Add exporter based on existing MjScene functionality

unity/Editor/Exporter/MjExporterEditorWindow.cs

@@ -0,0 +1,62 @@
+// Copyright 2019 DeepMind Technologies Limited
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+using System.IO;
+using System.Text;
+using System.Xml;
+using UnityEditor;
+using UnityEngine;
+
+namespace Mujoco
+{
+
+    // Mujoco scenes exporter window.
+    public class MjExporterEditorWindow : EditorWindow
+    {
+        [MenuItem("Assets/Export MuJoCo Scene")]
+        public static void Apply()
+        {
+            if (Application.isPlaying || MjScene.InstanceExists)
+            {
+                Debug.LogWarning("MJCF exporting only available outside Play mode, and with no MjScene in the hierarchy.");
+                return;
+            }
+
+            string path = EditorUtility.SaveFilePanel("Save MJCF model", "", "model", "xml");
+            if (string.IsNullOrEmpty(path)) return;
+
+            // Due to singleton pattern, this creates a MjScene GameObject we need to remove at the end.
+            var mjcf = MjScene.Instance.CreateScene(skipCompile: true);
+            try
+            {
+                using (var stream = File.Open(path, FileMode.Create))
+                {
+                    using (var writer = new XmlTextWriter(stream, new UTF8Encoding(false)))
+                    {
+                        writer.Formatting = Formatting.Indented;
+                        mjcf.WriteContentTo(writer);
+                        Debug.Log($"MJCF saved to {path}");
+                    }
+                }
+            }
+            catch (IOException ex)
+            {
+                Debug.LogWarning("Failed to save Xml to a file: " + ex.ToString());
+            }
+
+            // Removing the added MjScene.
+            DestroyImmediate(MjScene.Instance.gameObject);
+        }
+    }
+}

unity/Editor/Exporter/MjExporterEditorWindow.cs.meta

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+fileFormatVersion: 2
+guid: 7695f261c02fdc343a09b219c92a6aab
+MonoImporter:
+  externalObjects: {}
+  serializedVersion: 2
+  defaultReferences: []
+  executionOrder: 0
+  icon: {instanceID: 0}
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: