EventSystemDragExample.cs

using UnityEngine;
using UnityEngine.EventSystems;

/// <summary>
/// Example implementing pointer click and drag (using event system events)
/// </summary>
[RequireComponent(typeof(Collider))]
public class EventSystemDragExample : MonoBehaviour, IDragHandler, IBeginDragHandler, IEndDragHandler
{
    private static readonly Color dragColor = Color.green;

    [SerializeField]
    new Camera camera;

    [SerializeField]
    Vector3 dragOffset;

    [Header("Drag Plane"), SerializeField]
    Vector3 dragPlaneNormal = Vector3.up;

    [SerializeField]
    float planeDistance = 0;

    [SerializeField]
    BoxCollider dragBounds;

    private Plane plane;
    private Color originalColour;
    private Material material;

    private void Start()
    {
        if(camera == null)
        {
            camera = Camera.main;
        }

        var renderer = GetComponent<Renderer>();
        if(renderer != null)
        {
            material = renderer.material;
        }

        //Make plane with serialized settings
        plane = new(dragPlaneNormal, planeDistance);
    }

    public void OnBeginDrag(PointerEventData eventData)
    {
        //Change colour of material if one exists
        // This helps indicate that we began dragging
        if(material != null)
        {
            originalColour = material.color;
            material.color = dragColor;
        }
    }

    public void OnEndDrag(PointerEventData eventData)
    {
        //Restore colour of renderer if one exists
        if(material != null)
        {
            material.color = originalColour;
        }
    }

    /// <summary>
    /// Implements dragging this gameobject on an imaginary plane using event system pointer drag event
    /// </summary>
    /// <param name="eventData"></param>
    public void OnDrag(PointerEventData eventData)
    {
        //Using pointer screen position from event data, make a ray
        Ray ray = camera.ScreenPointToRay(eventData.position);

        //Perform a raycast with our plane
        //The distance from the ray origin to a plane intersection is returned
        if(plane.Raycast(ray, out float distance))
        {
            //Use raycast hit distance to project forward and get the point in world space where our raycast actually hit the collider
            Vector3 newWorldPosition = ray.GetPoint(distance) + dragOffset;

            //Clamp position inside bounds (if assigned)
            if(dragBounds != null)
            {
                newWorldPosition = dragBounds.ClosestPoint(newWorldPosition);
            }

            transform.position = newWorldPosition;
        }
    }

#if UNITY_EDITOR
    private void Reset()
    {
        camera = FindFirstObjectByType<Camera>();
    }
#endif
}