderParent.childCount; i++) {
            if (this.transform.position.x < placeholderParent.GetChild(i).transform.position.x) {
                newSiblingIndex = i;
                if (parent.transform.GetSiblingIndex() < newSiblingIndex) {
                    newSiblingIndex--;
                }
                break;
            }
        }

        placeholderParent.transform.SetSiblingIndex(newSiblingIndex);
    }

    public void OnBeginDrag(PointerEventData eventData) {
        startPosition = this.transform.position;

        placeholder = new GameObject();
        placeholder.transform.SetParent(this.transform.parent);
        LayoutElement le = placeholder.AddComponent<LayoutElement>();
        le.preferredHeight = this.GetComponent<LayoutElement>().preferredHeight;
        le.preferredWidth = this.GetComponent<LayoutElement>().preferredWidth;
        le.flexibleHeight = this.GetComponent<LayoutElement>().flexibleHeight;
        le.flexibleWidth = this.GetComponent<LayoutElement>().flexibleWidth;
        le.minHeight = this.GetComponent<LayoutElement>().minHeight;
        le.minWidth = this.GetComponent<LayoutElement>().minWidth;

        placeholder.transform.SetSiblingIndex(this.transform.GetSiblingIndex());

        parent = this.transform.parent;
        placeholderParent = parent;
        GetComponent<CanvasGroup>().blocksRaycasts = false;
    }

    public void OnEndDrag(PointerEventData eventData) {
        if (parent == null) {
            this.transform.position = startPosition;
            this.transform.SetSiblingIndex(placeholder.transform.GetSiblingIndex());
        } else {
            this.transform.SetParent(parent);
        }
        GetComponent<CanvasGroup>().blocksRaycasts = true;

        Destroy(placeholder);
    }
}