Added singleton pattern

Author: Habrador

Assets/Patterns/5. Singleton.meta

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+using System.Collections;
+using System.Collections.Generic;
+using UnityEngine;
+
+namespace SingletonPattern
+{
+    public class GameController : MonoBehaviour
+    {
+
+        void Start()
+        {
+            TestCSharpSingleton();
+
+            //TestUnitySingleton();
+        }
+
+
+        void Update()
+        {
+
+        }
+
+
+
+        private void TestCSharpSingleton()
+        {
+            //Is not working because this is a singleton with a private constructor
+            //SingletonCSharp singletonCSharp = new SingletonCSharp();
+
+            //This is working
+            SingletonCSharp instance = SingletonCSharp.Instance;
+
+            instance.TestSingleton();
+
+            SingletonCSharp instance2 = SingletonCSharp.Instance;
+
+            instance2.TestSingleton();
+        }
+
+
+
+        private void TestUnitySingleton()
+        {
+            SingletonUnity instance = SingletonUnity.Instance;
+
+            instance.TestSingleton();
+
+            SingletonUnity instance2 = SingletonUnity.Instance;
+
+            instance2.TestSingleton();
+        }
+    }
+}

Assets/Patterns/5. Singleton/Scripts.meta

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+using System.Collections;
+using System.Collections.Generic;
+using UnityEngine;
+
+//
+// Basic Singleton pattern implementation in C#
+//
+
+namespace SingletonPattern
+{
+    //This is the most simplest Singleton pattern. The problem is that it's not thread safe. If you want a thread safe Singleton, then visit the link: https://csharpindepth.com/articles/singleton
+    public class SingletonCSharp
+    {
+        //A static variable which holds a reference to the single created instance
+        private static SingletonCSharp instance = null;
+
+
+
+        //For testing that we only call the constructor once
+        private float randomNumber;
+
+
+
+        //A public static means of getting the reference to the single created instance, creating one if necessary
+        public static SingletonCSharp Instance
+        {
+            get
+            {
+                if (instance == null)
+                {
+                    instance = new SingletonCSharp();
+                }
+
+                return instance;
+            }
+        }
+
+
+
+        //A single constructor, which is private and parameterless (singletons are not allowed to have parameters)
+        //This prevents other classes from instantiating it and it also prevents subclassing (which both are violating the pattern)
+        //But some argue that you should be able to inherit from singletons...
+        private SingletonCSharp()
+        {
+            randomNumber = Random.Range(0f, 1f);
+        }
+
+
+
+        //For testing
+        public void TestSingleton()
+        {
+            Debug.Log($"Hello this is Singleton, my random number is: {randomNumber}");
+        }
+    }
+}

Assets/Patterns/5. Singleton/Scripts/GameController.cs

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+using System.Collections;
+using System.Collections.Generic;
+using UnityEngine;
+
+//
+// Basic Singleton pattern implementation in Unity
+//
+
+namespace SingletonPattern
+{
+    //A translation of the most basic Singleton pattern to Unity script which inherits from MonoBehaviour
+    public class SingletonUnity : MonoBehaviour
+    {
+        //A static variable which holds a reference to the single created instance
+        private static SingletonUnity instance = null;
+
+
+
+        //For testing that we only call the constructor once
+        private float randomNumber;
+
+
+
+        //A public static means of getting the reference to the single created instance, creating one if necessary
+        public static SingletonUnity Instance
+        {
+            get
+            {
+                if (instance == null)
+                {
+                    //If a script in Unity inherits from MonoBehaviour, we can't use the new keyword to create a new Singleton as we did before
+                    //So you have to manually add this script to a gameobject in the scene
+                    //But because we inherit from MonoBehaviour whem might have accidentally added several of them to the scene, which will cause trouble, so we have to make sure we have just one!
+
+                    //Find all singletons of this type in the scene
+                    SingletonUnity[] allSingletonsInScene = GameObject.FindObjectsOfType<SingletonUnity>();
+
+                    if (allSingletonsInScene != null && allSingletonsInScene.Length > 0)
+                    {
+                        //Destroy all but one singleton
+                        if (allSingletonsInScene.Length > 1)
+                        {
+                            Debug.LogWarning($"You have more than one SingletonUnity in the scene!");
+
+                            for (int i = 1; i < allSingletonsInScene.Length; i++)
+                            {
+                                Destroy(allSingletonsInScene[i].gameObject);
+                            }
+                        }
+
+                        //Now we should have just one singleton in the scene, so pick it
+                        instance = allSingletonsInScene[0];
+
+                        //Init the singleton
+                        instance.FakeConstructor();
+                    }
+                    //We have no singletons in the scene
+                    else
+                    {
+                        Debug.LogError($"You need to add the script SingletonUnity to a gameobject in the scene!");
+                    }
+                }
+
+                return instance;
+            }
+        }
+
+
+
+        //Because this script inherits from MonoBehaviour, we cant use a constructor, so we have to invent our own
+        private void FakeConstructor()
+        {
+            randomNumber = Random.Range(0f, 1f);
+        }
+
+
+
+        //For testing
+        public void TestSingleton()
+        {
+            Debug.Log($"Hello this is Singleton, my random number is: {randomNumber}");
+        }
+    }
+}