using UnityEngine;
using UnityEngine.EventSystems;
using UnityEngine.UI;

public class DraggableUI : MonoBehaviour, IDragHandler, IPointerDownHandler
{

    public RectTransform target;
    public Button button;
    private bool isLocked = false;
    private Vector3 _dragOffset;
    private float _dragScreenZ;

    private void Awake()
    {
        LockDrag();
    }

    public void OnPointerDown(PointerEventData eventData)
    {
        if (isLocked) return;

        Camera cam = ResolveCamera(eventData);
        if (cam == null) return;

        // 记录指针与物体的世界空间偏移，避免拖动开始时物体跳到指针中心。
        _dragScreenZ = cam.WorldToScreenPoint(transform.position).z;
        Vector3 pointerWorld = ScreenToWorld(cam, eventData.position, _dragScreenZ);
        _dragOffset = transform.position - pointerWorld;
    }

    public void OnDrag(PointerEventData eventData)
    {
        if (isLocked) return;

        Camera cam = ResolveCamera(eventData);
        if (cam == null) return;

        Vector3 pointerWorld = ScreenToWorld(cam, eventData.position, _dragScreenZ);
        Vector3 move = pointerWorld + _dragOffset;
        move.z = transform.position.z;
        transform.position = move;
    }

    void Update()
    {
        if (target == null || button == null) return;

        float distance = Vector3.Distance(transform.position, target.position);

        if (distance <= 1 && !isLocked)
        {
            button.interactable=true;
        }
        else
        {
            button.interactable=false;
        }
    }

    public void LockDrag()
    {
        isLocked = true;
    }

    public void UnlockDrag()
    {
        isLocked = false;
    }

    private static Camera ResolveCamera(PointerEventData eventData)
    {
        if (eventData != null && eventData.pressEventCamera != null)
            return eventData.pressEventCamera;
        return Camera.main;
    }

    private static Vector3 ScreenToWorld(Camera cam, Vector2 screenPosition, float screenZ)
    {
        return cam.ScreenToWorldPoint(new Vector3(screenPosition.x, screenPosition.y, screenZ));
    }
}