Added iOS UI dispatcher

Author: jamesmcroft

MADE.App.Views/Threading/UIDispatcher.iOS.cs

@@ -0,0 +1,79 @@
+// --------------------------------------------------------------------------------------------------------------------
+// <copyright file="UIDispatcher.iOS.cs" company="MADE Apps">
+//   Copyright (c) MADE Apps.
+// </copyright>
+// <summary>
+//   Defines a dispatcher that performs actions on the UI thread.
+// </summary>
+// --------------------------------------------------------------------------------------------------------------------
+
+#if __IOS__
+namespace MADE.App.Views.Threading
+{
+    using System;
+    using System.Threading.Tasks;
+
+    using Foundation;
+
+    /// <summary>
+    /// Defines a dispatcher that performs actions on the UI thread.
+    /// </summary>
+    public class UIDispatcher : IUIDispatcher
+    {
+        /// <summary>
+        /// Gets the platform specified UI object to use as a reference.
+        /// </summary>
+        public object Reference { get; private set; }
+
+        private NSObject NSObject => this.Reference as NSObject;
+
+        /// <summary>
+        /// Sets the <see cref="Foundation.NSObject"/> to use as a reference for dispatching actions on the UI thread.
+        /// </summary>
+        /// <param name="reference">
+        /// The <see cref="Foundation.NSObject"/> object.
+        /// </param>
+        public void SetReference(object reference)
+        {
+            this.Reference = null;
+
+            if (!(reference is NSObject activity))
+            {
+                return;
+            }
+
+            this.Reference = activity;
+        }
+
+        /// <summary>
+        /// Schedules the provided action on the UI thread from a worker thread.
+        /// </summary>
+        /// <param name="action">
+        /// The action to run on the dispatcher.
+        /// </param>
+        /// <returns>
+        /// An asynchronous operation.
+        /// </returns>
+        public async Task RunAsync(Action action)
+        {
+            UIDispatcher dispatcher = this;
+            dispatcher.CheckInitialized();
+            if (action == null)
+            {
+                return;
+            }
+
+            await Task.Run(() => { this.NSObject?.InvokeOnMainThread(action); });
+        }
+
+        private void CheckInitialized()
+        {
+            if (this.Reference == null)
+            {
+                throw new InvalidOperationException(
+                    "Cannot run an action as the NSObject has not be set as the reference in the SetReference method.");
+            }
+        }
+    }
+}
+#endif