1、代码 

public class UIManager
{
    private static UIManager instance = new UIManager();
    public static UIManager Instance => instance;

    //存储显示着的面板脚本（不是面板Gameobject），每显示一个面板就存入字典
    //隐藏的时候获取字典中对应面板 进行隐藏
    private Dictionary<string, BasePanel> panelDic = new Dictionary<string, BasePanel>();

    //得到Canvas用于设置为面板父对象
    private Transform canvasTrans;

    private UIManager()
    {
        //将Canvas作为预设体，UI管理器启动时创建Canvas
        GameObject canvas = GameObject.Instantiate(Resources.Load<GameObject>("UI/Canvas"));
        //得到创建的canvas
        canvasTrans = canvas.transform;
        //不移除Canvas，保证整个游戏中只有一个
        GameObject.DontDestroyOnLoad(canvas);
    }
    //显示面板
    public T ShowPanel<T>() where T:BasePanel
    {
        //保证泛型T的类型和预设体名字一样（规则）
        string panelName = typeof(T).Name;

        //判断字典中是否已经有这个面板
        if (panelDic.ContainsKey(panelName))
        {
            return panelDic[panelName] as T;
        }

        //如果字典中没有
        //根据面板名字动态创建预设体 设置父对象
        GameObject panelObj = GameObject.Instantiate(Resources.Load<GameObject>("UI/" + panelName));
        //把这个对象放到场景Canvas下面
        panelObj.transform.SetParent(canvasTrans,false);

        //
        T panel = panelObj.GetComponent<T>();
        //把这个面板脚本 存储到字典中
        panelDic.Add(panelName, panel);
        panel.ShowMe();

        return panel;
    }

    /// <summary>
    /// 隐藏面板
    /// </summary>
    /// <typeparam name="T">面板类型</typeparam>
    public void HidePanel<T>() where T : BasePanel
    {
        //根据泛型得名字
        string panelName = typeof(T).Name;
        //判断当前显示的面板有没有传入的面板
        if (panelDic.ContainsKey(panelName))
        {
            //面板淡出完毕后删除
            panelDic[panelName].HideMe(()=> 
            {
                GameObject.Destroy(panelDic[panelName].gameObject);
                //删除字典里面存储的面板脚本
                panelDic.Remove(panelName);
            });
        }
    }

    //得到面板
    public T GetPanel<T>() where T : BasePanel
    {
        string panelName = typeof(T).Name;
        if (panelDic.ContainsKey(panelName))
        {
            return panelDic[panelName] as T;
        }

        //没有对应面板返回空
        return null;
    }
}

2、如何使用

创建一个测试的UI面板（随便乱拼），将他作为预设体

创建一个测试类

public class TestMain : MonoBehaviour
{
    // Start is called before the first frame update
    void Start()
    {
        UIManager.Instance.ShowPanel<TestPanel>();
    }

    // Update is called once per frame
    void Update()
    {
        
    }
}

启动发现已经动态创建了Canvas和对应面板