add empty AnimationOnlyExportData constructor

+ check if exportData found no dependencies, and return null if it didn't

Author: vkovec

Packages/com.unity.formats.fbx/Editor/Scripts/FbxExporter.cs

@@ -2943,14 +2943,15 @@ public static IExportData GetExportData(GameObject rootObject, AnimationClip ani
                 if (exportOptions==null)
                     exportOptions = DefaultOptions;
                 Debug.Assert(exportOptions!=null);
-
-                var goToExport = new HashSet<GameObject>();
-                var animationClips = new Dictionary<AnimationClip, GameObject>();
-                var exportComponent = new Dictionary<GameObject, System.Type>();
-
-                var exportData = new AnimationOnlyExportData(animationClips, goToExport, exportComponent);
+                
+                var exportData = new AnimationOnlyExportData();
                 exportData.CollectDependencies(animationClip, rootObject, exportOptions);
-
+                
+                // could not find any dependencies, return null
+                if(exportData.Objects.Count <= 0)
+                {
+                    return null;
+                }
                 return exportData;
             }
 
@@ -2964,11 +2965,7 @@ internal static IExportData GetExportData(GameObject go, IExportOptions exportOp
                 var legacyAnim = go.GetComponentsInChildren<Animation>();
                 var genericAnim = go.GetComponentsInChildren<Animator>();
 
-                var goToExport = new HashSet<GameObject>();
-                var animationClips = new Dictionary<AnimationClip, GameObject>();
-                var exportComponent = new Dictionary<GameObject, System.Type>();
-
-                var exportData = new AnimationOnlyExportData(animationClips, goToExport, exportComponent);
+                var exportData = new AnimationOnlyExportData();
 
                 int depthFromRootAnimation = int.MaxValue;
                 Animation rootAnimation = null;

Packages/com.unity.formats.fbx/Editor/Scripts/IExportData.cs

@@ -18,7 +18,7 @@ public interface IExportData
     /// Export data containing what to export when
     /// exporting animation only.
     /// </summary>
-    internal struct AnimationOnlyExportData : IExportData
+    internal class AnimationOnlyExportData : IExportData
     {
         // map from animation clip to GameObject that has Animation/Animator
         // component containing clip
@@ -46,6 +46,14 @@ public AnimationOnlyExportData(
             this.defaultClip = null;
         }
 
+        public AnimationOnlyExportData()
+        {
+            this.animationClips = new Dictionary<AnimationClip, GameObject>();
+            this.goExportSet = new HashSet<GameObject>();
+            this.exportComponent = new Dictionary<GameObject, System.Type>();
+            this.defaultClip = null;
+        }
+
         /// <summary>
         /// collect all object dependencies for given animation clip
         /// </summary>