Add Singleton<T> class

CHANGELOG.md

@@ -28,6 +28,7 @@
 - X10D: Added `Vector4.WithY()`
 - X10D: Added `Vector4.WithZ()`
 - X10D: Added `Vector4.WithW()`
+- X10D.Unity: Added `Singleton<T>` class
 - X10D.Unity: Added `Color.Inverted()`
 - X10D.Unity: Added `Color.WithA()`
 - X10D.Unity: Added `Color.WithB()`

X10D.Unity.Tests/Assets/Tests/SingletonTests.cs

@@ -0,0 +1,45 @@
+using System.Collections;
+using NUnit.Framework;
+using UnityEngine;
+using UnityEngine.TestTools;
+
+namespace X10D.Unity.Tests
+{
+    public class SingletonTests
+    {
+        [UnityTest]
+        public IEnumerator Singleton_ShouldReturnNewInstance_WhenNoInstanceExists()
+        {
+            TestBehaviour instance = Singleton<TestBehaviour>.Instance;
+            Assert.IsTrue(instance);
+            Assert.IsTrue(instance.Flag);
+
+            yield break;
+        }
+
+        [UnityTest]
+        public IEnumerator Singleton_ShouldReturnSameInstance_WhenAccessedTwice()
+        {
+            TestBehaviour instance = Singleton<TestBehaviour>.Instance;
+            Assert.IsTrue(instance);
+            Assert.AreEqual(instance, Singleton<TestBehaviour>.Instance);
+
+            yield break;
+        }
+
+        [UnityTest]
+        public IEnumerator Singleton_ShouldReturnNewInstance_WhenDestroyed()
+        {
+            TestBehaviour instance = Singleton<TestBehaviour>.Instance;
+            Assert.IsTrue(instance);
+            Object.Destroy(instance);
+            yield return null;
+            Assert.IsFalse(instance);
+
+            // ReSharper disable once HeuristicUnreachableCode
+            instance = Singleton<TestBehaviour>.Instance;
+            Assert.IsNotNull(instance);
+            Assert.IsTrue(instance.Flag);
+        }
+    }
+}

X10D.Unity.Tests/Assets/Tests/SingletonTests.cs.meta

@@ -0,0 +1,3 @@
+fileFormatVersion: 2
+guid: 5c51198d124f40859bd9298d3241d5a6
+timeCreated: 1652428949

X10D.Unity.Tests/Assets/Tests/TestBehaviour.cs

@@ -0,0 +1,10 @@
+namespace X10D.Unity.Tests
+{
+    internal sealed class TestBehaviour : Singleton<TestBehaviour>
+    {
+        public bool Flag
+        {
+            get => true;
+        }
+    }
+}

X10D.Unity.Tests/Assets/Tests/TestBehaviour.cs.meta

@@ -0,0 +1,3 @@
+fileFormatVersion: 2
+guid: df932718d20948ecbe9b0de8aa7bbaf4
+timeCreated: 1652428898

X10D.Unity/src/Singleton.cs

@@ -0,0 +1,42 @@
+using UnityEngine;
+
+namespace X10D.Unity;
+
+/// <summary>
+///     Represents a class which implements the singleton pattern for a specific <see cref="MonoBehaviour" />. This class is not
+///     thread-safe.
+/// </summary>
+/// <typeparam name="T">The type of the singleton.</typeparam>
+public abstract class Singleton<T> : MonoBehaviour
+    where T : Singleton<T>
+{
+    private static Lazy<T> s_instanceLazy = new(CreateLazyInstanceInternal, false);
+
+    /// <summary>
+    ///     Gets the instance of the singleton.
+    /// </summary>
+    /// <value>The singleton instance.</value>
+    public static T Instance
+    {
+        get => s_instanceLazy.Value;
+    }
+
+    /// <summary>
+    ///     Called when the object is destroyed.
+    /// </summary>
+    protected virtual void OnDestroy()
+    {
+        s_instanceLazy = new Lazy<T>(CreateLazyInstanceInternal, false);
+    }
+
+    private static T CreateLazyInstanceInternal()
+    {
+        if (FindObjectOfType<T>() is { } instance)
+        {
+            return instance;
+        }
+
+        var gameObject = new GameObject {name = typeof(T).Name};
+        return gameObject.AddComponent<T>();
+    }
+}