Object created via drag&drop is deleted immediately when using RegisterCreatedObjectUndo

Hi,
I am trying to create a script, that allows the user to add an image to the scene via drag&drop (similar to dragging a prefab into the hierarchy). I used this post as a starting point and the script worked, but the new object in the scene was not registered as a change and therefore not saved.

I tried to use Undo.RegisterCreatedObjectUndo, but if I do that, the object appears for a split second and then gets deleted. This only happens when the function for creating the image object is called from EditorApplication.hierarchyWindowItemOnGUI. I tested calling the function from a menu entry and loading the image from the AssetDatabase and that works as intended. So I guess the problem stems from the way I process the drag&drop or something happens in/after EditorApplication.hierarchyWindowItemOnGUI.

You can see the behaviour here (link to GIF). It looks like this (link to GIF) if I remove RegisterCreatedObjectUndo (notice how the item is deselected immediately) and like this (link to GIF) when I call the function from a menu entry.

Thanks

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEditor;
4. using UnityEngine;
5.  
6. [InitializeOnLoad]
7. public class DragImage : Editor
8. {
9.  
10.     static DragImage()
11.     {
12.         EditorApplication.hierarchyWindowItemOnGUI += HierarchyWindowCallback;      
13.     }
14.  
15.     static void HierarchyWindowCallback(int id, Rect pos)
16.     {
17.         if (Event.current.type == EventType.DragExited)
18.         {
19.             if(DragDropCallback()) {
20.                 Event.current.Use();
21.             }
22.         }
23.     }  
24.  
25.     static bool DragDropCallback()
26.     {
27.         DragAndDrop.AcceptDrag();
28.         List<Texture2D> images = new List<Texture2D>();
29.         foreach (Object obj in DragAndDrop.objectReferences)
30.         {
31.             if (obj.GetType() == typeof(Texture2D))
32.             {
33.                 Texture2D tex = (Texture2D)obj;
34.                 images.Add(tex);
35.             }
36.         }
37.  
38.         foreach(Texture2D img in images)
39.         {
40.             CreateImage(img);
41.         }
42.         return images.Count>0;
43.     }
44.  
45.     [MenuItem("GameObject/3D Object/TestImage")]
46.     static void CreateInMenu()
47.     {
48.         CreateImage(AssetDatabase.LoadAssetAtPath<Texture2D>("Assets/test.png"));
49.     }
50.  
51.  
52.     static void CreateImage(Texture2D tex)
53.     {
54.         GameObject imgObject = GameObject.CreatePrimitive(PrimitiveType.Plane);
55.         imgObject.name = "Image";
56.         imgObject.transform.localScale = new Vector3(0.1f * ((float)tex.width) / ((float)tex.height), 1f, 0.1f);
57.         imgObject.transform.rotation = Quaternion.Euler(90f, 0f, 180f);
58.         imgObject.transform.position = new Vector3(0f, 1.5f, 0f);
59.         MeshRenderer renderer = imgObject.GetComponent<MeshRenderer>();
60.         Material mat = new Material(Shader.Find("Standard"));
61.         mat.SetTexture("_MainTex", tex);
62.         mat.SetColor("_Color", Color.grey);
63.         mat.SetFloat("_Glossiness", 0.0f);
64.         renderer.material = mat;
65.         Undo.RegisterCreatedObjectUndo(imgObject, "Create image");
66.         Selection.activeObject = imgObject;
67.     }
68. }
69.  

 

The following code should work as expected. However, since it's a callback for "items in the hierarchy window", it only accepts the drop if the mouse is located on an item.

Code (CSharp):

1. static void HierarchyWindowCallback(int id, Rect pos)
2. {
3.     if (!pos.Contains(Event.current.mousePosition))
4.         return;
5.  
6.     var eventType = Event.current.type;
7.     if (eventType == EventType.DragUpdated || eventType == EventType.DragPerform)
8.     {
9.         DragAndDrop.visualMode = DragAndDropVisualMode.Copy;
10.  
11.         if (eventType == EventType.DragPerform)
12.         {
13.             DragAndDrop.AcceptDrag();
14.             DragDropCallback();
15.         }
16.  
17.         Event.current.Use();
18.     }
19. }

 

Thanks for your answer. I did not realize, that the DragPerform event is only triggered, when you drag onto an existing item. Now it works when I drag onto an existing item, but that's not very practical, so I switched to SceneView.onSceneGUIDelegate (to drag&drop into the scene view).

My final code looks like this:

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEditor;
4. using UnityEngine;
5.  
6. [InitializeOnLoad]
7. public class DragImage : Editor
8. {
9.  
10.     static DragImage()
11.     {
12.         SceneView.onSceneGUIDelegate += SceneViewCallback;
13.     }
14.  
15.     static void SceneViewCallback(SceneView view)
16.     {
17.         if (Event.current.type == EventType.DragUpdated || Event.current.type == EventType.DragPerform)
18.         {
19.             DragAndDrop.visualMode = DragAndDropVisualMode.Copy;
20.         }
21.         if (Event.current.type == EventType.DragPerform)
22.         {
23.             DragAndDrop.AcceptDrag();
24.             if (DragDropCallback())
25.             {
26.                 Event.current.Use();
27.             }
28.         }
29.     }
30.        
31.  
32.     static bool DragDropCallback()
33.     {      
34.         List<Texture2D> images = new List<Texture2D>();
35.         foreach (Object obj in DragAndDrop.objectReferences)
36.         {
37.             if (obj.GetType() == typeof(Texture2D))
38.             {
39.                 Texture2D tex = (Texture2D)obj;
40.                 images.Add(tex);
41.             }
42.         }
43.  
44.         foreach(Texture2D img in images)
45.         {
46.             CreateImage(img);
47.         }
48.         return images.Count>0;
49.     }
50.  
51.     static void CreateImage(Texture2D tex)
52.     {
53.         GameObject imgObject = GameObject.CreatePrimitive(PrimitiveType.Plane);
54.         imgObject.name = "Image";
55.         imgObject.transform.localScale = new Vector3(0.1f * ((float)tex.width) / ((float)tex.height), 1f, 0.1f);
56.         imgObject.transform.rotation = Quaternion.Euler(90f, 0f, 180f);
57.         imgObject.transform.position = new Vector3(0f, 1.5f, 0f);
58.         MeshRenderer renderer = imgObject.GetComponent<MeshRenderer>();
59.         Material mat = new Material(Shader.Find("Standard"));
60.         mat.SetTexture("_MainTex", tex);
61.         mat.SetColor("_Color", Color.grey);
62.         mat.SetFloat("_Glossiness", 0.0f);
63.         renderer.material = mat;
64.         Undo.RegisterCreatedObjectUndo(imgObject, "Create image");
65.         Selection.activeObject = imgObject;
66.     }
67. }
68.