cmdpal: A sample for tracking load/unload for pages (#39265)

Committing this sample for posterity's sake.

This is the product of an experiment: "can an extension know when it's
page was opened / closed?"

And without changing the actual SDK, this **is** possible. It relies on
the fact that CmdPal (the host) needs to register for the `ItemsChanged`
event on list pages, and it does that when the page is loaded, and it
unregisters itself when the page is closed.

This does require manually implementing `IListPage` - you can't just
`override` the `event` in the base class ([that's
illegal](https://learn.microsoft.com/en-us/dotnet/fundamentals/code-analysis/quality-rules/ca1070)),
and using `new` to shadow it doesn't work either (probably for cswinrt
reasons).

This is the best I came up with.

src/modules/cmdpal/ext/SamplePagesExtension/OnLoadPage.cs

@@ -0,0 +1,82 @@
+// 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 System;
+using System.Collections.Generic;
+using System.Globalization;
+using Microsoft.CommandPalette.Extensions;
+using Microsoft.CommandPalette.Extensions.Toolkit;
+using Windows.Foundation;
+
+namespace SamplePagesExtension;
+
+internal sealed partial class OnLoadPage : IListPage
+{
+    private readonly List<ListItem> _items = new();
+
+    public IIconInfo Icon => new IconInfo("\uE8AB"); // switch
+
+    public string Title => "Load/Unload sample";
+
+    public string PlaceholderText => "This page changes each time you load it";
+
+    public ICommandItem EmptyContent => new CommandItem() { Icon = new IconInfo("\uE8AB"), Title = "This page starts empty", Subtitle = "but go back and open it again" };
+
+    public IFilters Filters => null;
+
+    public IGridProperties GridProperties => null;
+
+    public bool HasMoreItems => false;
+
+    public string SearchText => string.Empty;
+
+    public bool ShowDetails => false;
+
+    public OptionalColor AccentColor => default;
+
+    public bool IsLoading => false;
+
+    public string Id => string.Empty;
+
+    public string Name => "Open";
+
+#pragma warning disable CS0067 // The event is never used
+    public event TypedEventHandler<object, IPropChangedEventArgs> PropChanged;
+
+    private event TypedEventHandler<object, IItemsChangedEventArgs> InternalItemsChanged;
+#pragma warning restore CS0067 // The event is never used
+
+    public event TypedEventHandler<object, IItemsChangedEventArgs> ItemsChanged
+    {
+        add
+        {
+            InternalItemsChanged += value;
+            var nowString = DateTime.Now.ToString("T", CultureInfo.CurrentCulture);
+            var item = new ListItem(new NoOpCommand())
+            {
+                Title = $"Loaded {nowString}",
+                Icon = new IconInfo("\uECCB"), // Radio button on
+            };
+            _items.Add(item);
+        }
+
+        remove
+        {
+            InternalItemsChanged -= value;
+            var nowString = DateTime.Now.ToString("T", CultureInfo.CurrentCulture);
+            var item = new ListItem(new NoOpCommand())
+            {
+                Title = $"Unloaded {nowString}",
+                Icon = new IconInfo("\uECCA"), // Radio button off
+            };
+            _items.Add(item);
+        }
+    }
+
+    public IListItem[] GetItems() => _items.ToArray();
+
+    public void LoadMore()
+    {
+    }
+}

src/modules/cmdpal/ext/SamplePagesExtension/SamplesListPage.cs

@@ -32,6 +32,11 @@ public partial class SamplesListPage : ListPage
             Title = "Dynamic List Page Command",
             Subtitle = "Changes the list of items in response to the typed query",
         },
+        new ListItem(new OnLoadPage())
+        {
+            Title = "Demo of OnLoad/OnUnload",
+            Subtitle = "Changes the list of items every time the page is opened / closed",
+        },
 
         // Content pages
         new ListItem(new SampleContentPage())