feat(SDK): allow additional tracked controller usage

The HTC Trackers are now supported in a basic way by allowing for
additional tracked controllers to be supported by the SteamVR SDK
Bridge. The tracker support still requires manual configuration of
the tracker objects and only works with SteamVR. But until more
tracking solutions come out, this will be ok as the HTC trackers
can only be used with SteamVR at the moment anyway.

To set a tracker up:

 * Follow HTC Tracker instructions to set object up with SteamVR
 * Add `VRTK_TrackedController` to tracker object
 * Add `VRTK_ControllerTracker` to tracker object
 * The tracker object is now usable by VRTK
 * Attach any controller scripts directly to the tracker object

Assets/VRTK/SDK/SteamVR/SDK_SteamVRController.cs

@@ -21,6 +21,8 @@ public class SDK_SteamVRController
 #if VRTK_DEFINE_SDK_STEAMVR
         private SteamVR_TrackedObject cachedLeftTrackedObject;
         private SteamVR_TrackedObject cachedRightTrackedObject;
+        private Dictionary<GameObject, SteamVR_TrackedObject> cachedTrackedObjectsByGameObject = new Dictionary<GameObject, SteamVR_TrackedObject>();
+        private Dictionary<uint, SteamVR_TrackedObject> cachedTrackedObjectsByIndex = new Dictionary<uint, SteamVR_TrackedObject>();
         private ushort maxHapticVibration = 3999;
 
         /// <summary>
@@ -130,6 +132,12 @@ public override GameObject GetControllerByIndex(uint index, bool actual = false)
                     return (actual ? sdkManager.actualRightController : sdkManager.scriptAliasRightController);
                 }
             }
+
+            if (cachedTrackedObjectsByIndex.ContainsKey(index) && cachedTrackedObjectsByIndex[index] != null)
+            {
+                return cachedTrackedObjectsByIndex[index].gameObject;
+            }
+
             return null;
         }
 
@@ -860,6 +868,8 @@ private void SetTrackedControllerCaches(bool forceRefresh = false)
             {
                 cachedLeftTrackedObject = null;
                 cachedRightTrackedObject = null;
+                cachedTrackedObjectsByGameObject.Clear();
+                cachedTrackedObjectsByIndex.Clear();
             }
 
             var sdkManager = VRTK_SDKManager.instance;
@@ -879,17 +889,30 @@ private void SetTrackedControllerCaches(bool forceRefresh = false)
         private SteamVR_TrackedObject GetTrackedObject(GameObject controller)
         {
             SetTrackedControllerCaches();
-            SteamVR_TrackedObject trackedObject = null;
 
             if (IsControllerLeftHand(controller))
             {
-                trackedObject = cachedLeftTrackedObject;
+                return cachedLeftTrackedObject;
             }
             else if (IsControllerRightHand(controller))
             {
-                trackedObject = cachedRightTrackedObject;
+                return cachedRightTrackedObject;
+            }
+
+            if (cachedTrackedObjectsByGameObject.ContainsKey(controller) && cachedTrackedObjectsByGameObject[controller] != null)
+            {
+                return cachedTrackedObjectsByGameObject[controller];
+            }
+            else
+            {
+                var trackedObject = controller.GetComponent<SteamVR_TrackedObject>();
+                if (trackedObject != null)
+                {
+                    cachedTrackedObjectsByGameObject.Add(controller, trackedObject);
+                    cachedTrackedObjectsByIndex.Add((uint)trackedObject.index, trackedObject);
+                }
+                return trackedObject;
             }
-            return trackedObject;
         }
 
         private bool IsButtonPressed(uint index, ButtonPressTypes type, ulong button)

Assets/VRTK/Scripts/Internal/VRTK_ControllerTracker.cs

@@ -8,12 +8,8 @@ public class VRTK_ControllerTracker : MonoBehaviour
 
         protected virtual void OnEnable()
         {
-            var actualController = VRTK_DeviceFinder.GetActualController(gameObject);
-            if (actualController)
-            {
-                trackedController = actualController.GetComponent<VRTK_TrackedController>();
-            }
-
+            GameObject actualController = VRTK_DeviceFinder.GetActualController(gameObject);
+            trackedController = (actualController != null ? actualController.GetComponent<VRTK_TrackedController>() : GetComponent<VRTK_TrackedController>());
             Update();
         }
 

Assets/VRTK/Scripts/Internal/VRTK_TrackedController.cs

@@ -57,6 +57,10 @@ protected virtual VRTKTrackedControllerEventArgs SetEventPayload(uint previousIn
         protected virtual void OnEnable()
         {
             aliasController = VRTK_DeviceFinder.GetScriptAliasController(gameObject);
+            if (aliasController == null)
+            {
+                aliasController = gameObject;
+            }
 
             if (enableControllerCoroutine != null)
             {
@@ -97,7 +101,7 @@ protected virtual void Update()
 
             VRTK_SDK_Bridge.ControllerProcessUpdate(index);
 
-            if (aliasController && gameObject.activeInHierarchy && !aliasController.activeSelf)
+            if (aliasController != null && gameObject.activeInHierarchy && !aliasController.activeSelf)
             {
                 aliasController.SetActive(true);
             }