Pilfer TypedEventHandlerExtensions from WCT (#404)

I think this pilfers the `InvokeAsync` as mentioned in #333

WE can't use the actual version for the same reason we can't use that one TryEnqueue extension - the WCT MD Text block we're on uses a different version of the WCT than the latest version, so the two get confused. 

We can remove this pilfered copy after either #245 or #219

Closes #333

.github/actions/spell-check/expect.txt

@@ -1709,6 +1709,7 @@ TOPDOWNDIB
 TOUCHEVENTF
 TOUCHINPUT
 TRACEHANDLE
+TResult
 tracelogging
 tracerpt
 trackbar
@@ -1720,6 +1721,7 @@ triaging
 trl
 trx
 tsa
+TSender
 TServer
 tstoi
 TStr

src/modules/cmdpal/Microsoft.CmdPal.UI/Controls/IconBox.cs

@@ -2,6 +2,7 @@
 // The Microsoft Corporation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using Microsoft.CmdPal.UI.Deferred;
 using Microsoft.CmdPal.UI.ViewModels;
 using Microsoft.UI.Dispatching;
 using Microsoft.UI.Xaml;
@@ -90,14 +91,14 @@ public partial class IconBox : ContentControl
             {
                 // TODO GH #239 switch back when using the new MD text block
                 // _ = @this._queue.EnqueueAsync(() =>
-                @this._queue.TryEnqueue(new(() =>
+                @this._queue.TryEnqueue(new(async () =>
                 {
                     var requestedTheme = @this.ActualTheme;
                     var eventArgs = new SourceRequestedEventArgs(e.NewValue, requestedTheme);
 
                     if (@this.SourceRequested != null)
                     {
-                        @this.SourceRequested.Invoke(@this, eventArgs);
+                        await @this.SourceRequested.InvokeAsync(@this, eventArgs);
 
                         @this.Source = eventArgs.Value;
 
@@ -131,7 +132,7 @@ public partial class IconBox : ContentControl
                                 // The range of MDL2 Icons isn't explicitly defined, but
                                 // we're using this based off the table on:
                                 // https://docs.microsoft.com/en-us/windows/uwp/design/style/segoe-ui-symbol-font
-                                var isMDL2Icon = ch >= '\uE700' && ch <= '\uF8FF';
+                                var isMDL2Icon = ch is >= '\uE700' and <= '\uF8FF';
                                 if (!isMDL2Icon)
                                 {
                                     @this.Padding = new Thickness(-4);

src/modules/cmdpal/Microsoft.CmdPal.UI/Helpers/TypedEventHandlerExtensions.cs

@@ -0,0 +1,72 @@
+// Copyright (c) Microsoft Corporation
+// The Microsoft Corporation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using CommunityToolkit.Common.Deferred;
+using Windows.Foundation;
+
+// Pilfered from CommunityToolkit.WinUI.Deferred
+namespace Microsoft.CmdPal.UI.Deferred;
+
+/// <summary>
+/// Extensions to <see cref="TypedEventHandler{TSender, TResult}"/> for Deferred Events.
+/// </summary>
+public static class TypedEventHandlerExtensions
+{
+    /// <summary>
+    /// Use to invoke an async <see cref="TypedEventHandler{TSender, TResult}"/> using <see cref="DeferredEventArgs"/>.
+    /// </summary>
+    /// <typeparam name="S">Type of sender.</typeparam>
+    /// <typeparam name="R"><see cref="EventArgs"/> type.</typeparam>
+    /// <param name="eventHandler"><see cref="TypedEventHandler{TSender, TResult}"/> to be invoked.</param>
+    /// <param name="sender">Sender of the event.</param>
+    /// <param name="eventArgs"><see cref="EventArgs"/> instance.</param>
+    /// <returns><see cref="Task"/> to wait on deferred event handler.</returns>
+#pragma warning disable CA1715 // Identifiers should have correct prefix
+#pragma warning disable SA1314 // Type parameter names should begin with T
+    public static Task InvokeAsync<S, R>(this TypedEventHandler<S, R> eventHandler, S sender, R eventArgs)
+#pragma warning restore SA1314 // Type parameter names should begin with T
+#pragma warning restore CA1715 // Identifiers should have correct prefix
+        where R : DeferredEventArgs => InvokeAsync(eventHandler, sender, eventArgs, CancellationToken.None);
+
+    /// <summary>
+    /// Use to invoke an async <see cref="TypedEventHandler{TSender, TResult}"/> using <see cref="DeferredEventArgs"/> with a <see cref="CancellationToken"/>.
+    /// </summary>
+    /// <typeparam name="S">Type of sender.</typeparam>
+    /// <typeparam name="R"><see cref="EventArgs"/> type.</typeparam>
+    /// <param name="eventHandler"><see cref="TypedEventHandler{TSender, TResult}"/> to be invoked.</param>
+    /// <param name="sender">Sender of the event.</param>
+    /// <param name="eventArgs"><see cref="EventArgs"/> instance.</param>
+    /// <param name="cancellationToken"><see cref="CancellationToken"/> option.</param>
+    /// <returns><see cref="Task"/> to wait on deferred event handler.</returns>
+#pragma warning disable CA1715 // Identifiers should have correct prefix
+#pragma warning disable SA1314 // Type parameter names should begin with T
+    public static Task InvokeAsync<S, R>(this TypedEventHandler<S, R> eventHandler, S sender, R eventArgs, CancellationToken cancellationToken)
+#pragma warning restore SA1314 // Type parameter names should begin with T
+#pragma warning restore CA1715 // Identifiers should have correct prefix
+        where R : DeferredEventArgs
+    {
+        if (eventHandler == null)
+        {
+            return Task.CompletedTask;
+        }
+
+        var tasks = eventHandler.GetInvocationList()
+            .OfType<TypedEventHandler<S, R>>()
+            .Select(invocationDelegate =>
+            {
+                cancellationToken.ThrowIfCancellationRequested();
+
+                invocationDelegate(sender, eventArgs);
+
+#pragma warning disable CS0618 // Type or member is obsolete
+                var deferral = eventArgs.GetCurrentDeferralAndReset();
+
+                return deferral?.WaitForCompletion(cancellationToken) ?? Task.CompletedTask;
+#pragma warning restore CS0618 // Type or member is obsolete
+            })
+            .ToArray();
+
+        return Task.WhenAll(tasks);
+    }
+}