UNI-23143: handle an FbxPrefab that's not the root of its prefab

Previously we would nuke everything above the FbxPrefab in its hierarchy.

Now we handle it properly: instantiate the entire prefab, find the branch that
gets sync'd to the FBX, update just that branch, then apply the entire prefab
back.

With unit test.

Author: Benoit Hudson

Assets/FbxExporters/Editor/UnitTests/FbxPrefabTest.cs

@@ -434,6 +434,44 @@ public void TestTransformAndReparenting()
             Assert.That (original.transform.GetChild (0).GetChild (0).name, Is.EqualTo ("building3"));
             Assert.That (original.transform.GetChild (0).GetChild (0).localScale, Is.EqualTo (new Vector3 (.5f, .5f, .5f)));
         }
+
+        [Test]
+        public void TestFbxPrefabNotAtRoot()
+        {
+            // UNI-23143: what happens if you have an FbxPrefab that's not at the root of its prefab?
+            // Use case: table and chair are two separate fbx files, but we
+            // combine them into one prefab.
+
+            // Create a cube fbx/prefab pair.
+            var cube = GameObject.CreatePrimitive(PrimitiveType.Cube);
+            var cubeMesh = cube.GetComponent<MeshFilter>().sharedMesh;
+            var cubePath = GetRandomFbxFilePath();
+            var cubeInstance = ConvertToModel.Convert(cube, fbxFullPath: cubePath);
+            var cubeName = cubeInstance.name;
+
+            // Create a prefab that has the cube as a child.
+            // Test the test: make sure the structure is what we expect.
+            var root = new GameObject("root");
+            cubeInstance.transform.parent = root.transform;
+            var rootPrefab = PrefabUtility.CreatePrefab(GetRandomPrefabAssetPath(), root);
+            Assert.That(rootPrefab);
+            Assert.That(rootPrefab.GetComponent<MeshFilter>(), Is.Null);
+            Assert.That(rootPrefab.transform.GetChild(0).name == cubeName);
+            Assert.AreEqual(24, rootPrefab.transform.GetChild(0).GetComponent<MeshFilter>().sharedMesh.vertexCount);
+
+            // Turn the cube into a quad by exporting over its fbx file.
+            var quad = GameObject.CreatePrimitive(PrimitiveType.Quad);
+            var quadMesh = quad.GetComponent<MeshFilter>().sharedMesh;
+            ModelExporter.ExportObject(cubePath, quad);
+
+            // The actual test:
+            // Make sure the rootPrefab has the structure we expect.
+            // Before fixing the bug, the root of the prefab was destroyed and recreated, so the very first test here failed:
+            Assert.That(rootPrefab);
+            Assert.That(rootPrefab.GetComponent<MeshFilter>(), Is.Null);
+            Assert.That(rootPrefab.transform.GetChild(0).name == cubeName);
+            Assert.AreEqual(4, rootPrefab.transform.GetChild(0).GetComponent<MeshFilter>().sharedMesh.vertexCount);
+        }
     }
 
     public class FbxPrefabHelpersTest

Assets/FbxExporters/FbxPrefab.cs

@@ -920,12 +920,20 @@ void CompareAndUpdate()
             // We could optimize this out if we had nothing to do, but then the
             // OnUpdate handler wouldn't always get called, and that makes for
             // confusing API.
-            var prefabInstance = UnityEditor.PrefabUtility.InstantiatePrefab(this.gameObject) as GameObject;
-            if (!prefabInstance) {
+
+            // This FbxPrefab may not be at the root of its prefab. We instantiate the root of the prefab, then
+            // we find the corresponding FbxPrefab.
+            var prefabRoot = UnityEditor.PrefabUtility.FindPrefabRoot(this.gameObject);
+            var prefabInstanceRoot = UnityEditor.PrefabUtility.InstantiatePrefab(prefabRoot) as GameObject;
+            if (!prefabInstanceRoot) {
                 throw new System.Exception(string.Format("Failed to instantiate {0}; is it really a prefab?",
                             this.gameObject));
             }
-            var fbxPrefabInstance = prefabInstance.GetComponent<FbxPrefab>();
+            var fbxPrefabInstance = prefabInstanceRoot.GetComponentsInChildren<FbxPrefab>().FirstOrDefault(
+                fbxPrefab => UnityEditor.PrefabUtility.GetPrefabParent(fbxPrefab) == this);
+            if (!fbxPrefabInstance) {
+                throw new System.Exception(string.Format("Internal error: couldn't find the right FbxPrefab after instantiating."));
+            }
 
             // Do ALL the things (potentially nothing).
             var updatedObjects = updates.ImplementUpdates(fbxPrefabInstance);
@@ -941,10 +949,10 @@ void CompareAndUpdate()
             fbxPrefabInstance.m_fbxHistory = newFbxRepString;
 
             // Save the changes back to the prefab.
-            UnityEditor.PrefabUtility.ReplacePrefab(prefabInstance, this.transform);
+            UnityEditor.PrefabUtility.ReplacePrefab(prefabInstanceRoot, prefabRoot);
 
             // Destroy the prefabInstance.
-            GameObject.DestroyImmediate(prefabInstance);
+            GameObject.DestroyImmediate(prefabInstanceRoot);
         }
 
         /// <summary>