概要

using UnityEngine;
using UnityEngine.EventSystems;

public class Example :
    MonoBehaviour,
    IPointerDownHandler,
    IDragHandler
{
    [SerializeField] private Canvas m_canvas;

    private Vector3 m_offset;

    void IPointerDownHandler.OnPointerDown( PointerEventData eventData )
    {
        RectTransformUtility.ScreenPointToLocalPointInRectangle
        (
            rect: m_canvas.GetComponent<RectTransform>(),
            screenPoint: eventData.position,
            cam: m_canvas.worldCamera,
            localPoint: out var localPoint
        );

        m_offset = transform.position - m_canvas.transform.TransformPoint( localPoint );
    }

    void IDragHandler.OnDrag( PointerEventData eventData )
    {
        RectTransformUtility.ScreenPointToLocalPointInRectangle
        (
            rect: m_canvas.GetComponent<RectTransform>(),
            screenPoint: eventData.position,
            cam: m_canvas.worldCamera,
            localPoint: out var localPoint
        );

        transform.position = m_canvas.transform.TransformPoint( localPoint ) + m_offset;
    }
}