fix: MonoSingleton (#46)

Runtime/Patterns/Singleton/MonoSingleton.cs

@@ -2,15 +2,17 @@
 
 namespace StansAssets.Foundation.Patterns
 {
-	/// <summary>
+    /// <summary>
     /// Singleton pattern implementation.
     /// Can be used with classes extended from a MonoBehaviour.
     /// Once instance is found or created, game object will be marked as DontDestroyOnLoadю
     /// </summary>
     public abstract class MonoSingleton<T> : MonoBehaviour where T : MonoBehaviour
-	{
-		static T s_Instance;
-		static bool s_ApplicationIsQuitting;
+    {
+        static T s_Instance;
+        static bool s_ApplicationIsQuitting;
+        static bool s_IsDestroyed;
+
 
         protected virtual void Awake()
         {
@@ -25,11 +27,13 @@ protected virtual void Awake()
         /// </summary>
         public static T Instance
         {
-			get
-			{
-				if (s_ApplicationIsQuitting)
-				{
-                    Debug.LogError($"{typeof(T)} [MonoSingleton] is already destroyed. Please check HasInstance before accessing instance in the destructor.");
+            get
+            {
+                if (s_ApplicationIsQuitting)
+                {
+                    Debug.LogError(
+                        $"{typeof(T)} [MonoSingleton] is already destroyed. " +
+                        $"Please check {nameof(HasInstance)} or {nameof(IsDestroyed)} before accessing instance in the destructor.");
                     return null;
                 }
 
@@ -39,9 +43,10 @@ public static T Instance
                     if (s_Instance == null)
                         Instantiate();
                 }
-				return s_Instance;
-			}
-		}
+
+                return s_Instance;
+            }
+        }
 
         /// <summary>
         /// Methods will create new object Instantiate
@@ -57,33 +62,36 @@ public static void Instantiate()
         }
 
         /// <summary>
-        /// True if Singleton Instance exists
+        /// Returns `true` if Singleton Instance exists.
         /// </summary>
-        public static bool HasInstance => !IsDestroyed;
+        public static bool HasInstance => s_Instance != null;
 
         /// <summary>
-        /// True if Singleton Instance doesn't exist
+        /// If this property returns `true` it means that object with explicitly destroyed.
+        /// This could happen if Destroy function  was called for this object or if it was
+        /// automatically destroyed during the `ApplicationQuit`.
         /// </summary>
-        public static bool IsDestroyed => s_Instance == null;
+        public static bool IsDestroyed => s_IsDestroyed;
 
         /// <summary>
-		/// When Unity quits, it destroys objects in a random order.
-		/// In principle, a Singleton is only destroyed when application quits.
-		/// If any script calls Instance after it have been destroyed,
-		/// it will create a buggy ghost object that will stay on the Editor scene
-		/// even after stopping playing the Application. Really bad!
-		/// So, this was made to be sure we're not creating that buggy ghost object.
-		/// </summary>
-		protected virtual void OnDestroy ()
+        /// When Unity quits, it destroys objects in a random order.
+        /// In principle, a Singleton is only destroyed when application quits.
+        /// If any script calls Instance after it have been destroyed,
+        /// it will create a buggy ghost object that will stay on the Editor scene
+        /// even after stopping playing the Application. Really bad!
+        /// So, this was made to be sure we're not creating that buggy ghost object.
+        /// </summary>
+        protected virtual void OnDestroy()
         {
-			s_Instance = null;
-			s_ApplicationIsQuitting = true;
-		}
+            s_Instance = null;
+            s_IsDestroyed = true;
+        }
 
-		protected virtual void OnApplicationQuit ()
-		{
-			s_Instance = null;
-			s_ApplicationIsQuitting = true;
-		}
-	}
-}
+        protected virtual void OnApplicationQuit()
+        {
+            s_Instance = null;
+            s_IsDestroyed = true;
+            s_ApplicationIsQuitting = true;
+        }
+    }
+}

Runtime/Patterns/Singleton/Singleton.cs

@@ -1,15 +1,15 @@
 namespace StansAssets.Foundation.Patterns
 {
-	/// <summary>
-	/// Singleton pattern implementation.
-	/// </summary>
-	public abstract class Singleton<T>  where T : Singleton<T>, new()
-	{
-		static T s_Instance;
+    /// <summary>
+    /// Singleton pattern implementation.
+    /// </summary>
+    public abstract class Singleton<T> where T : Singleton<T>, new()
+    {
+        static T s_Instance;
 
-		/// <summary>
-		/// Returns a singleton class instance.
-		/// </summary>
-		public static T Instance => s_Instance ?? (s_Instance = new T());
-	}
-}
+        /// <summary>
+        /// Returns a singleton class instance.
+        /// </summary>
+        public static T Instance => s_Instance ?? (s_Instance = new T());
+    }
+}