Show added and removed actors in prefab diff view.

Source/Editor/CustomEditors/Dedicated/ActorEditor.cs

@@ -240,6 +240,12 @@ namespace FlaxEditor.CustomEditors.Dedicated
                 node.TextColor = Color.OrangeRed;
                 node.Text = Utilities.Utils.GetPropertyNameUI(removed.PrefabObject.GetType().Name);
             }
+            // Removed Actor
+            else if (editor is RemovedActorDummy removedActor)
+            {
+                node.TextColor = Color.OrangeRed;
+                node.Text = $"{removedActor.PrefabObject.Name} ({Utilities.Utils.GetPropertyNameUI(removedActor.PrefabObject.GetType().Name)})";
+            }
             // Actor or Script
             else if (editor.Values[0] is SceneObject sceneObject)
             {
@@ -295,11 +301,35 @@ namespace FlaxEditor.CustomEditors.Dedicated
                 // Not used
             }
         }
+        
+        private class RemovedActorDummy : CustomEditor
+        {
+            /// <summary>
+            /// The removed prefab object (from the prefab default instance).
+            /// </summary>
+            public Actor PrefabObject;
+
+            /// <summary>
+            /// The prefab instance's parent.
+            /// </summary>
+            public Actor ParentActor;
+
+            /// <summary>
+            /// The order of the removed actor in the parent.
+            /// </summary>
+            public int OrderInParent;
+
+            /// <inheritdoc />
+            public override void Initialize(LayoutElementsContainer layout)
+            {
+                // Not used
+            }
+        }
 
         private TreeNode ProcessDiff(CustomEditor editor, bool skipIfNotModified = true)
         {
-            // Special case for new Script added to actor
-            if (editor.Values[0] is Script script && !script.HasPrefabLink)
+            // Special case for new Script or child actor added to actor
+            if ((editor.Values[0] is Script script && !script.HasPrefabLink) || (editor.Values[0] is Actor a && !a.HasPrefabLink))
                 return CreateDiffNode(editor);
 
             // Skip if no change detected
@@ -359,6 +389,41 @@ namespace FlaxEditor.CustomEditors.Dedicated
                 }
             }
 
+            // Compare child actors for removed actors.
+            if (editor is ActorEditor && editor.Values.HasReferenceValue && editor.Values.ReferenceValue is Actor prefabObjectActor)
+            {
+                var thisActor = editor.Values[0] as Actor;
+                for (int i = 0; i < prefabObjectActor.ChildrenCount; i++)
+                {
+                    var prefabActorChild = prefabObjectActor.Children[i];
+                    if (thisActor == null)
+                        continue;
+                    bool isRemoved = true;
+                    for (int j = 0; j < thisActor.ChildrenCount; j++)
+                    {
+                        var actorChild = thisActor.Children[j];
+                        if (actorChild.PrefabObjectID == prefabActorChild.PrefabObjectID)
+                        {
+                            isRemoved = false;
+                            break;
+                        }
+                    }
+                    if (isRemoved)
+                    {
+                        var dummy = new RemovedActorDummy
+                        {
+                            PrefabObject = prefabActorChild,
+                            ParentActor = thisActor,
+                            OrderInParent = prefabActorChild.OrderInParent,
+                        };
+                        var child = CreateDiffNode(dummy);
+                        if (result == null)
+                            result = CreateDiffNode(editor);
+                        result.AddChild(child);
+                    }
+                }
+            }
+
             return result;
         }
 
@@ -459,6 +524,25 @@ namespace FlaxEditor.CustomEditors.Dedicated
                 return;
             }
 
+            // Special case for reverting removed Actors
+            if (editor is RemovedActorDummy removedActor)
+            {
+                Editor.Log("Reverting removed actor changes to prefab (adding it)");
+
+                var parentActor = removedActor.ParentActor;
+                var restored = parentActor.AddChild(removedActor.PrefabObject.GetType());
+                var prefabId = parentActor.PrefabID;
+                var prefabObjectId = removedActor.PrefabObject.PrefabObjectID;
+                string data = JsonSerializer.Serialize(removedActor.PrefabObject);
+                JsonSerializer.Deserialize(restored, data);
+                if (Presenter.Owner is PropertiesWindow propertiesWindow)
+                    Editor.Instance.SceneEditing.Spawn(restored, parentActor, removedActor.OrderInParent);
+                else if (Presenter.Owner is PrefabWindow prefabWindow)
+                    prefabWindow.Spawn(restored, parentActor, removedActor.OrderInParent);
+                Actor.Internal_LinkPrefab(FlaxEngine.Object.GetUnmanagedPtr(restored), ref prefabId, ref prefabObjectId);
+                return;
+            }
+
             // Special case for new Script added to actor
             if (editor.Values[0] is Script script && !script.HasPrefabLink)
             {
@@ -470,6 +554,27 @@ namespace FlaxEditor.CustomEditors.Dedicated
 
                 return;
             }
+            
+            // Special case for new Actor added to actor
+            if (editor.Values[0] is Actor a && !a.HasPrefabLink)
+            {
+                Editor.Log("Reverting added actor changes to prefab (removing it)");
+
+                // TODO: Keep previous selection.
+                if (Presenter.Owner is PropertiesWindow propertiesWindow)
+                {
+                    var editorInstance = Editor.Instance.SceneEditing;
+                    editorInstance.Select(a);
+                    editorInstance.Delete();
+                }
+                else if (Presenter.Owner is PrefabWindow prefabWindow)
+                {
+                    prefabWindow.Select(prefabWindow.Graph.Root.Find(a));
+                    prefabWindow.Delete();
+                }
+
+                return;
+            }
 
             editor.RevertToReferenceValue();
         }