Playing with unity editor

UnityEditorTests.cs

using UnityEditor;
using UnityEngine;

[ExecuteInEditMode]
public class TestEditor : MonoBehaviour
{
    public UnityEngine.Object prefab;

    public GameObject parent;

    public Brush brush;
    
    private void OnEnable()
    {
        SceneView.duringSceneGui += OnSceneGUI;
       // Selection.selectionChanged += OnSelectionChanged;
    }

    private bool _selectionChanged;

    private void OnSceneGUI(SceneView view)
    {
        var e = Event.current;
        // Debug.Log(e);
        
        var mousePosition = e.mousePosition;
        
        if ((e.type == EventType.MouseDrag || e.type == EventType.MouseDown) && e.button == 1)
        {
            var position = HandleUtility.GUIPointToWorldRay (mousePosition).origin;

            if (e.alt)
            {
                Debug.Log("ERASE!");
                // delete instance
                var go = HandleUtility.PickGameObject(mousePosition, true);
                if (go != null && go != this.gameObject)
                {
                    if (Selection.Contains(go))
                    {
                        Selection.activeObject = null;
                    }
                    
                    GameObject.DestroyImmediate(go);
                }
            }
            else
            {
                if (brush.CanPaint(position))
                {
                    var instance = PrefabUtility.InstantiatePrefab(prefab, parent.transform) as GameObject;
                    instance.transform.position = position;
                    LockObject(instance, true);
//                    instance.hideFlags = HideFlags.NotEditable | HideFlags.HideInHierarchy;
//                    instance.transform.GetChild(0).hideFlags = HideFlags.NotEditable | HideFlags.HideInHierarchy;
                    brush.Paint(position);
                }                
            }
            
            e.Use();
        }
        
        
        void LockObject(GameObject targetObject, bool recursive)
        {
            targetObject.hideFlags = targetObject.hideFlags | HideFlags.NotEditable;
            if (recursive)
            {
                foreach (Transform child in targetObject.transform)
                    LockObject(child.gameObject, true);
            }
        }
 
        void UnlockObject(GameObject targetObject, bool recursive)
        {
            targetObject.hideFlags = targetObject.hideFlags & ~HideFlags.NotEditable;
            if (recursive)
            {
                foreach (Transform child in targetObject.transform)
                    UnlockObject(child.gameObject, true);
            }
        }
    }

    private void OnDisable()
    {
        SceneView.duringSceneGui -= OnSceneGUI;
        // Selection.selectionChanged -= OnSelectionChanged;
    }
}

////

using System.Collections;
using System.Collections.Generic;
using UnityEditor;
using UnityEngine;


[InitializeOnLoad]
public class TestCustomEditor
{
    private static bool _selectionChanged;
    
    static TestCustomEditor()
    {
        EditorApplication.update += TestUpdate;       
        Selection.selectionChanged += OnSelectionChanged;
    }

    private static void OnSelectionChanged()
    {
       // return;
        _selectionChanged = true;
    }
    
    private static bool ObjectIsLocked(GameObject testObject)
    {
        if (testObject == null)
            return false;
        return ((int) testObject.hideFlags & (int)HideFlags.NotEditable) != 0;
    }

    private static void TestUpdate()
    {
        // Debug.Log("Update!!");
        if (_selectionChanged)
        {
            _selectionChanged = false;
            
            var selectedObjects = Selection.objects;
            var newSelectedObjects = new List<Object>();
        
            for (var i = 0; i < selectedObjects.Length; i++)
            {
                if (ObjectIsLocked(selectedObjects[i] as GameObject))
                {
                    Debug.Log("Removed from selection!");
                    continue;
                }

                Debug.Log("Added to selection");
                newSelectedObjects.Add(selectedObjects[i]);
            }

            Selection.objects = newSelectedObjects.ToArray();
        }
    }
}

///

using UnityEditor;
using UnityEngine;

[CreateAssetMenu(menuName = "Gemserk/Brush")]
public class Brush : ScriptableObject
{
    // paint per second?
    public float delay;
    
    public float distance;

    private double _lastTime;

    private Vector2 _lastPosition;

    public bool CanPaint(Vector2 position)
    {
        if (EditorApplication.timeSinceStartup - _lastTime > delay)
            return true;

        return Vector2.SqrMagnitude(position - _lastPosition) > distance * distance;
    }

    public void Paint(Vector2 position)
    {
        _lastPosition = position;
        _lastTime = EditorApplication.timeSinceStartup;
    }
}