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postIt: SettingsPage is a singleton, navigation is idempotent

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Two related changes that close the loop on the SettingsPage
push semantics.

1. The SettingsPage used to be registered as Transient. Each
   click on the Paramètres button resolved a fresh instance,
   re-bound it to the Settings singleton, and pushed it onto
   the navigation stack. Repeated clicks accumulated stacked
   instances, each fully bound, and the user had to tap Back
   N times to leave. The fix is to register the page as a
   Singleton in the DI container. There is now one and only
   one SettingsPage ContentPage for the lifetime of the app:
     - its DataContext is wired once, at composition time
       (just after the ViewLocator is added to DataTemplates),
       not on every push;
     - the OpenSettingsRequested handler is a pure navigation
       concern, with no DI resolution and no rebinding;
     - the in-memory Settings state is preserved across visits
       (any in-flight edit stays in the same instance).

2. The OpenSettingsRequested handler is guarded so that if the
   SettingsPage is already at the top of NavigationStack, the
   push is a no-op. NavigationPage.PushAsync does not
   deduplicate; without the guard, calling it twice with the
   same instance pushes it a second time, and the user has to
   tap Back twice to leave. The guard is a reference comparison
   on NavigationStack[Count - 1] against the singleton
   instance, which is correct precisely because the page is
   a singleton.

doc/architecture/postit.md is updated to match: the DI table
reflects the new lifetime, and the 'Garde anti-empilement'
section is rewritten from 'to be implemented' to the actual
implementation, including the rationale for reference
comparison and the cross-dependency between the singleton
lifetime and the guard.

The Settings-singleton invariant (in the same doc) is
unchanged: Settings is still a singleton, and adding a
transient override would still be the bug it always was.
The new SettingsPage singleton sits alongside it cleanly.

Build: 0 errors. Tests: 3/3 SettingsLoadTests green.
This commit is contained in:
Paul Schneider 2026-07-09 22:00:00 +01:00
commit 1733dababb
2 changed files with 68 additions and 24 deletions
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40
doc/architecture/postit.md
View file

@ -113,7 +113,8 @@ le DI est construit. Ordre, dans cet ordre :
| `Settings` | **Singleton** | État partagé (`Loaded`, `IsDirty`, `Authentication`) — doit être unique. |
| `YavscApiClient` | Singleton | Porte le `TokenStore` et le cache de tokens ; un seul par process. |
| `BlogApiClient` | Singleton | Mapper stateless, partagé. |
| `MainPage` / `SettingsPage` / `HomePage` / `SignaturePage` | Transient | Résolution à la demande par le `ViewLocator`. |
| `SettingsPage` | **Singleton** | Une seule instance pour la vie de l'app : le `DataContext` est câblé une fois au boot, le push est idempotent (cf. section *Garde anti-empilement* ci-dessous). |
| `MainPage` / `HomePage` / `SignaturePage` | Transient | Résolution à la demande par le `ViewLocator`. |
| `MainPageViewModel` / `HomePageViewModel` / `SignaturePageViewModel` | Transient | VM reconstruites à chaque navigation ; pas d'état partagé à conserver. |
| `SessionStatusViewModel` + `SessionStatusBanner` | Singleton + Transient | Le VM est un singleton (survit à la navigation), le bandeau est transient (réinstancié quand la fenêtre le recrée). |
@ -139,22 +140,31 @@ posé sur `MainWindow.axaml`. La pile est gérée par les
### Garde anti-empilement
Le handler `OpenSettingsRequested` doit garantir qu'une seule
`SettingsPage` est au sommet de la pile à un instant donné.
Sans garde, plusieurs clics sur **Paramètres** empilent
plusieurs instances (chacune résolue en `Transient`), et
l'utilisateur doit appuyer N fois sur **Retour** pour sortir.
`NavigationPage.PushAsync` n'est pas idempotent : pousser deux
fois la même instance l'empile deux fois, et l'utilisateur doit
taper **Retour** N fois pour sortir. Le handler
`OpenSettingsRequested` est gardé pour bloquer ce cas :
L'invariant à implémenter dans le handler :
```csharp
var settingsPage = provider.GetRequiredService<SettingsPage>();
var stack = w.NavRoot.NavigationStack;
if (stack.Count > 0 && ReferenceEquals(stack[stack.Count - 1], settingsPage))
{
return; // déjà au sommet, no-op silencieux
}
_ = w.NavRoot.PushAsync(settingsPage);
```
> Si la page du sommet de `NavRoot.NavigationStack` est déjà
> une `SettingsPage`, ne pas empiler une nouvelle instance
> (no-op silencieux). Sinon, `PushAsync` une nouvelle instance
> comme aujourd'hui.
Le détail d'implémentation (lecture de la pile, gestion des
cas "SettingsPage est plus bas dans la pile") reste à coder
quand l'UI le demandera.
La comparaison est par référence, pas par type : on ne veut
empêcher qu'un push de *cette* instance particulière, pas
celui d'une éventuelle autre `SettingsPage` (il n'en existe
qu'une, mais l'invariant est plus clair comme ça). La garde
repose sur le fait que `SettingsPage` est un singleton ; si on
repassait en `Transient`, `ReferenceEquals` resterait correct
mais la pertinence de la garde s'évaporerait (chaque push
apporterait une nouvelle instance et l'anti-empilement
reposerait sur l'invariant « la même est déjà au sommet »,
qui ne tiendrait plus).
## ViewModels et invariants d'état

52
src/PostIt/PostIt/App.axaml.cs
View file

@ -60,7 +60,18 @@ public partial class App : Application
// Vues
services.AddTransient<MainPage>();
services.AddTransient<SettingsPage>();
// SettingsPage is a singleton: there must be one and only one
// instance of the settings UI for the lifetime of the app.
// This guarantees that (a) the bindings always reflect the
// current in-memory Settings state, (b) the page already has
// its DataContext wired up at composition-root time (see
// below), and (c) the OpenSettingsRequested handler is a
// pure push with a no-op-if-already-on-top guard, never a
// re-resolution from DI. Transient would let the user
// accumulate stale SettingsPage instances on the navigation
// stack, each bound to a fresh SettingsViewModel and missing
// any in-flight edits.
services.AddSingleton<SettingsPage>();
services.AddTransient<HomePage>();
services.AddTransient<SignaturePage>();
@ -93,6 +104,17 @@ public partial class App : Application
DataTemplates.Clear();
DataTemplates.Add(new ViewLocator(provider));
// Wire the Settings singleton onto the SettingsPage singleton
// once, at composition time. The page is registered as a
// singleton (see above) precisely so this binding is stable
// for the lifetime of the app: every push to / pop from the
// navigation stack finds the same ContentPage with the same
// DataContext, and the TwoWay bindings inside the page keep
// mutating the same in-memory Settings instance that the rest
// of the app reads (OidcClientOptions construction, etc.).
provider.GetRequiredService<SettingsPage>().DataContext =
provider.GetRequiredService<Settings>();
if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
{
var homePage = provider.GetRequiredService<HomePage>();
@ -130,18 +152,30 @@ public partial class App : Application
};
// When the user clicks the "Paramètres" button on the
// session banner, push the SettingsPage on top of the
// current navigation stack. Resolved from DI so the
// ViewLocator + service-locator dance stays out of the
// VM, and bound to the same Settings singleton the rest
// of the app is using (the one we Load()'d at startup).
// Two-way bindings on the page mutate that singleton
// in place; callers re-read on next access.
// session banner, push the SettingsPage singleton on top
// of the current navigation stack. The DataContext is
// already wired at composition time (see the
// provider.GetRequiredService<SettingsPage>().DataContext
// assignment above), so this handler is a pure
// navigation concern.
//
// Anti-empilement guard: if the SettingsPage is already
// at the top of the stack, do nothing. NavigationPage's
// PushAsync does not deduplicate; calling it twice with
// the same instance would push it a second time and the
// user would have to tap Back twice to leave. Reference
// comparison is correct here because SettingsPage is a
// singleton — there is exactly one instance to compare
// against.
sessionStatus.OpenSettingsRequested += () =>
{
var w = (MainWindow)((IClassicDesktopStyleApplicationLifetime)ApplicationLifetime!).MainWindow!;
var settingsPage = provider.GetRequiredService<SettingsPage>();
settingsPage.DataContext = provider.GetRequiredService<Settings>();
var stack = w.NavRoot.NavigationStack;
if (stack.Count > 0 && ReferenceEquals(stack[stack.Count - 1], settingsPage))
{
return;
}
_ = w.NavRoot.PushAsync(settingsPage);
};