unity 拖拽UI

using UnityEngine;
using UnityEngine.EventSystems;
/// 

/// 拖拽UI（UI不能设置全屏，否则无效）
/// 
public class DragPanel : MonoBehaviour, IPointerDownHandler, IDragHandler
{
    // 鼠标起点
    private Vector2 originalLocalPointerPosition;

    // 面板起点
    private Vector3 originalPanelLocalPosition;

    // 当前面板
    public RectTransform panelRectTransform;

    // 父节点,这个最好是UI父节点，因为它的矩形大小刚好是屏幕大小
    public RectTransform parentRectTransform;

    private int siblingIndex = 99;

    private void Awake()
    {
        //panelRectTransform = transform.parent as RectTransform;
        //parentRectTransform = panelRectTransform.parent as RectTransform;
    }

    // 鼠标按下
    public void OnPointerDown(PointerEventData data)
    {
        siblingIndex++;
        panelRectTransform.transform.SetSiblingIndex(siblingIndex);
        // 记录当前面板起点
        originalPanelLocalPosition = panelRectTransform.localPosition;
        // 通过屏幕中的鼠标点，获取在父节点中的鼠标点
        // parentRectTransform:父节点
        // data.position:当前鼠标位置
        // data.pressEventCamera:当前事件的摄像机
        // originalLocalPointerPosition:获取当前鼠标起点
        RectTransformUtility.ScreenPointToLocalPointInRectangle(parentRectTransform, data.position, data.pressEventCamera, out originalLocalPointerPosition);
    }

    // 拖动
    public void OnDrag(PointerEventData data)
    {
        //Debug.Log("22222222");
        if (panelRectTransform == null || parentRectTransform == null)
            return;
        Vector2 localPointerPosition;
        // 获取本地鼠标位置
        if (RectTransformUtility.ScreenPointToLocalPointInRectangle(parentRectTransform, data.position, data.pressEventCamera, out localPointerPosition))
        {
            // 移动位置 = 本地鼠标当前位置 - 本地鼠标起点位置
            Vector3 offsetToOriginal = localPointerPosition - originalLocalPointerPosition;
            // 当前面板位置 = 面板起点 + 移动位置
            panelRectTransform.localPosition = originalPanelLocalPosition + offsetToOriginal;
        }
        ClampToWindow();
    }

    // 限制当前面板在父节点中的区域位置
    private void ClampToWindow()
    {
        // 面板位置
        Vector3 pos = panelRectTransform.localPosition;

        // 如果是UI父节点，设置面板大小为0，那么最大最小位置为正负屏幕的一半
        Vector3 minPosition = parentRectTransform.rect.min - panelRectTransform.rect.min;
        Vector3 maxPosition = parentRectTransform.rect.max - panelRectTransform.rect.max;

        pos.x = Mathf.Clamp(panelRectTransform.localPosition.x, minPosition.x, maxPosition.x);
        pos.y = Mathf.Clamp(panelRectTransform.localPosition.y, minPosition.y, maxPosition.y);

        panelRectTransform.localPosition = pos;
    }
}