Application context selector

Status: ✅ Implemented

Intent

Fix a value at APPLICATION level — the active hotel in a PMS, the company in a multi-tenant ERP, the business unit, the fiscal year — so every screen of the app works against it without asking again.

Problem

Without an app-level context, every listing and form needs its own “which hotel?” filter, users re-select it on every screen, and nothing guarantees two screens agree on the value.

Solution

Annotate a field of the @UI app class with @AppContext. It renders as a compact selector on the app header (next to the theme toggle), visible on every screen:

@UI("")
@Title("My first Mateu app")
public class BackofficeApp {

    @AppContext(label = "Hotel")
    HotelSelector hotel;      // options from a LookupOptionsSupplier

    @AppContext
    Environment environment;  // or simply an enum: its constants are the options
}

The field’s type provides the choices:

An enum — its constants become the options.
Any class implementing LookupOptionsSupplier — searched once with an empty text (first page) when the app shell is built:

public class HotelSelector implements LookupOptionsSupplier {
    @Override
    public ListingData<Option> search(String fieldName, String searchText,
                                      Pageable pageable, HttpRequest httpRequest) {
        return ListingData.of(hotels.stream()
                .map(h -> new Option(h.id(), h.name()))
                .toArray(Option[]::new));
    }
}

The Hotel context selector on the app header

Reading the context

The selected option’s value lives in the app state under the field’s name and travels with EVERY request, so any screen or action reads it from the HttpRequest:

@Button
public Message greet(HttpRequest httpRequest) {
    var hotel = httpRequest.appContext("hotel");   // "2", or null when nothing is selected
    ...
}

An AutoCrud typically uses it inside repository()/search(...) to scope the data — the widget stays fully decoupled from the screens.

Behaviour

Compact select or searchable picker — up to 7 options render as a plain select; more options render as a button opening a searchable panel: typing filters the loaded options and asks the server for matches beyond the first page (the _appcontext-search-<field> action, answered by the field’s LookupOptionsSupplier or the enum constants).
Persistent — the selection (and its label) is stored client-side (one entry per origin) and survives reloads, navigation and new tabs.
Uniform reactivity — picking a different value reloads the current route, so whatever screen the user is on rebuilds against the new context. No per-screen wiring.
Cross-tab sync — when another tab of the same origin changes the context, every open tab reloads and rebuilds against the new value.
Explicit app state wins — the persisted context is merged under any appState entries the components set programmatically.
Several @AppContext fields render several selectors (e.g. hotel + fiscal year).

Other servers

The pattern is wire-level, so the .NET and Python backends emit the same selectors:

[App("Backoffice")]
public class BackofficeApp
{
    [AppContext("Hotel")]
    public IReadOnlyList<OptionDto> Hotel() => hotels.Select(h => new OptionDto(h.Id, h.Name)).ToList();

    [AppContext] public Environment Environment { get; set; }   // enum property
}

@app("Backoffice")
class BackofficeApp:
    @app_context("Hotel")
    def hotel(self):
        return [(h.id, h.name) for h in hotels]   # or Option(...) objects, or an Enum return type

Native renderers

The native shells render the same wire field:

React Native — selectors at the top of the drawer; session-scoped, picking a value remounts the current screen.

Like the web picker, a selector with more than 7 options gains a search box: typing filters the loaded options and (debounced) asks the server for matches beyond the loaded page via the same _appcontext-search-<field> action — React Native shows it under the drawer selector.

Principles served

Workflow over screens — set once, applies everywhere
Recoverability — the context survives reloads; no re-selection tax
Single source of truth — screens can’t disagree on the active context