fix: Fixed custom icons not being set for generated classes in Unity 2021

AssetPreview.GetMiniThumbnail stopped providing persistent textures, so it can't be used to get a custom texture anymore. .meta file parsing is used instead now.

Author: SolidAlloy

Editor/Util/IconFinder.cs

@@ -0,0 +1,81 @@
+namespace GenericUnityObjects.Editor.Util
+{
+    using System;
+    using System.IO;
+    using NUnit.Framework;
+    using UnityEditor;
+    using UnityEngine;
+
+    internal static class IconFinder
+    {
+        public static bool TryGetCustomIcon(string genericTypeGUID, out Texture2D customIcon, bool isScriptableObject)
+        {
+            customIcon = null;
+
+            string assetPath = AssetDatabase.GUIDToAssetPath(genericTypeGUID);
+
+            if (string.IsNullOrEmpty(assetPath))
+                return false;
+
+            if (TryGetCustomIcon(assetPath, out customIcon))
+                return true;
+
+            // If generated type inherits from MonoBehaviour, a default script icon must be set, but for scriptable
+            // objects, it is not the case.
+            return ! isScriptableObject && TryGetDefaultIconFromMonoBehaviourScript(assetPath, out customIcon);
+        }
+
+        private static bool TryGetDefaultIconFromMonoBehaviourScript(string assetPath, out Texture2D customIcon)
+        {
+            var monoScript = AssetDatabase.LoadAssetAtPath<MonoScript>(assetPath);
+
+            if (monoScript is null)
+            {
+                customIcon = null;
+                return false;
+            }
+
+            customIcon = AssetPreview.GetMiniThumbnail(monoScript);
+            return true;
+        }
+
+        private static bool TryGetCustomIcon(string assetPath, out Texture2D customIcon)
+        {
+            // Unity doesn't expose any method to get icon guid or Texture2D from MonoScript, so we have to parse the .meta file manually.
+            // AssetPreview.GetMiniThumbnail returns the icon, but in 2021 it has the DontSave flag and can't be used to set as a custom icon for an asset.
+
+            string iconLine = GetIconLine(assetPath);
+
+            if (iconLine == null)
+            {
+                customIcon = null;
+                return false;
+            }
+
+            string guid = GetGUIDFromIconLine(iconLine);
+            string texturePath = AssetDatabase.GUIDToAssetPath(guid);
+            customIcon = AssetDatabase.LoadAssetAtPath<Texture2D>(texturePath);
+            Assert.IsNotNull(customIcon);
+            return true;
+        }
+
+        private static string GetGUIDFromIconLine(string iconLine)
+        {
+            int guidIndex = iconLine.IndexOf("guid: ", StringComparison.Ordinal);
+            return iconLine.Substring(guidIndex + 6, 32);
+        }
+
+        private static string GetIconLine(string assetPath)
+        {
+            string[] metaContentLines = File.ReadAllLines($"{assetPath}.meta");
+
+            foreach (string line in metaContentLines)
+            {
+                if (line.StartsWith("  icon: "))
+                    return line;
+            }
+
+            return null;
+        }
+    }
+}

Editor/Util/IconFinder.cs.meta

@@ -25,34 +25,13 @@ public static void AddAssemblyForIconChange(string genericScriptGUID, string ass
             if (string.IsNullOrEmpty(genericScriptGUID))
                 return;
 
-            if (TryGetCustomIcon(genericScriptGUID, out Texture2D texture, isScriptableObject))
+            if (IconFinder.TryGetCustomIcon(genericScriptGUID, out Texture2D texture, isScriptableObject))
                 Instance.AddAssembly(assemblyGUID, texture);
         }
 
-        private static bool TryGetCustomIcon(string genericTypeGUID, out Texture2D customIcon, bool isScriptableObject)
-        {
-            customIcon = null;
-
-            string assetPath = AssetDatabase.GUIDToAssetPath(genericTypeGUID);
-
-            if (string.IsNullOrEmpty(assetPath))
-                return false;
-
-            var monoScript = AssetDatabase.LoadAssetAtPath<MonoScript>(assetPath);
-
-            if (monoScript is null)
-                return false;
-
-            customIcon = AssetPreview.GetMiniThumbnail(monoScript);
-
-            // If generated type inherits from MonoBehaviour, a default script icon must be set, but for scriptable
-            // objects, it is not the case.
-            return ! (isScriptableObject && customIcon.name.Contains("cs Script Icon"));
-        }
-
         private void AddAssembly(string guid, Texture2D texture)
         {
-            if ( ! _dict.TryGetValue(texture, out List<string> guids))
+            if ( ! _dict.TryGetValue(texture, out var guids))
             {
                 guids = new List<string>();
                 _dict.Add(texture, guids);