Add singleton for detecting screen size changes

Author: adamgraham

Runtime/ScreenSizeListener.cs

@@ -0,0 +1,114 @@
+using UnityEngine;
+
+namespace Zigurous.UI
+{
+    /// <summary>
+    /// Listens for changes in the screen size.
+    /// </summary>
+    public sealed class ScreenSizeListener : MonoBehaviour
+    {
+        private static volatile ScreenSizeListener _instance;
+        private static object _lock = new object();
+        private static bool _isUnloading = false;
+
+        /// <summary>
+        /// The current instance of the class.
+        /// The instance will be created if it does not already exist.
+        /// </summary>
+        public static ScreenSizeListener Instance
+        {
+            get
+            {
+                if (_isUnloading) {
+                    return null;
+                }
+
+                if (_instance == null)
+                {
+                    lock (_lock)
+                    {
+                        _instance = FindObjectOfType<ScreenSizeListener>();
+
+                        if (_instance == null)
+                        {
+                            GameObject singleton = new GameObject();
+                            singleton.name = typeof(ScreenSizeListener).Name;
+                            singleton.hideFlags = HideFlags.HideInHierarchy | HideFlags.HideInInspector;
+                            singleton.AddComponent<ScreenSizeListener>();
+                            DontDestroyOnLoad(singleton);
+                        }
+                    }
+                }
+
+                return _instance;
+            }
+        }
+
+        /// <summary>
+        /// Checks if the singleton has been initialized and an instance is
+        /// available to use.
+        /// </summary>
+        public static bool HasInstance => _instance != null;
+
+        /// <summary>
+        /// A function delegate invoked when the screen size changes.
+        /// </summary>
+        /// <param name="width">The new width of the screen.</param>
+        /// <param name="height">The new height of the screen.</param>
+        public delegate void OnResize(int width, int height);
+
+        /// <summary>
+        /// The callback invoked when the screen size changes.
+        /// </summary>
+        public OnResize resized;
+
+        /// <summary>
+        /// The current width of the screen.
+        /// </summary>
+        public int width { get; private set; }
+
+        /// <summary>
+        /// The current height of the screen.
+        /// </summary>
+        public int height { get; private set; }
+
+        private ScreenSizeListener() {}
+
+        private void Awake()
+        {
+            _isUnloading = false;
+
+            if (_instance == null) {
+                _instance = this;
+            } else {
+                Destroy(this);
+            }
+        }
+
+        private void OnDestroy()
+        {
+            this.resized = null;
+
+            _isUnloading = true;
+
+            if (_instance == this) {
+                _instance = null;
+            }
+        }
+
+        private void Update()
+        {
+            if (Screen.width != this.width || Screen.height != this.height)
+            {
+                this.width = Screen.width;
+                this.height = Screen.height;
+
+                if (this.resized != null) {
+                    this.resized.Invoke(this.width, this.height);
+                }
+            }
+        }
+
+    }
+
+}