Unity 球体旋转 到固定位置

今天解决了一个小白的问题。
说来惭愧，一直对四元数不是很理解，之前有一个滑动屏幕旋转球体的问题，使用欧拉角旋转来处理，万向锁的问题解决不了，尝试了很多方法都失败了，今天无意中看到一篇帖子（如上）说到如何将球体旋转到固定位置，看完代码突然感觉有了希望，于是，问题解决了，我把代码贴出来。

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.Serialization;

public class SphereRotate : MonoBehaviour
{
    public delegate void OnDragStartHandler();
    public OnDragStartHandler onDragStartHandler;

    [Tooltip("sphere target")]
    public Transform target;

    private bool _rotateable = false;

    [Tooltip("sphere rotate speed")]
    [Range(0, 10)]
    public float speed = 1f;

    private void Start()
    {
        if (target)
            _rotateable = true;
    }

    void OnEnable()
    {
        EasyTouch.On_TouchDown += OnDragHandler;
        EasyTouch.On_TouchStart += EasyTouch_On_DragStart;
    }

    void OnDisable()
    {
        EasyTouch.On_TouchDown -= OnDragHandler;
        EasyTouch.On_TouchStart -= EasyTouch_On_DragStart;
    }

    private void OnDragHandler(Gesture gesture)
    {
        if (!_rotateable) return;
        //屏幕向量
        Vector2 deltaPos = gesture.deltaPosition;
        float magnitude = deltaPos.magnitude;
        //转换到世界空间的向量，默认高度
        Vector3 moveDir = new Vector3(deltaPos.x, 0, deltaPos.y);
        // 球进行旋转即以 moveDir和Vector3.up向量组成任意平面垂直的向量（法向量）为旋转轴
        // moveDir 和 Vector3.up 叉乘，获取 moveDir、Vector3.up 组成平面的法向量
        Vector3 cross = Vector3.Cross(moveDir, Vector3.up);
        float angle = -magnitude * speed;
        //Debug.Log("deltaPos:" + deltaPos + ",angle:" + angle);
        Quaternion qua = Quaternion.AngleAxis(angle, cross);
        target.transform.rotation = qua * target.transform.rotation;
    }

    //开始滑动
    void EasyTouch_On_DragStart(Gesture gesture)
    {
        if (!_rotateable) return;
        if (onDragStartHandler != null)
            onDragStartHandler();
    }
}