PostIt: route VM navigation through ViewLocator

src/PostIt.Tests/MainPageButtonsTests.cs

@@ -5,6 +5,7 @@ using Avalonia.Headless.XUnit;
 using Avalonia.Input;
 using Avalonia.Interactivity;
 using CommunityToolkit.Mvvm.Input;
+using Microsoft.Extensions.DependencyInjection;
 using Yavsc.Api.Client;
 using Yavsc.Blogspot;
 using PostIt.Services;
@@ -78,7 +79,18 @@ public class MainPageButtonsTests
     {
         var api = new ThrowingApi();
         var blog = new BlogApiClient(api, "http://localhost/");
-        var vm = new MainPageViewModel(blog);
+        // Minimal DI graph: only what MainPageViewModel resolves
+        // when the user clicks a navigation button. Today that's
+        // SignaturePageViewModel (for the [DEV] Signature toolbar
+        // shortcut). Anything the SignaturePage or its VM touch
+        // transitively must be registered here too — the test
+        // refuses to share App.BuildServices() because that one
+        // constructs a real YavscApiClient pointing at the host's
+        // token store, which is exactly the noise we want out of
+        // a UI-driving test.
+        var services = new ServiceCollection();
+        services.AddTransient<SignaturePageViewModel>();
+        var vm = new MainPageViewModel(blog, services: services.BuildServiceProvider());
         if (selectedPost is not null) vm.SelectedPost = selectedPost;
         return vm;
     }
@@ -183,16 +195,20 @@ public class MainPageButtonsTests
     [AvaloniaFact]
     public void Signature_dev_button_click_pushes_a_page_onto_nav_stack()
     {
-        // Arrange: the "[DEV] Signature" button uses XAML's
-        // Click="OpenSignatureDev" attribute, so we don't bind
-        // a Command here — we drive the click directly. The
-        // handler resolves App.ServiceProvider, which is null
-        // in a unit test, and early-returns; that is the
-        // failure mode the test pins.
+        // Arrange: the "[DEV] Signature" button is bound to the
+        // MainPageViewModel.OpenSignatureDevCommand [RelayCommand].
+        // The click must push SignaturePage on top of NavRoot.
+        // The ServiceCollection registered in MakeViewModel provides
+        // SignaturePageViewModel so the command can resolve it via
+        // DI and assign it to CurrentViewModel; the ViewLocator
+        // then maps SignaturePageViewModel -> SignaturePage and
+        // the binding pushes the page.
         var vm = MakeViewModel();
         var (window, page) = MountMainPage(vm);
 
         var signatureButton = page.OpenSignatureDevButton;
+        Assert.NotNull(signatureButton.Command);
+        Assert.True(signatureButton.Command.CanExecute(null));
 
         // Act
         var stackBefore = ClickAndCapture(window, signatureButton);

src/PostIt/PostIt/App.axaml

@@ -2,10 +2,8 @@
              xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
              xmlns:local="using:PostIt"
              x:Class="PostIt.App">
-          
-    <Application.DataTemplates>
-        <local:ViewLocator/>
-    </Application.DataTemplates>    
+
+    <!-- ViewLocator is registered in App.axaml.cs with the real DI container. -->
 
     <Application.Styles>
         <FluentTheme />

src/PostIt/PostIt/App.axaml.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Linq;
 using System.Threading.Tasks;
 using Microsoft.Extensions.DependencyInjection;
 using Avalonia;
@@ -48,11 +49,11 @@ public partial class App : Application
         // build is ever reconfigured to skip the early check.
         if (TryHandOffCustomSchemeUrl()) return;
 
-        var serviceProvider = BuildServices();
-        AttachServiceProvider(serviceProvider);
-        var settings = serviceProvider.GetRequiredService<Settings>();
-        var sessionStatus = serviceProvider.GetRequiredService<SessionStatusViewModel>();
-        var api = serviceProvider.GetRequiredService<YavscApiClient>();
+        this.ServiceProvider = BuildServices();
+        AttachServiceProvider(ServiceProvider);
+        var settings = ServiceProvider.GetRequiredService<Settings>();
+        var sessionStatus = ServiceProvider.GetRequiredService<SessionStatusViewModel>();
+        var api = ServiceProvider.GetRequiredService<YavscApiClient>();
 
         DataTemplates.Clear();
         DataTemplates.Add(new ViewLocator(ServiceProvider));
@@ -148,7 +149,7 @@ public partial class App : Application
                 _ = w.NavRoot.PushAsync(settingsPage);
             };
 
-            window.Opened += async (_, _) => await BootAsync(ServiceProvider, api);
+            window.Opened += async (_, _) => await BootAsync(this.ServiceProvider, api);
         }
         else if (ApplicationLifetime is ISingleViewApplicationLifetime singleView)
         {
@@ -320,4 +321,41 @@ public partial class App : Application
         return true;
     }
 
+    internal void PushPage(ViewModelBase vm)
+    {
+        if (window is null)
+        {
+            throw new InvalidOperationException("MainWindow is not initialized yet.");
+        }
+
+        var template = DataTemplates.FirstOrDefault(t => t.Match(vm));
+        if (template is null)
+        {
+            throw new InvalidOperationException($"No IDataTemplate found for {vm.GetType().Name}.");
+        }
+
+        var view = template.Build(vm);
+        if (view is null)
+        {
+            throw new InvalidOperationException(
+                $"Template for {vm.GetType().Name} returned <null>.");
+        }
+
+        if (view is not Page page)
+        {
+            throw new InvalidOperationException(
+                $"Template for {vm.GetType().Name} returned {view.GetType().Name}, expected a Page.");
+        }
+
+        page.DataContext = vm;
+
+        // Avoid stacking the same singleton page twice (e.g. SettingsPage).
+        var stack = window.NavRoot.NavigationStack;
+        if (stack.Count > 0 && ReferenceEquals(stack[stack.Count - 1], page))
+        {
+            return;
+        }
+
+        _ = window.NavRoot.PushAsync(page);
+    }
 }

src/PostIt/PostIt/ViewModels/MainPageViewModel.cs

@@ -2,11 +2,14 @@ using System;
 using System.Collections.ObjectModel;
 using System.Linq;
 using System.Threading.Tasks;
+using Avalonia;
 using CommunityToolkit.Mvvm.ComponentModel;
 using CommunityToolkit.Mvvm.Input;
+using Microsoft.Extensions.DependencyInjection;
 using Yavsc.Blogspot;
 using Yavsc.Api.Client;
 using PostIt.Services;
+using PostIt.Views;
 
 namespace PostIt.ViewModels;
 
@@ -47,9 +50,6 @@ public partial class MainPageViewModel : ViewModelBase
     [ObservableProperty]
     public partial bool DraftIsPublished { get; set; }
 
-    [ObservableProperty]
-    public partial ViewModelBase? CurrentViewModel { get; set; }
-
     public Settings SettingsModel { get; }
 
     [ObservableProperty]
@@ -81,9 +81,46 @@ public partial class MainPageViewModel : ViewModelBase
     /// </summary>
     public BlogApiClient? BlogClient { get; }
 
+    /// <summary>
+    /// DI container the VM uses to resolve navigation targets
+    /// (other ViewModels) when the user clicks a toolbar button
+    /// that opens a sub-screen. Owned by <c>App.ServiceProvider</c>
+    /// in production; injected directly in tests. The VM resolves
+    /// <em>ViewModels</em> via this provider, never Views — the
+    /// actual <see cref="Control"/> to push is decided by
+    /// <see cref="ViewLocator"/> at bind time, per CONTRIBUTING.md
+    /// §"Navigation (PostIt)".
+    /// </summary>
+    public IServiceProvider? Services { get; }
+
+    private SignaturePageViewModel? _signatureModel;
+
+    /// <summary>
+    /// Resolved on first access. Lazy so the test path (which
+    /// never pushes <c>SignaturePage</c>) does not require a
+    /// fully-built DI graph just to construct the VM. Mirrors the
+    /// pattern of <see cref="SettingsModel"/> for the Settings case.
+    /// </summary>
+    public SignaturePageViewModel SignatureModel =>
+        _signatureModel ??= ResolveSignatureModel();
+
     public override bool CanNavigateNext { get => throw new NotImplementedException(); protected set => throw new NotImplementedException(); }
     public override bool CanNavigatePrevious { get => throw new NotImplementedException(); protected set => throw new NotImplementedException(); }
 
+    private SignaturePageViewModel ResolveSignatureModel()
+    {
+        var sp = Services ?? (Application.Current as App)?.ServiceProvider;
+        if (sp is null)
+        {
+            throw new InvalidOperationException(
+                "Cannot resolve SignaturePageViewModel: no IServiceProvider " +
+                "was injected and App.ServiceProvider is null. This is a " +
+                "test-time wiring bug — the test must construct an " +
+                "IServiceProvider that registers SignaturePageViewModel.");
+        }
+        return sp.GetRequiredService<SignaturePageViewModel>();
+    }
+
 
     public MainPageViewModel()
     {
@@ -115,7 +152,6 @@ public partial class MainPageViewModel : ViewModelBase
         DraftTitle = string.Empty;
         DraftArticle = string.Empty;
         DraftIsPublished = false;
-        CurrentViewModel = this;
     }
 
     /// <summary>Save is enabled as soon as the user has typed
@@ -135,10 +171,11 @@ public partial class MainPageViewModel : ViewModelBase
     /// <see cref="BlogApiClient"/>. Production code uses the
     /// (Settings, BlogApiClient) overload below.
     /// </summary>
-    public MainPageViewModel(BlogApiClient blogClient, Settings? settings = null)
+    public MainPageViewModel(BlogApiClient blogClient, Settings? settings = null, IServiceProvider? services = null)
     {
         SettingsModel = new Settings();
         BlogClient = blogClient ?? throw new ArgumentNullException(nameof(blogClient)); ;
+        Services = services;
 
         Init(settings);
     }
@@ -309,10 +346,22 @@ public partial class MainPageViewModel : ViewModelBase
         });
     }
 
+    /// <summary>
+    /// DEV ONLY: open the signature capture page. The production
+    /// entry point is a SignalR push from Yavsc.Org ("devis
+    /// received, sign here"); this command is the dev-time
+    /// shortcut to reach the page without that infrastructure.
+    /// Aligned on the same nav-via-CurrentViewModel pattern as
+    /// <see cref="OpenSettings"/>: the VM resolves the target VM
+    /// through <see cref="Services"/>, the <c>ViewLocator</c> picks
+    /// the matching <c>Control</c> at bind time. No
+    /// <c>Click</code> handler, no <c>App.ServiceProvider</c>
+    /// access from the view layer.
+    /// </summary>
     [RelayCommand]
-    internal void OpenSettings()
+    internal void OpenSignatureDev()
     {
-        CurrentViewModel = SettingsModel;
+        ((App)App.Current).PushPage(SignatureModel);
     }
 
     private ViewModelBase? GetACLViewModel(BlogPostDto selectedPost)
@@ -386,7 +435,7 @@ public partial class MainPageViewModel : ViewModelBase
     public void ManageAcl()
     {
         if (SelectedPost is null) return;
-        CurrentViewModel = GetACLViewModel(SelectedPost);
+        ((App)App.Current).PushPage(GetACLViewModel(SelectedPost));
     }
 
     /// <summary>

src/PostIt/PostIt/ViewModels/SessionStatusViewModel.cs

@@ -2,6 +2,7 @@ using System;
 using System.Threading.Tasks;
 using CommunityToolkit.Mvvm.ComponentModel;
 using CommunityToolkit.Mvvm.Input;
+using Microsoft.Extensions.DependencyInjection;
 using PostIt.Services;
 
 namespace PostIt.ViewModels;
@@ -144,9 +145,10 @@ public partial class SessionStatusViewModel : ViewModelBase
     }
 
     [RelayCommand]
-    public async System.Threading.Tasks.Task OpenSettingsCommand()
+    internal void OpenSettings()
     {
-        OpenSettingsRequested?.Invoke();
-        await System.Threading.Tasks.Task.CompletedTask;
+        var app = (App)App.Current;
+        app.PushPage(app.ServiceProvider.GetRequiredService<Settings>());
     }
+
 }

src/PostIt/PostIt/Views/MainPage.axaml

@@ -59,8 +59,8 @@
                     MainPage.axaml.cs once the SignalR handler lands.
                 -->
                 <Button x:Name="OpenSignatureDevButton"
+                        Command="{Binding OpenSignatureDev}"
                         Content="[DEV] Signature"
-                        Click="OpenSignatureDev"
                         ToolTip.Tip="DEV ONLY — to remove when SignalR handler lands" />
             </StackPanel>
         </StackPanel>

src/PostIt/PostIt/Views/MainPage.axaml.cs

@@ -1,11 +1,4 @@
-using System;
-using Avalonia;
 using Avalonia.Controls;
-using Avalonia.Interactivity;
-using Microsoft.Extensions.DependencyInjection;
-using PostIt.ViewModels;
-using Yavsc.Blogspot;
-using Yavsc.Api.Client;
 
 namespace PostIt.Views;
 
@@ -15,34 +8,4 @@ public partial class MainPage : ContentPage
     {
         InitializeComponent();
     }
-
-    MainPageViewModel? _vm;
-
-
-    /// <summary>
-    /// DEV ONLY: temporary shortcut to open the signature capture
-    /// page from the blog editor. The production entry point is a
-    /// SignalR push from Yavsc.Org ("devis received, sign here"),
-    /// which is the only path that carries the devis identifier
-    /// needed to bind the capture to a specific contract.
-    ///
-    /// Remove this method and the corresponding button in
-    /// MainPage.axaml.cs once the SignalR handler lands.
-    /// </summary>
-    private void OpenSignatureDev(object? sender, RoutedEventArgs e)
-    {
-        // Resolve via the App's DI container so the page gets
-        // the canonical services (Api client, settings, ...).
-        var app = Application.Current as App;
-        var services = app?.ServiceProvider;
-        if (services is null) return;
-
-        var page = services.GetRequiredService<SignaturePage>();
-        page.DataContext = services.GetRequiredService<SignaturePageViewModel>();
-
-        if (this.VisualRoot is MainWindow window)
-        {
-            _ = window.NavRoot.PushAsync(page);
-        }
-    }
-}
+}