add StartupSingleton

Assets/Scripts/Utilities/StartupSingleton.cs

@@ -0,0 +1,39 @@
+using System;
+using UnityEngine;
+
+namespace SanAndreasUnity.Utilities
+{
+    public class StartupSingleton<T> : MonoBehaviour
+        where T : StartupSingleton<T>
+    {
+        public static T Singleton { get; private set; }
+
+        private void Awake()
+        {
+            if (Singleton != null)
+            {
+                throw new Exception($"Awake() method called twice for singleton of type {this.GetType().Name}");
+            }
+
+            Singleton = (T) this;
+
+            this.OnSingletonAwake();
+        }
+
+        protected virtual void OnSingletonAwake()
+        {
+        }
+
+        private void Start()
+        {
+            if (this != Singleton)
+                return;
+
+            this.OnSingletonStart();
+        }
+
+        protected virtual void OnSingletonStart()
+        {
+        }
+    }
+}

Assets/Scripts/Utilities/StartupSingleton.cs.meta

@@ -0,0 +1,11 @@
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+guid: 43e2ad434a43a4a4f8369aee19fed4b4
+MonoImporter:
+  externalObjects: {}
+  serializedVersion: 2
+  defaultReferences: []
+  executionOrder: 0
+  icon: {instanceID: 0}
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: