fix(Interaction): make force stop interactions wait for end of frame

Previously, it was possible to get the force stop interactions into an
infinite loop by calling `ForceStopTouching` or `ForceStopInteracting`
from within an event listener from the Stop Using event which would
in turn could emit the Stop Using event again and therefore getting
stuck in an infinite loop.

The fix now ensures that any interactable object force stop events
are done at the end of the frame giving the controller scripts time
to correctly clean up.

Assets/SteamVR_Unity_Toolkit/Scripts/VRTK_InteractableObject.cs

@@ -11,6 +11,7 @@
 namespace VRTK
 {
     using UnityEngine;
+    using System.Collections;
     using System.Collections.Generic;
 
     public struct InteractableObjectEventArgs
@@ -200,7 +201,10 @@ public virtual void StopTouching(GameObject previousTouchingObject)
         {
             OnInteractableObjectUntouched(SetInteractableObjectEvent(previousTouchingObject));
             touchingObject = null;
-            StopUsingOnControllerChange(previousTouchingObject);
+            if(gameObject.activeInHierarchy)
+            {
+                StartCoroutine(StopUsingOnControllerChange(previousTouchingObject));
+            }
         }
 
         public virtual void Grabbed(GameObject currentGrabbingObject)
@@ -224,22 +228,9 @@ public virtual void Ungrabbed(GameObject previousGrabbingObject)
             grabbedSnapHandle = null;
             grabbingObject = null;
             LoadPreviousState();
-            StopUsingOnControllerChange(previousGrabbingObject);
-        }
-
-        private void StopUsingOnControllerChange(GameObject previousController)
-        {
-            var usingObject = previousController.GetComponent<VRTK_InteractUse>();
-            if (usingObject)
+            if (gameObject.activeInHierarchy)
             {
-                if (holdButtonToUse)
-                {
-                    usingObject.ForceStopUsing();
-                }
-                else
-                {
-                    usingObject.ForceResetUsing();
-                }
+                StartCoroutine(StopUsingOnControllerChange(previousGrabbingObject));
             }
         }
 
@@ -370,24 +361,9 @@ public bool IsValidInteractableController(GameObject actualController, AllowedCo
 
         public void ForceStopInteracting()
         {
-            if (touchingObject != null && touchingObject.activeInHierarchy)
-            {
-                touchingObject.GetComponent<VRTK_InteractTouch>().ForceStopTouching();
-                forcedDropped = true;
-            }
-
-            if (grabbingObject != null && grabbingObject.activeInHierarchy)
-            {
-                grabbingObject.GetComponent<VRTK_InteractTouch>().ForceStopTouching();
-                grabbingObject.GetComponent<VRTK_InteractGrab>().ForceRelease();
-                forcedDropped = true;
-            }
-
-            if (usingObject != null && usingObject.activeInHierarchy)
+            if (gameObject.activeInHierarchy)
             {
-                usingObject.GetComponent<VRTK_InteractTouch>().ForceStopTouching();
-                usingObject.GetComponent<VRTK_InteractUse>().ForceStopUsing();
-                forcedDropped = true;
+                StartCoroutine(ForceStopInteractingAtEndOfFrame());
             }
         }
 
@@ -691,5 +667,46 @@ private void OnTeleported(object sender, DestinationMarkerEventArgs e)
                 transform.position = grabbingObject.transform.position;
             }
         }
+
+        private IEnumerator StopUsingOnControllerChange(GameObject previousController)
+        {
+            yield return new WaitForEndOfFrame();
+            var usingObject = previousController.GetComponent<VRTK_InteractUse>();
+            if (usingObject)
+            {
+                if (holdButtonToUse)
+                {
+                    usingObject.ForceStopUsing();
+                }
+                else
+                {
+                    usingObject.ForceResetUsing();
+                }
+            }
+        }
+
+        private IEnumerator ForceStopInteractingAtEndOfFrame()
+        {
+            yield return new WaitForEndOfFrame();
+            if (touchingObject != null && touchingObject.activeInHierarchy)
+            {
+                touchingObject.GetComponent<VRTK_InteractTouch>().ForceStopTouching();
+                forcedDropped = true;
+            }
+
+            if (grabbingObject != null && grabbingObject.activeInHierarchy)
+            {
+                grabbingObject.GetComponent<VRTK_InteractTouch>().ForceStopTouching();
+                grabbingObject.GetComponent<VRTK_InteractGrab>().ForceRelease();
+                forcedDropped = true;
+            }
+
+            if (usingObject != null && usingObject.activeInHierarchy)
+            {
+                usingObject.GetComponent<VRTK_InteractTouch>().ForceStopTouching();
+                usingObject.GetComponent<VRTK_InteractUse>().ForceStopUsing();
+                forcedDropped = true;
+            }
+        }
     }
 }