Add user sensor bindings.

Stop force-setting nusersensor, instead allow MuJoCo to infer the size.

PiperOrigin-RevId: 494783883
Change-Id: Ia2a1b1803b2904f2dded7b790d01eb6f1920dbaf

unity/Runtime/Components/MjScene.cs

@@ -401,12 +401,17 @@ public class MjScene : MonoBehaviour {
     MjRoot.AppendChild(GenerateMjcfSection(
         doc, components.Where(component => component is MjBaseConstraint), "equality"));
 
-    MjRoot.AppendChild(GenerateMjcfSection(
-        doc, components.Where(component => component is MjActuator), "actuator"));
-
-    MjRoot.AppendChild(GenerateMjcfSection(
-        doc, components.Where(component => component is MjBaseSensor), "sensor"));
+    MjRoot.AppendChild(
+        GenerateMjcfSection(doc,
+                            components.Where(component => component is MjActuator)
+                                .OrderBy(component => component.transform.GetSiblingIndex()),
+                            "actuator"));
 
+    MjRoot.AppendChild(
+        GenerateMjcfSection(doc,
+                            components.Where(component => component is MjBaseSensor)
+                                .OrderBy(component => component.transform.GetSiblingIndex()),
+                            "sensor"));
     // Generate the Mjcf of the runtime dependencies added to the context.
     _generationContext.GenerateMjcf(MjRoot);
     return doc;

unity/Runtime/Components/Sensors/MjUserSensor.cs

@@ -0,0 +1,46 @@
+// 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;
+using System.Linq;
+using System.Xml;
+using UnityEngine;
+
+namespace Mujoco {
+
+public class MjUserSensor : MjBaseSensor {
+
+  public String Name;
+  public int Dimension = 0;
+  public String UserData;
+
+  protected override XmlElement ToMjcf(XmlDocument doc) {
+    if (Dimension == 0) {
+      throw new MissingFieldException("Dimension should be larger than 0.");
+    }
+    var mjcf = doc.CreateElement("user");
+    mjcf.SetAttribute("name", Name);
+    mjcf.SetAttribute("dim", $"{Dimension}");
+    // TODO: add validation that UserData is a space-separated list of floating numbers?
+    mjcf.SetAttribute("user", UserData);
+    return mjcf;
+  }
+
+  protected override void FromMjcf(XmlElement mjcf) {
+    Name = mjcf.GetAttribute("name");
+    int.TryParse(mjcf.GetAttribute("dim"), out Dimension);
+    UserData = mjcf.GetAttribute("user");
+  }
+}
+}

unity/Runtime/Components/Sensors/MjUserSensor.cs.meta

@@ -0,0 +1,11 @@
+fileFormatVersion: 2
+guid: a0b10d0a10ca3ad8cb5fa2b6bcd331c1
+MonoImporter:
+  externalObjects: {}
+  serializedVersion: 2
+  defaultReferences: []
+  executionOrder: 0
+  icon: {instanceID: 0}
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: