unity 编辑器二次开发，在非运行状态下，在scene场景点击某个物体出现一条线，起始点是该物体，终点在鼠标位置，当鼠标点击其他球体后线的终点放到最后被点击的物体上

这个功能可以通过编写Unity Editor插件来实现。以下是一个示例脚本，可以实现在非运行状态下，点击场景中的物体出现一条线，并且当鼠标点击其他物体时，线的终点会自动切换到最后被点击的物体上。

using UnityEditor;
using UnityEngine;

[InitializeOnLoad]
public class SceneClickHandler : EditorWindow
{
    private static SceneClickHandler _instance;
    private GameObject _startObject;
    private GameObject _endObject;
    private Vector3 _mousePosition;

    static SceneClickHandler()
    {
        SceneView.duringSceneGui += OnSceneGUI;
    }

    [MenuItem("Custom/Scene Click Handler")]
    public static void ShowWindow()
    {
        _instance = GetWindow<SceneClickHandler>();
        _instance.titleContent = new GUIContent("Scene Click Handler");
    }

    private static void OnSceneGUI(SceneView sceneView)
    {
        if (Event.current.type == EventType.MouseDown && Event.current.button == 0)
        {
            Ray ray = HandleUtility.GUIPointToWorldRay(Event.current.mousePosition);
            RaycastHit hit;
            if (Physics.Raycast(ray, out hit))
            {
                if (_instance._startObject == null)
                {
                    _instance._startObject = hit.collider.gameObject;
                    _instance._endObject = null;
                }
                else if (_instance._endObject == null || _instance._endObject != hit.collider.gameObject)
                {
                    _instance._endObject = hit.collider.gameObject;
                }
            }
        }
        else if (Event.current.type == EventType.MouseMove)
        {
            _instance._mousePosition = Event.current.mousePosition;
            SceneView.RepaintAll();
        }

        if (_instance._startObject != null)
        {
            Handles.DrawLine(_instance._startObject.transform.position, _instance._mousePosition);

            if (_instance._endObject != null && _instance._endObject != _instance._startObject)
            {
                Handles.DrawLine(_instance._startObject.transform.position, _instance._endObject.transform.position);
            }
        }
    }
}

将以上代码保存为名为`SceneClickHandler.cs`的文件，然后将它放到`Assets/Editor`目录下。接着在Unity编辑器中选择`Custom/Scene Click Handler`菜单项，就可以打开插件窗口了。在非运行状态下，在场景中点击物体，就会出现一条线，并且当鼠标点击其他物体时，线的终点会自动切换到最后被点击的物体上。