From 048e0a22f712beb12e5832b9e2dc273c2ba98d5f Mon Sep 17 00:00:00 2001
From: Oliver Booth <me@olivr.me>
Date: Fri, 13 May 2022 08:54:00 +0100
Subject: [PATCH] Add Singleton<T> class

---
 CHANGELOG.md                                  |  1 +
 .../Assets/Tests/SingletonTests.cs            | 45 +++++++++++++++++++
 .../Assets/Tests/SingletonTests.cs.meta       |  3 ++
 .../Assets/Tests/TestBehaviour.cs             | 10 +++++
 .../Assets/Tests/TestBehaviour.cs.meta        |  3 ++
 X10D.Unity/src/Singleton.cs                   | 42 +++++++++++++++++
 6 files changed, 104 insertions(+)
 create mode 100644 X10D.Unity.Tests/Assets/Tests/SingletonTests.cs
 create mode 100644 X10D.Unity.Tests/Assets/Tests/SingletonTests.cs.meta
 create mode 100644 X10D.Unity.Tests/Assets/Tests/TestBehaviour.cs
 create mode 100644 X10D.Unity.Tests/Assets/Tests/TestBehaviour.cs.meta
 create mode 100644 X10D.Unity/src/Singleton.cs

diff --git a/CHANGELOG.md b/CHANGELOG.md
index ac2db5b..3f6acc9 100644
--- a/CHANGELOG.md
+++ b/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()`
diff --git a/X10D.Unity.Tests/Assets/Tests/SingletonTests.cs b/X10D.Unity.Tests/Assets/Tests/SingletonTests.cs
new file mode 100644
index 0000000..9bf7a24
--- /dev/null
+++ b/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);
+        }
+    }
+}
diff --git a/X10D.Unity.Tests/Assets/Tests/SingletonTests.cs.meta b/X10D.Unity.Tests/Assets/Tests/SingletonTests.cs.meta
new file mode 100644
index 0000000..3f086ce
--- /dev/null
+++ b/X10D.Unity.Tests/Assets/Tests/SingletonTests.cs.meta
@@ -0,0 +1,3 @@
+fileFormatVersion: 2
+guid: 5c51198d124f40859bd9298d3241d5a6
+timeCreated: 1652428949
\ No newline at end of file
diff --git a/X10D.Unity.Tests/Assets/Tests/TestBehaviour.cs b/X10D.Unity.Tests/Assets/Tests/TestBehaviour.cs
new file mode 100644
index 0000000..a603e1d
--- /dev/null
+++ b/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;
+        }
+    }
+}
diff --git a/X10D.Unity.Tests/Assets/Tests/TestBehaviour.cs.meta b/X10D.Unity.Tests/Assets/Tests/TestBehaviour.cs.meta
new file mode 100644
index 0000000..bc63104
--- /dev/null
+++ b/X10D.Unity.Tests/Assets/Tests/TestBehaviour.cs.meta
@@ -0,0 +1,3 @@
+fileFormatVersion: 2
+guid: df932718d20948ecbe9b0de8aa7bbaf4
+timeCreated: 1652428898
\ No newline at end of file
diff --git a/X10D.Unity/src/Singleton.cs b/X10D.Unity/src/Singleton.cs
new file mode 100644
index 0000000..8e506db
--- /dev/null
+++ b/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>();
+    }
+}