Hot (un)load extensions on (un)install (microsoft#375)

Closes microsoft#370 

The DevHome code was great for "I need something that can lookup extensions and enumerate all of them".

However, the DevHome code is a very blunt hammer when it comes to extensions. The only thing it tracks is "packages changed", and if it gets one of those, it just blows away all the extensions and rebuilds them. Yikes. 

This PR changes `ExtensionService` to be a scalpel. We'll keep `_installedExtensions` fresh. When we get a package install, we'll add only that package's extension to our cache, and let the `TopLevelCommandManager` know. Similarly for updates and uninstalls. 

That way, we can exactly change the top-level list as needed, rather than bluntly forcing all the extensions to reload. 

In the middle of all this, I fixed a bug where uninstalling an extension, then reloading would just fail to load extensions. This is because the old code would clear out the **whole** list of extensions when _one_ was uninstalled. That created a race where we'd be parsing the new list of all the extensions (from the reload), get an uninstall event, clear the list, then InvalidOperation as the list of extensions was modified during enumeration. 

There's a bunch more locking in here. This might drive-by microsoft#324 but hard to be sure. 

Related to microsoft#89

Author: zadjii-msft

src/modules/cmdpal/Microsoft.CmdPal.Common/Services/IExtensionService.cs

@@ -2,9 +2,9 @@
 // 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.Threading.Tasks;
+using Windows.Foundation;
 
 namespace Microsoft.CmdPal.Common.Services;
 
@@ -19,7 +19,9 @@ public interface IExtensionService
 
     Task SignalStopExtensionsAsync();
 
-    public event EventHandler OnExtensionsChanged;
+    public event TypedEventHandler<IExtensionService, IEnumerable<IExtensionWrapper>>? OnExtensionAdded;
+
+    public event TypedEventHandler<IExtensionService, IEnumerable<IExtensionWrapper>>? OnExtensionRemoved;
 
     public void EnableExtension(string extensionUniqueId);
 

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/CommandPaletteHost.cs

@@ -6,6 +6,7 @@
 using System.Diagnostics;
 using Microsoft.CmdPal.Common.Services;
 using Microsoft.CmdPal.Extensions;
+using Microsoft.CmdPal.Extensions.Helpers;
 using Windows.Foundation;
 
 namespace Microsoft.CmdPal.UI.ViewModels;
@@ -173,4 +174,11 @@ public void ProcessHideStatusMessage(IStatusMessage message)
     }
 
     public void SetHostHwnd(ulong hostHwnd) => HostingHwnd = hostHwnd;
+
+    public void DebugLog(string message)
+    {
+#if DEBUG
+        this.ProcessLogMessage(new LogMessage(message));
+#endif
+    }
 }

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/Commands/MainListPage.cs

@@ -2,7 +2,6 @@
 // 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.Collections.ObjectModel;
 using System.Collections.Specialized;
 using System.Diagnostics;
 using CommunityToolkit.Mvvm.Messaging;
@@ -23,8 +22,7 @@ public partial class MainListPage : DynamicListPage,
 {
     private readonly IServiceProvider _serviceProvider;
 
-    private readonly ObservableCollection<TopLevelCommandItemWrapper> _commands;
-
+    private readonly TopLevelCommandManager _tlcManager;
     private IEnumerable<IListItem>? _filteredItems;
 
     private bool _appsLoading = true;
@@ -36,12 +34,9 @@ public MainListPage(IServiceProvider serviceProvider)
         Icon = new(Path.Combine(AppDomain.CurrentDomain.BaseDirectory.ToString(), "Assets\\StoreLogo.scale-200.png"));
         _serviceProvider = serviceProvider;
 
-        var tlcManager = _serviceProvider.GetService<TopLevelCommandManager>()!;
-        tlcManager.PropertyChanged += TlcManager_PropertyChanged;
-
-        // reference the TLC collection directly... maybe? TODO is this a good idea ot a terrible one?
-        _commands = tlcManager.TopLevelCommands;
-        _commands.CollectionChanged += Commands_CollectionChanged;
+        _tlcManager = _serviceProvider.GetService<TopLevelCommandManager>()!;
+        _tlcManager.PropertyChanged += TlcManager_PropertyChanged;
+        _tlcManager.TopLevelCommands.CollectionChanged += Commands_CollectionChanged;
 
         WeakReferenceMessenger.Default.Register<ClearSearchMessage>(this);
 
@@ -60,7 +55,7 @@ private void TlcManager_PropertyChanged(object? sender, System.ComponentModel.Pr
         }
     }
 
-    private void Commands_CollectionChanged(object? sender, NotifyCollectionChangedEventArgs e) => RaiseItemsChanged(_commands.Count);
+    private void Commands_CollectionChanged(object? sender, NotifyCollectionChangedEventArgs e) => RaiseItemsChanged(_tlcManager.TopLevelCommands.Count);
 
     public override IListItem[] GetItems()
     {
@@ -70,9 +65,20 @@ public override IListItem[] GetItems()
             _startedAppLoad = true;
         }
 
-        return string.IsNullOrEmpty(SearchText)
-            ? _commands.Select(tlc => tlc).Where(tlc => !string.IsNullOrEmpty(tlc.Title)).ToArray()
-            : _filteredItems?.ToArray() ?? [];
+        if (string.IsNullOrEmpty(SearchText))
+        {
+            lock (_tlcManager.TopLevelCommands)
+            {
+                return _tlcManager.TopLevelCommands.Select(tlc => tlc).Where(tlc => !string.IsNullOrEmpty(tlc.Title)).ToArray();
+            }
+        }
+        else
+        {
+            lock (_tlcManager.TopLevelCommands)
+            {
+                return _filteredItems?.ToArray() ?? [];
+            }
+        }
     }
 
     public override void UpdateSearchText(string oldSearch, string newSearch)
@@ -89,40 +95,43 @@ public override void UpdateSearchText(string oldSearch, string newSearch)
             }
         }
 
-        // This gets called on a background thread, because ListViewModel
-        // updates the .SearchText of all extensions on a BG thread.
-        foreach (var command in _commands)
+        var commands = _tlcManager.TopLevelCommands;
+        lock (commands)
         {
-            command.TryUpdateFallbackText(newSearch);
-        }
+            // This gets called on a background thread, because ListViewModel
+            // updates the .SearchText of all extensions on a BG thread.
+            foreach (var command in commands)
+            {
+                command.TryUpdateFallbackText(newSearch);
+            }
 
-        // Cleared out the filter text? easy. Reset _filteredItems, and bail out.
-        if (string.IsNullOrEmpty(newSearch))
-        {
-            _filteredItems = null;
-            RaiseItemsChanged(_commands.Count);
-            return;
-        }
+            // Cleared out the filter text? easy. Reset _filteredItems, and bail out.
+            if (string.IsNullOrEmpty(newSearch))
+            {
+                _filteredItems = null;
+                RaiseItemsChanged(commands.Count);
+                return;
+            }
 
-        // If the new string doesn't start with the old string, then we can't
-        // re-use previous results. Reset _filteredItems, and keep er moving.
-        if (!newSearch.StartsWith(oldSearch, StringComparison.CurrentCultureIgnoreCase))
-        {
-            _filteredItems = null;
-        }
+            // If the new string doesn't start with the old string, then we can't
+            // re-use previous results. Reset _filteredItems, and keep er moving.
+            if (!newSearch.StartsWith(oldSearch, StringComparison.CurrentCultureIgnoreCase))
+            {
+                _filteredItems = null;
+            }
 
-        // If we don't have any previous filter results to work with, start
-        // with a list of all our commands & apps.
-        if (_filteredItems == null)
-        {
-            IEnumerable<IListItem> commands = _commands;
-            IEnumerable<IListItem> apps = AllAppsCommandProvider.Page.GetItems();
-            _filteredItems = commands.Concat(apps);
-        }
+            // If we don't have any previous filter results to work with, start
+            // with a list of all our commands & apps.
+            if (_filteredItems == null)
+            {
+                IEnumerable<IListItem> apps = AllAppsCommandProvider.Page.GetItems();
+                _filteredItems = commands.Concat(apps);
+            }
 
-        // Produce a list of everything that matches the current filter.
-        _filteredItems = ListHelpers.FilterList<IListItem>(_filteredItems, SearchText, ScoreTopLevelItem);
-        RaiseItemsChanged(_filteredItems.Count());
+            // Produce a list of everything that matches the current filter.
+            _filteredItems = ListHelpers.FilterList<IListItem>(_filteredItems, SearchText, ScoreTopLevelItem);
+            RaiseItemsChanged(_filteredItems.Count());
+        }
     }
 
     private bool ActuallyLoading()