unity 三级下拉菜单（树形下拉菜单UGUI）

1. 最新项目需求要求做一个下拉菜单列表（树形下拉菜单）。但UGUI并没有原生的插件，只能自己实现。
   思路每个级的ItemPanel（一个单级菜单条）样式是一样的，于是需要一个panel来装所有的itemPanel，在整个菜单 顶部panel上添加vertically layout group组件，使下面的子菜单从上到下依次排列。文章最后会附上 demo下载地址
2. 效果图
   
3. 具体实现新建Canvas->Panel,
   
   粘出部分代码，详细可以下载demo

控制端

public class PullDownList : MonoBehaviour
{
    private List itemPanelList;
    public GameObject itemPanel;
 
 
    private void Awake()
    {
        itemPanelList = new List();
    }
    // Use this for initialization
    void Start()
    {
        for (int i = 0; i < 10; i++)
        {
            GameObject newItemPanel = Instantiate(itemPanel);
            itemPanelList.Add(newItemPanel);
            newItemPanel.GetComponent().SetBaseParent(this.transform);
            newItemPanel.GetComponent().InitPanelContent(new ItemBean("一级菜单" + i, i));
        }
 
        for (int i = 0; i < 5; i++)
        {
            GameObject newItemPanel2 = Instantiate(itemPanel);
            itemPanelList.Add(newItemPanel2);
            newItemPanel2.GetComponent().SetItemParent(itemPanelList[i].GetComponent());
            newItemPanel2.GetComponent().InitPanelContent(new ItemBean("二级菜单" + i, i));
        }
 
        for (int i = 0; i < 2; i++)
        {
            GameObject newItemPanel3 = Instantiate(itemPanel);
            itemPanelList.Add(newItemPanel3);
            newItemPanel3.GetComponent().SetItemParent(itemPanelList[11].GetComponent());
            newItemPanel3.GetComponent().InitPanelContent(new ItemBean("三级菜单" + i, i));
        }
    }   
}

itempanel端

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;
 
public class ItemPanelBase : MonoBehaviour
{
    private List childList;//子物体集合
    [HideInInspector]
    public Button downArrow;//下箭头按钮
    public Sprite down, right,dot;
    public bool isOpen { get; set; }//子物体开启状态
    private Vector2 startSize;//起始大小
 
    private void Awake()
    {
        childList = new List();
        downArrow = this.transform.Find("ContentPanel/ArrowButton").GetComponent();
        downArrow.onClick.AddListener(() =>
        {
            if (isOpen)
            {
                CloseChild();
                isOpen = false;
            }
            else
            {
                OpenChild();
                isOpen = true;
            }
        });
        startSize = this.GetComponent().sizeDelta;
        isOpen = false;
    }
 
    //添加子物体到集合
    private void AddChild(ItemPanelBase parentItemPanelBase)
    {
        childList.Add(parentItemPanelBase);
        if (childList.Count >= 1)
        {
            downArrow.GetComponent().sprite = right;
        }
    }
 
    /// 

    /// 设置父物体，父物体不为一级菜单
    /// 
    /// 
    public void SetItemParent(ItemPanelBase parentItemPanelBase)
    {
        this.transform.parent = parentItemPanelBase.transform;
        parentItemPanelBase.AddChild(this);
        this.GetComponent().padding = new RectOffset((int)parentItemPanelBase.downArrow.GetComponent().sizeDelta.x, 0, 0, 0);
        if (parentItemPanelBase.isOpen)
        {
            
            this.GetComponent().AddParentSize((int)this.gameObject.GetComponent().sizeDelta.y);
        }
        else
        {
            this.transform.gameObject.SetActive(false);        
        }
    }
 
    /// 

    /// 设置父物体，父物体为一级菜单
    /// 
    /// 
    public void SetBaseParent(Transform tran)
    {
        this.transform.parent = tran;
    }
 
    /// 

    /// 增加一个子物体后更新Panel大小
    /// 
    /// 
    public void UpdateRectTranSize(int change)
    {
        this.gameObject.GetComponent().sizeDelta = new Vector2(startSize.x, this.gameObject.GetComponent().sizeDelta.y + change);
    }
    /// 

    /// 增加父物体高度
    /// 
    /// 
    /// 
    public void AddParentSize(int change)
    {
        if (this.transform.parent.GetComponent() != null)
        {
            this.transform.parent.GetComponent().UpdateRectTranSize(change);
            this.transform.parent.GetComponent().AddParentSize(change);
        }
    }
 
    /// 

    /// 关闭子物体列表
    /// 
    public void CloseChild()
    {
        if (childList.Count == 0) return;
        foreach (ItemPanelBase child in childList)
        {
            child.gameObject.SetActive(false);
            child.GetComponent().AddParentSize(-(int)child.gameObject.GetComponent().sizeDelta.y);
        }
        downArrow.GetComponent().sprite = right;
    }
 
    /// 

    /// 打开子物体列表
    /// 
    public void OpenChild()
    {
        if (childList.Count == 0) return;
        foreach (ItemPanelBase child in childList)
        {
            child.gameObject.SetActive(true);
            child.GetComponent().AddParentSize((int)child.gameObject.GetComponent().sizeDelta.y);
        }
        downArrow.GetComponent().sprite = down;
    }
 
    //填充Item数据
    public virtual void InitPanelContent(ItemBeanbase itemBeanbase) { }
 
}

5. 最后附上项目地址 demo下载地址