Is it safe to get a StyleSheet from the default reference in the EditorWindow?

I'd like to avoid loading files from hardcoded paths, so I thought it might be possible to load a style sheet by setting a default reference in the inspector of my custom editor window script.

Code (CSharp):

1. public class MyEditorWindow : EditorWindow
2. {
3.     // This reference can be applied via the inspector
4.     // of the script as a "default reference".
5.     [SerializeField]
6.     private StyleSheet styleSheet;
7.  
8.     private void OnEnable()
9.     {
10.         // Instead of loading by path, always use the default reference.
11.         if (styleSheet != null)
12.             base.rootVisualElement.styleSheets.Add(styleSheet);
13.     }
14. }

In my first tests, this worked great. I can open my window, the default reference exists and I can add the stylesheet. I can also make changes to the sheet and they are instantly reflected in the window.

However, sometimes, I get a MissingReferenceException saying that the StyleSheet ScriptableObject was destroyed (the C++ representation). My C# "styleSheet" member is not null, but somewhere a style sheet object apparently was deleted by Unity. The stack trace pointed deep into the UIElements code, no lines from my own script, I believe:

Spoiler: Exception Details

MissingReferenceException: The object of type 'StyleSheet' has been destroyed but you are still trying to access it.
Your script should either check if it is null or you should not destroy the object.
UnityEngine.Object.get_name () (at C:/buildslave/unity/build/Runtime/Export/Scripting/UnityEngineObject.bindings.cs:189)
UnityEngine.UIElements.StyleSheets.StyleSheetCache.GetPropertyID (UnityEngine.UIElements.StyleSheet sheet, UnityEngine.UIElements.StyleRule rule, System.Int32 index) (at C:/buildslave/unity/build/Modules/UIElements/StyleSheets/StyleSheetCache.cs:333)
UnityEngine.UIElements.StyleSheets.StyleSheetCache.GetPropertyIDs (UnityEngine.UIElements.StyleSheet sheet, System.Int32 ruleIndex) (at C:/buildslave/unity/build/Modules/UIElements/StyleSheets/StyleSheetCache.cs:253)
UnityEngine.UIElements.VisualTreeStyleUpdaterTraversal.ProcessMatchedRules (UnityEngine.UIElements.VisualElement element, System.Collections.Generic.List`1[T] matchingSelectors) (at C:/buildslave/unity/build/Modules/UIElements/VisualTreeStyleUpdater.cs:313)
UnityEngine.UIElements.VisualTreeStyleUpdaterTraversal.TraverseRecursive (UnityEngine.UIElements.VisualElement element, System.Int32 depth) (at C:/buildslave/unity/build/Modules/UIElements/VisualTreeStyleUpdater.cs:248)
UnityEngine.UIElements.StyleSheets.HierarchyTraversal.Recurse (UnityEngine.UIElements.VisualElement element, System.Int32 depth) (at C:/buildslave/unity/build/Modules/UIElements/HierarchyTraversal.cs:24)
UnityEngine.UIElements.VisualTreeStyleUpdaterTraversal.TraverseRecursive (UnityEngine.UIElements.VisualElement element, System.Int32 depth) (at C:/buildslave/unity/build/Modules/UIElements/VisualTreeStyleUpdater.cs:271)
UnityEngine.UIElements.StyleSheets.HierarchyTraversal.Recurse (UnityEngine.UIElements.VisualElement element, System.Int32 depth) (at C:/buildslave/unity/build/Modules/UIElements/HierarchyTraversal.cs:24)
UnityEngine.UIElements.VisualTreeStyleUpdaterTraversal.TraverseRecursive (UnityEngine.UIElements.VisualElement element, System.Int32 depth) (at C:/buildslave/unity/build/Modules/UIElements/VisualTreeStyleUpdater.cs:271)
UnityEngine.UIElements.StyleSheets.HierarchyTraversal.Traverse (UnityEngine.UIElements.VisualElement element) (at C:/buildslave/unity/build/Modules/UIElements/HierarchyTraversal.cs:11)
UnityEngine.UIElements.VisualTreeStyleUpdater.ApplyStyles () (at C:/buildslave/unity/build/Modules/UIElements/VisualTreeStyleUpdater.cs:123)
UnityEngine.UIElements.VisualTreeStyleUpdater.Update () (at C:/buildslave/unity/build/Modules/UIElements/VisualTreeStyleUpdater.cs:106)
UnityEngine.UIElements.VisualTreeUpdater.UpdateVisualTree () (at C:/buildslave/unity/build/Modules/UIElements/VisualTreeUpdater.cs:70)
UnityEngine.UIElements.Panel.Repaint (UnityEngine.Event e) (at C:/buildslave/unity/build/Modules/UIElements/Panel.cs:589)
UnityEngine.UIElements.UIElementsUtility.DoDispatch (UnityEngine.UIElements.BaseVisualElementPanel panel) (at C:/buildslave/unity/build/Modules/UIElements/UIElementsUtility.cs:240)
UnityEngine.UIElements.UIElementsUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr) (at C:/buildslave/unity/build/Modules/UIElements/UIElementsUtility.cs:78)
UnityEngine.GUIUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr) (at C:/buildslave/unity/build/Modules/IMGUI/GUIUtility.cs:179)

Do I have to load my sheet from path in OnEnable because of limitations with this approach or am I seeing exceptions because of a more general issue with style sheets sometimes being destroyed?

 

How about using

Resources.Load<StyleSheet>("path/style");

. Then place styles in a path like

Assets/Path/Editor/Resources/path/style.uss

The part up to Editor/Resources can be anything and that would normally be what a user would want to change if you were making a tool to be used in other projects.

[edit] However, thinking about it now. I did also get an exception related to the style sheet now and then while making changes to the code and/or style.

 

Yes, I will do that tonight!

I've observed my tool for a couple of days now and found that the posted exception was also thrown occasionally when I loaded the style sheet by path. So, I assume the issue is not related to the approach of using default references.

That would work, but I generally found this more difficult to work with if there is only a single file to reference. In our team we like to split our content into functional components and not by category or type. This means I would be adding multiple (many) Resources folders to the project for single files etc. The default references approach actually works perfectly for this type of thing and after looking into the issue I think the exception is not even related to the use of the default reference.

 

Hi Damian, I suspect I'm running into that exact issue. I've followed the execution path and came to this:

Code (CSharp):

1. // UnityEngine.UIElements.VisualElement
2. internal void AddStyleSheetPath(string sheetPath)
3. {
4.     StyleSheet styleSheet = Panel.LoadResource(sheetPath, typeof(StyleSheet), scaledPixelsPerPoint) as StyleSheet;
5.     if (styleSheet == null)
6.     {
7.         Debug.LogWarning($"Style sheet not found for path \"{sheetPath}\"");
8.     }
9.     else
10.     {
11.         styleSheets.Add(styleSheet);
12.     }
13. }

Obviously, I'm getting the LogWarning for all the elements in the graph when the domain is reloaded. Is there somewhere I can follow the bug and the progress? Is there a quick workaround at the moment, such as trying to reload the graph?

 

Hi Antoine,

I'm using 2019.3.3f1.

I'm in the process now of trying to load the USS myself, OR, whether I could strip the USS code, and insert it into my own USS file.

I'm also in the process of trying to load the resource manually, but I don't think this is how loading a resource works:
var nodess = Resources.Load<StyleSheet> ( "StyleSheets/GraphView/Node" ); // with and without the .uss.

I'm running in to a lot of "internal" methods and properties which is really slowing me down. I'm either using reflection in some cases, or rewriting methods.

But I shouldn't complain. I mean, the names space does actually have the word "experimental" in it! haha