Fix test failures and more improvements

Author: MaxWang-MS

Assets/MRTK/Core/Utilities/Async/AwaiterExtensions.cs

@@ -141,12 +141,23 @@ private static void RunOnUnityScheduler(Action action)
         {
             if (SynchronizationContext.Current == SyncContextUtility.UnitySynchronizationContext)
             {
+                // Take the opportunity to ensure AsyncCoroutineRunner is running when we are on the Unity thread
+                if (!AsyncCoroutineRunner.IsInstanceRunning)
+                {
+                    var _ = AsyncCoroutineRunner.Instance;
+                }
                 action();
             }
             else
             {
-                // Make sure there is an instance of AsyncCoroutineRunner before calling AsyncCoroutineRunner.Post
-                var _ = AsyncCoroutineRunner.Instance;
+                // Make sure there is a running instance of AsyncCoroutineRunner before calling AsyncCoroutineRunner.Post
+                // If not warn the user. Note we cannot call AsyncCoroutineRunner.Instance here as that getter contains
+                // calls to Unity functions that can only be run on the Unity thread
+                if (!AsyncCoroutineRunner.IsInstanceRunning)
+                {
+                    Debug.LogWarning("There is no active AsyncCoroutineRunner when an action is posted. Place a GameObject " +
+                        "at the root of the scene and attach the AsyncCoroutineRunner script to make it function properly.");
+                }
                 AsyncCoroutineRunner.Post(action);
             }
         }

Assets/MRTK/Core/Utilities/Async/Internal/AsyncCoroutineRunner.cs

@@ -43,6 +43,8 @@ internal sealed class AsyncCoroutineRunner : MonoBehaviour
     {
         private static AsyncCoroutineRunner instance;
 
+        private static bool isInstanceRunning = false;
+
         private static readonly Queue<Action> Actions = new Queue<Action>();
 
         internal static AsyncCoroutineRunner Instance
@@ -51,10 +53,17 @@ internal static AsyncCoroutineRunner Instance
             {
                 if (instance == null)
                 {
-                    instance = FindObjectOfType<AsyncCoroutineRunner>();
+                    AsyncCoroutineRunner[] instances = FindObjectsOfType<AsyncCoroutineRunner>();
+                    Debug.Assert(instances.Length <= 1, "[AsyncCoroutineRunner] There should only be one AsyncCoroutineRunner in the scene.");
+                    instance = instances.Length == 1 ? instances[0] : null;
+                    if (instance != null && !instance.enabled)
+                    {
+                        Debug.LogWarning("[AsyncCoroutineRunner] Found a disabled AsyncCoroutineRunner component. Enabling the component.");
+                        instance.enabled = true;
+                    }
                 }
 
-                // FindObjectOfType() only search for active objects. The FindObjectOfType(bool includeInactive) variant is not available to Unity 2019.4 and earlier so cannot be used.
+                // FindObjectOfType() only search for objects attached to active GameObjects. The FindObjectOfType(bool includeInactive) variant is not available to Unity 2019.4 and earlier so cannot be used.
                 // We instead search for GameObject called AsyncCoroutineRunner and see if it has the component attached.
                 if (instance == null)
                 {
@@ -117,9 +126,12 @@ internal static void Post(Action task)
             }
         }
 
+        internal static bool IsInstanceRunning => isInstanceRunning;
+
         private void Update()
         {
-            Debug.Assert(Instance != null);
+            Debug.Assert(Instance == this, "[AsyncCoroutineRunner] There should only be one AsyncCoroutineRunner in the scene.");
+            isInstanceRunning = true;
 
             int actionCount;
 
@@ -140,5 +152,19 @@ private void Update()
                 next();
             }
         }
+
+        private void OnDisable()
+        {
+            if (instance == this)
+            {
+                isInstanceRunning = false;
+            }
+        }
+
+        private void Awake()
+        {
+            Debug.Assert(Instance == this, "[AsyncCoroutineRunner] There should only be one AsyncCoroutineRunner in the scene.");
+            isInstanceRunning = true;
+        }
     }
 }