概要

using UnityEngine;
using UnityEngine.EventSystems;

public class Example :
    MonoBehaviour,
    IPointerDownHandler,
    IDragHandler
{
    private Vector3 m_offset;

    void IPointerDownHandler.OnPointerDown( PointerEventData eventData )
    {
        var worldPoint = Camera.main.ScreenToWorldPoint( eventData.position );
        worldPoint.z = 0;

        m_offset = transform.position - worldPoint;
    }

    void IDragHandler.OnDrag( PointerEventData eventData )
    {
        var worldPoint = Camera.main.ScreenToWorldPoint( eventData.position );
        worldPoint.z = 0;

        transform.position = worldPoint + m_offset;
    }
}