Fix preserving objects order in prefab instances on apply (based on the prefab)

Fixes #346

Source/Engine/Level/Prefabs/Prefab.Apply.cpp

@@ -367,6 +367,17 @@ bool PrefabInstanceData::SynchronizePrefabInstances(Array<PrefabInstanceData>& p
                     Level::callActorEvent(Level::ActorEventType::OnActorActiveChanged, actor, nullptr);
                     Level::callActorEvent(Level::ActorEventType::OnActorOrderInParentChanged, actor, nullptr);
                 }
+
+                // Preserve order in parent (values from prefab are used)
+                if (i != 0)
+                {
+                    auto prefab = Content::Load<Prefab>(prefabId);
+                    const auto defaultInstance = prefab ? prefab->GetDefaultInstance(obj->GetPrefabObjectID()) : nullptr;
+                    if (defaultInstance)
+                    {
+                        obj->SetOrderInParent(defaultInstance->GetOrderInParent());
+                    }
+                }
             }
         }
 
@@ -784,6 +795,20 @@ bool Prefab::ApplyAllInternal(Actor* targetActor, bool linkTargetActorObjectToPr
                 obj->Deserialize(diffDataDocument[dataIndex], modifier.Value);
 
                 sceneObjects->Add(obj);
+
+                // Synchronize order of the scene objects with the serialized data (eg. user reordered actors in prefab editor and applied changes)
+                if (i != 0)
+                {
+                    for (int32 j = 0; j < targetObjects->Count(); j++)
+                    {
+                        SceneObject* targetObject = targetObjects->At(j);
+                        if (targetObject->GetPrefabObjectID() == obj->GetID())
+                        {
+                            obj->SetOrderInParent(targetObject->GetOrderInParent());
+                            break;
+                        }
+                    }
+                }
             }
             else
             {

Source/Engine/Level/SceneObjectsFactory.cpp

@@ -257,6 +257,16 @@ void SceneObjectsFactory::SynchronizePrefabInstances(Array<SceneObject*>& sceneO
             // Reparent
             obj->SetParent(actualParent, false);
         }
+
+        // Preserve order in parent (values from prefab are used)
+        if (i != 0)
+        {
+            const auto defaultInstance = prefab ? prefab->GetDefaultInstance(obj->GetPrefabObjectID()) : nullptr;
+            if (defaultInstance)
+            {
+                obj->SetOrderInParent(defaultInstance->GetOrderInParent());
+            }
+        }
     }
 
     // Check all actors with prefab linkage for adding missing objects

Source/Engine/Serialization/JsonTools.cpp

@@ -100,6 +100,8 @@ void ChangeIds(rapidjson_flax::Value& obj, rapidjson_flax::Document& document, c
 
 void JsonTools::ChangeIds(Document& doc, const Dictionary<Guid, Guid>& mapping)
 {
+    if (mapping.IsEmpty())
+        return;
     ::ChangeIds(doc, doc, mapping);
 }