import 'dart:async';

import 'package:flutter/foundation.dart';

import '../data/app_config.dart';
import '../data/doormile_api.dart';
import '../data/location_service.dart';
import '../data/models.dart';

/// Single source of truth for the customer app.
///
/// Screens read from here and call these methods; nothing mutates a [Booking]
/// directly.
///
/// **Every booking on screen came from the server.** This class has no way to
/// invent one and no way to move one along: a stage changes when the backend
/// says it changed and [refreshBooking] reads it back.
class AppState extends ChangeNotifier {
  AppState({DoormileApi? api, LocationService? location})
    : api = api ?? DoormileApi.instance,
      location = location ?? LocationService.instance {
    this.api.onSessionLost = onSessionLost;
    loadLimits();
  }

  final DoormileApi api;
  final LocationService location;

  // ------------------------------------------------------------- session

  Customer? customer;

  /// **The authoritative pickup point.** Display address *and* the coordinates
  /// the Miler is sent to, in one object.
  ///
  /// All three ways of choosing a pickup — the device fix, a map drag, and a
  /// search result — write this single field, so nothing downstream has to
  /// reconcile "the address" against "the pin".
  Place? pickup;

  /// True once we have both an address and coordinates to send a Miler to.
  /// The booking step gates on this rather than on the address alone.
  bool get hasPickupPoint => pickup?.hasLocation ?? false;

  /// True while a pin is being turned into an address. The pickup card shows a
  /// placeholder rather than a stale street name.
  bool resolvingPickup = false;

  /// Why we could not read the device location, if we could not. Null when the
  /// pin came from GPS or from the customer.
  LocationDenial? locationDenial;

  /// Where the **customer** is, as opposed to where the pickup pin is.
  ///
  /// Kept separate on purpose: the pin moves when they drag the map, this does
  /// not. It is what the blue dot renders at, and it survives a drag so they
  /// can always see how far they have moved the pin from themselves.
  double? deviceLat;
  double? deviceLng;

  /// Horizontal accuracy in metres, when the platform reported it — the halo
  /// around the dot.
  double? deviceAccuracy;

  bool get hasDeviceLocation => deviceLat != null && deviceLng != null;

  /// Guards against an earlier reverse-geocode landing after a later drag.
  int _pinSeq = 0;

  /// Where the map should open: the current pin, else the service-area default.
  double get mapLat => pickup?.lat ?? LocationService.fallbackLat;
  double get mapLng => pickup?.lng ?? LocationService.fallbackLng;

  bool get isSignedIn => customer != null;

  /// True until the stored session has been looked for. The app shows its
  /// launch state rather than the sign-in screen while this is set, so a
  /// returning customer never sees a login form flash past.
  bool restoringSession = true;

  void signIn(Customer c) {
    customer = c;
    notifyListeners();
    unawaited(refreshOrders());
  }

  /// Looks for a persisted session at launch.
  ///
  /// Never throws: being offline at launch is not being signed out, and the
  /// stored customer stands until a call actually comes back 401.
  Future<void> restoreSession() async {
    try {
      final restored = await api.restoreSession();
      if (restored != null) {
        customer = restored;
        unawaited(refreshOrders());
      } else if (AppConfig.devAutoLogin) {
        // ── Offline builds open on Home, not on a login screen ──
        //
        // The fake signs anybody in — there is no server to refuse them — so
        // stopping at the entrance would ask for a phone number and a code
        // that mean nothing, purely to arrive somewhere the build was always
        // going to allow. Any 4-digit code still works if someone wants to walk
        // the screen; this is the default rather than the only way through.
        //
        // Real builds are untouched: `useDevData` is false in every one of
        // them, so the login screen a customer meets is the real one.
        customer = await api.verifyOtp('+91 9876543210', '1234');
        unawaited(refreshOrders());
      } else if (AppConfig.hasAutoLogin) {
        await _autoSignIn();
      }
    } catch (e) {
      // Same rule as the limits call: this decides which screen opens, so it
      // fails to "signed out", never to a crash.
      debugPrint('[SESSION] restore failed: $e');
    } finally {
      restoringSession = false;
      notifyListeners();
    }
  }

  /// Signs in for real, without the login screen — `DM_LOGIN_AS`.
  ///
  /// This is a genuine sign-in, not a bypass: it runs the same two calls the
  /// login and code screens run, and what it ends up holding is a token the
  /// server issued. That is the whole point of it existing next to
  /// the deleted offline mock — a build that skips the entrance still has to
  /// end up as a real customer, or every booking it makes is a fake one.
  ///
  /// Failure is not fatal and never throws: the app simply lands on the login
  /// screen, which is exactly where someone whose code was refused should be.
  Future<void> _autoSignIn() async {
    final identifier = AppConfig.autoLoginIdentifier;
    final code = AppConfig.autoLoginCode;

    // ── Verify first, request second ──
    //
    // Requesting a code is not free: with no SMS gateway registered the server
    // generates a fresh one, writes it to its log and REPLACES whatever was in
    // Redis. So a code someone had just read out of that log would be
    // invalidated by the very call meant to use it.
    //
    // So this tries the code it was given first. That path is the one that
    // works with a log-read code, and with a fixed `CX_STAGING_OTP` it simply
    // fails once on a cold Redis and falls through to the request below.
    try {
      customer = await api.verifyOtp(identifier, code);
      unawaited(refreshOrders());
      debugPrint('[AUTH] DM_LOGIN_AS — signed in as $identifier');
      return;
    } catch (_) {
      // Expected when no code has been issued for this identifier yet.
    }

    try {
      await api.sendOtp(identifier);
      customer = await api.verifyOtp(identifier, code);
      unawaited(refreshOrders());
      debugPrint('[AUTH] DM_LOGIN_AS — signed in as $identifier after a request');
    } catch (e) {
      debugPrint(
        '[AUTH] DM_LOGIN_AS failed for $identifier: $e — '
        'the code was refused, so the login screen opens instead',
      );
    }
  }

  /// Called by the API layer when a refresh fails — the chain is dead, so the
  /// customer is signed out wherever they happen to be standing.
  void onSessionLost() {
    if (customer == null) return;
    debugPrint('[SESSION] lost, signing out');
    signOut();
  }

  Future<void> signOut() async {
    customer = null;
    orders.clear();
    pickup = null;
    deviceLat = null;
    deviceLng = null;
    deviceAccuracy = null;
    locationDenial = null;
    ordersCursor = null;
    clearCaches();
    notifyListeners();
    try {
      await api.signOut();
    } on ApiException catch (e) {
      // Local sign-out has already happened and must stand.
      debugPrint('[SESSION] sign-out call failed: $e');
    }
  }

  /// Reads the device location and turns it into an address.
  ///
  /// Never throws and never leaves the customer stuck: a refused permission
  /// falls back to the service-area centre with [locationDenial] set, so the
  /// screen can explain it and offer search instead.
  Future<void> detectPickupLocation() async {
    final seq = ++_pinSeq;
    resolvingPickup = true;
    notifyListeners();

    final fix = await location.current();
    if (seq != _pinSeq) return;
    locationDenial = fix.denial;

    if (fix.hasFix) {
      deviceLat = fix.lat;
      deviceLng = fix.lng;
      deviceAccuracy = fix.accuracy;
    }

    final lat = fix.lat ?? pickup?.lat ?? LocationService.fallbackLat;
    final lng = fix.lng ?? pickup?.lng ?? LocationService.fallbackLng;
    await _resolvePin(lat, lng, seq);
  }

  /// Called when the map settles under the pin. The newest drag always wins.
  Future<void> setPickupFromPin(double lat, double lng) async {
    final seq = ++_pinSeq;
    resolvingPickup = true;
    notifyListeners();
    await _resolvePin(lat, lng, seq);
  }

  Future<void> _resolvePin(double lat, double lng, int seq) async {
    Place resolved;
    try {
      resolved = await api.reverseGeocode(lat: lat, lng: lng);
    } on ApiException {
      resolved = Place(
        title: 'Pinned location',
        sub: 'Address unavailable — the Miler will confirm it at pickup',
        lat: lat,
        lng: lng,
      );
    }
    if (seq != _pinSeq) return;
    pickup = resolved;
    resolvingPickup = false;
    notifyListeners();
  }

  /// Adopts a place chosen from search. Cancels any in-flight pin resolution,
  /// so the newest customer intent always wins — same rule as a drag.
  void setPickup(Place place) {
    _pinSeq++;
    // A search result without coordinates cannot be a pin. The backend
    // contract requires them (§6.2); if one arrives without, keep the address
    // and hold the previous pin rather than dropping the customer somewhere
    // else on the map.
    assert(
      place.hasLocation,
      'searchPlaces returned a Place with no coordinates: ${place.title}',
    );
    pickup = place.hasLocation
        ? place
        : Place(
            title: place.title,
            sub: place.sub,
            lat: pickup?.lat,
            lng: pickup?.lng,
          );
    resolvingPickup = false;
    locationDenial = null;
    notifyListeners();
  }

  /// Opens OS settings after a permanent refusal.
  Future<void> openLocationSettings() => location.openSettings();

  // ------------------------------------------------- service delegation
  //
  // Screens call these, never `DoormileApi.instance`. That keeps the injected
  // instance authoritative and means no widget can tell whether an answer came
  // from the backend.

  Future<OtpChallenge> sendOtp(String identifier) => api.sendOtp(identifier);

  Future<OtpChallenge> signUp({
    required String name,
    required String phone,
    String? email,
  }) => api.signUp(name: name, phone: phone, email: email);

  /// Verifies the code and adopts the session in one step, so no screen has to
  /// remember to call [signIn] afterwards.
  Future<Customer> verifyOtp(
    String identifier,
    String code, {
    String? name,
  }) async {
    final verified = await api.verifyOtp(identifier, code, name: name);
    signIn(verified);
    return verified;
  }

  /// Biased towards the pickup point the customer is working with, so a search
  /// for a street name answers with the one in their city first.
  Future<List<Place>> searchPlaces(String query) {
    final near = draftPickup ?? pickup;
    return api.searchPlaces(query, lat: near?.lat, lng: near?.lng);
  }

  /// Backend policy on cancellation; the UI only asks.
  bool isCancellable(JourneyStage stage) => api.isCancellable(stage);

  /// The window a booking was placed in.
  ///
  /// Served from whatever [loadSlots] last returned, which is why the index is
  /// never cleared: a booking made an hour ago must still be able to name its
  /// window after the slot list has rolled over.
  PickupSlot? slotById(String? id) =>
      id == null ? null : _slotIndex[id] ?? api.slotById(id);

  final Map<String, PickupSlot> _slotIndex = {};

  // --------------------------------------------------------------- limits

  /// Caps on one pickup. Backend-owned; the UI never hardcodes them.
  BookingLimits limits = const BookingLimits();

  /// Caps are allowed to vary by city, so the pickup point goes with the ask.
  Future<void> loadLimits() async {
    try {
      limits = await api.getBookingLimits(pickup: pickup);
      notifyListeners();
    } catch (e) {
      // The safe cap is already in place; a failed fetch must not block
      // booking. Catching everything, not just ApiException: this runs from
      // the constructor, and a malformed response is not worth a dead app.
      debugPrint('[LIMITS] keeping defaults: $e');
    }
  }

  // -------------------------------------------------------- booking draft

  /// The in-progress pickup. One destination is the common case; the customer
  /// can add more, all collected in the same visit.
  ///
  /// A setter rather than a field, because the map editor writes it and then
  /// pops: as a plain field the send screen underneath kept its last frame and
  /// went on showing the address the customer had just corrected.
  Place? get draftPickup => _draftPickup;
  set draftPickup(Place? place) {
    if (_draftPickup == place) return;
    _draftPickup = place;
    notifyListeners();
  }

  Place? _draftPickup;

  List<DestinationGroup> draftDestinations = [DestinationGroup()];
  String? draftSlotId;
  FareEstimate? draftFare;

  int get draftTotalPackages =>
      draftDestinations.fold(0, (sum, d) => sum + d.packageCount);

  bool get canAddDestination =>
      draftDestinations.length < limits.maxDestinations &&
      draftTotalPackages < limits.maxPackages &&
      draftDestinations.every((d) => d.isComplete);

  bool canAddPackageTo(int index) => draftTotalPackages < limits.maxPackages;

  /// Every destination has a serviceable state and district.
  bool get draftReady =>
      draftDestinations.isNotEmpty &&
      draftDestinations.every((d) => d.isComplete);

  /// One key per booking *intent*, held across every retry of it.
  ///
  /// A retried `POST /customer/bookings` carrying the same key replays the
  /// original response instead of creating a second pickup; a new key is a new
  /// booking. So it is minted once here, when the customer starts filling the
  /// form, and cleared only when a booking actually lands.
  String? _bookingIdempotencyKey;

  void startBooking() {
    draftPickup = pickup;
    draftDestinations = [DestinationGroup()];
    draftSlotId = null;
    draftFare = null;
    districtCache.clear();
    // Slots carry their own date. A session left open across midnight would
    // otherwise offer yesterday's windows, and booking one is refused — so the
    // list is re-read for every booking rather than kept for the session.
    _invalidateSlots();
    _bookingIdempotencyKey = null;
    notifyListeners();
  }

  /// Whether "Send to another place" is offered at all.
  ///
  /// Multi-destination is server-gated while the Miler app still keys its work
  /// on a booking-level id: a fanned-out pickup would give the rider one stop
  /// for three parcels. The backend advertises the cap, the client obeys it,
  /// and a cap of 1 hides the control rather than showing a dead one.
  bool get allowsMultipleDestinations => limits.allowsMultipleDestinations;

  void selectStateFor(int index, ServiceArea area) {
    draftDestinations[index].destination
      ..stateCode = area.code
      ..stateName = area.name
      ..districtCode = null
      ..districtName = null;
    draftDestinations[index].district = null;
    draftFare = null;
    notifyListeners();
  }

  void selectDistrictFor(int index, District district) {
    draftDestinations[index]
      ..destination.districtCode = district.code
      ..destination.districtName = district.name
      ..district = district;
    notifyListeners();
    _loadFare();
  }

  void setPackageCount(int index, int count) {
    final group = draftDestinations[index];
    final others = draftTotalPackages - group.packageCount;
    group.packageCount = count.clamp(1, limits.maxPackages - others);
    notifyListeners();
    _loadFare();
  }

  /// Returns false when the caps say no, so the caller can explain why.
  bool addDestination() {
    if (!canAddDestination) return false;
    draftDestinations.add(DestinationGroup());
    notifyListeners();
    return true;
  }

  void removeDestination(int index) {
    if (draftDestinations.length <= 1) return;
    draftDestinations.removeAt(index);
    notifyListeners();
    _loadFare();
  }

  /// Republish after something mutated the draft in place (the details sheet).
  void touch() => notifyListeners();

  void selectSlot(PickupSlot slot) {
    draftSlotId = slot.id;
    notifyListeners();
  }

  void setDraftPin(int index, MapPin? pin) {
    draftDestinations[index].details.pin = pin;
    notifyListeners();
  }

  /// Priced as soon as the route is known, so Review has it ready.
  Future<void> _loadFare() async {
    final from = draftPickup ?? pickup;
    if (from == null || !draftReady) return;
    try {
      draftFare = await api.estimateFare(
        pickup: from,
        destinations: draftDestinations,
      );
      notifyListeners();
    } on ApiException {
      // A missing estimate is not worth blocking the booking over.
    }
  }

  Future<Booking> confirmBooking() async {
    final key = _bookingIdempotencyKey ??= _newIdempotencyKey();
    try {
      final booking = await api.createBooking(
        pickup: draftPickup ?? pickup!,
        destinations: draftDestinations,
        slotId: draftSlotId,
        fare: draftFare,
        idempotencyKey: key,
      );
      // The intent is spent. A further booking needs a new key or the server
      // would replay this one.
      _bookingIdempotencyKey = null;
      orders.insert(0, booking);
      activeReference = booking.reference;
      notifyListeners();
      return booking;
    } on ApiException catch (e) {
      // The slot went stale — filled up, or its window passed while the form
      // was open. Both recover by re-reading the list, so drop the cached one
      // here and let the screen send them back to slot selection.
      if (e.needsFreshSlots) {
        _invalidateSlots();
        draftSlotId = null;
        notifyListeners();
      }
      rethrow;
    }
  }

  String _newIdempotencyKey() {
    final now = DateTime.now().microsecondsSinceEpoch.toRadixString(36);
    final salt = Object().hashCode.toRadixString(36);
    return '$now-$salt';
  }

  // -------------------------------------------------------------- orders

  final List<Booking> orders = [];
  String? activeReference;
  BookingStatus ordersTab = BookingStatus.active;

  Booking? byReference(String? reference) {
    if (reference == null) return null;
    for (final o in orders) {
      if (o.reference == reference) return o;
    }
    return null;
  }

  Booking? get trackedBooking => byReference(activeReference);

  List<Booking> get activeBookings =>
      orders.where((o) => o.status == BookingStatus.active).toList();

  List<Booking> ofStatus(BookingStatus status) =>
      orders.where((o) => o.status == status).toList();

  /// Orders as the customer owns them: one row per pickup before collection,
  /// one row per destination once the backend has created those orders.
  List<OrderEntry> entriesOf(BookingStatus status) => [
    for (final booking in ofStatus(status))
      if (!booking.hasOrders)
        OrderEntry(booking)
      else
        for (var i = 0; i < booking.destinations.length; i++)
          OrderEntry(booking, i),
  ];

  List<OrderEntry> get recentEntries => [
    for (final booking in orders.where((o) => o.status != BookingStatus.active))
      if (!booking.hasOrders)
        OrderEntry(booking)
      else
        for (var i = 0; i < booking.destinations.length; i++)
          OrderEntry(booking, i),
  ].take(3).toList();

  /// Everything no longer running, newest first — Home's "Recent".
  List<Booking> get recent =>
      orders.where((o) => o.status != BookingStatus.active).take(3).toList();

  /// Keyset cursor for the next page of orders; null when the last page has
  /// been read or none has been read yet.
  String? ordersCursor;

  /// True while [loadMoreOrders] is in flight, so a scroll cannot start two.
  bool loadingMoreOrders = false;

  bool get hasMoreOrders => ordersCursor != null && ordersCursor!.isNotEmpty;

  /// Pull-to-refresh on Home and Orders — reads the first page again.
  ///
  /// The list is the server's: whatever comes back replaces what is on screen,
  /// including an empty page, which is a real answer and not a failure.
  Future<void> refreshOrders() async {
    try {
      final page = await api.getBookingPage();
      // An empty first page is a real answer: this customer has no bookings.
      // The list is the server's, so it is adopted whole rather than merged.
      orders
        ..clear()
        ..addAll(page.bookings);
      ordersCursor = page.nextCursor;
    } on ApiException {
      // A failed refresh leaves the last good list on screen.
    }
    notifyListeners();
  }

  /// Appends the next keyset page. Keyset rather than offset, so a booking
  /// arriving at the top mid-scroll cannot push a row into a second page and
  /// show it twice.
  Future<void> loadMoreOrders() async {
    if (!hasMoreOrders || loadingMoreOrders) return;
    loadingMoreOrders = true;
    notifyListeners();
    try {
      final page = await api.getBookingPage(cursor: ordersCursor);
      final seen = orders.map((o) => o.reference).toSet();
      orders.addAll(page.bookings.where((b) => !seen.contains(b.reference)));
      ordersCursor = page.nextCursor;
    } on ApiException {
      // Keep the rows already on screen; the cursor stays put so the next
      // scroll retries the same page rather than skipping it.
    } finally {
      loadingMoreOrders = false;
      notifyListeners();
    }
  }

  void openTracking(String reference) {
    activeReference = reference;
    notifyListeners();
    unawaited(refreshBooking(reference));
  }

  /// Re-reads one booking. Null means the server answered `304` — nothing
  /// moved — so the copy already on screen stands.
  Future<void> refreshBooking(String reference) async {
    try {
      final fresh = await api.getBooking(reference);
      if (fresh == null) return;
      final at = orders.indexWhere((o) => o.reference == reference);
      if (at == -1) {
        orders.insert(0, fresh);
      } else {
        orders[at] = fresh;
      }
      notifyListeners();
    } on ApiException {
      // Keep showing what we have.
    }
  }

  void setOrdersTab(BookingStatus status) {
    ordersTab = status;
    notifyListeners();
  }

  /// Sends the optional address and recipient for an existing booking.
  ///
  /// Accepted until the parcels are collected; after that the server answers
  /// `409` and the Miler's record at the door is the one that counts.
  Future<void> saveDestinationDetails(Booking booking, int index) async {
    if (index < 0 || index >= booking.destinations.length) return;
    final group = booking.destinations[index];
    await api.updateDestinationDetails(
      booking.reference,
      // The server's own position for this stop, which is what the route
      // addresses. The list position is only a fallback for a payload that
      // did not send one.
      group.index ?? index,
      group.details,
    );
    notifyListeners();
  }

  // ---------------------------------------------------------------- push

  /// Registers this device for milestone notifications.
  ///
  /// Nothing calls this yet: no push SDK is wired in, so the app has no token
  /// to hand over. The endpoint is reached through here so that adding one is
  /// a two-line change rather than a new path through the state layer.
  Future<void> registerPushToken(String token) async {
    try {
      await api.registerDevice(token);
    } on ApiException catch (e) {
      debugPrint('[PUSH] could not register device: $e');
    }
  }

  Future<void> unregisterPushToken(String token) async {
    try {
      await api.unregisterDevice(token);
    } on ApiException catch (e) {
      debugPrint('[PUSH] could not unregister device: $e');
    }
  }

  // ------------------------------------------------- stage transitions

  /// What the tracking screen's QA stepper calls.
  ///
  /// It asks the server's own helper to move the booking, then re-reads it —
  /// so what appears on screen is the stage the **server** recorded, never one
  /// the client applied to its own copy.
  Future<void> advanceStage(Booking booking, JourneyStage target) async {
    try {
      await api.setStage(booking.reference, target);
      await refreshBooking(booking.reference);
    } on ApiException catch (e) {
      debugPrint('[OPS] stage override refused: $e');
    }
  }

  Future<void> cancelBooking(Booking booking, String? reason) async {
    await api.cancelBooking(booking.reference, reason);
    booking
      ..status = BookingStatus.cancelled
      ..cancelReason = reason
      ..cancellable = false;
    notifyListeners();
  }

  // ------------------------------------------------- serviceability cache

  List<ServiceArea>? statesCache;
  final Map<String, List<District>> districtCache = {};
  List<PickupSlot>? slotsCache;

  /// Only states we can actually serve reach the picker. A state with nothing
  /// open is not an option the customer should have to read past.
  Future<List<ServiceArea>> loadStates({bool refresh = false}) async {
    if (refresh) statesCache = null;
    final all = statesCache ??= await api.getServiceableStates();
    return all.where((s) => s.hasOpenDistricts).toList();
  }

  /// Districts the picker offers: serviceable ones only. The full response is
  /// cached so [upcomingDistricts] can name what is opening without putting
  /// disabled tiles in front of anyone.
  Future<List<District>> loadDistricts(
    String stateCode, {
    bool refresh = false,
  }) async {
    if (refresh) districtCache.remove(stateCode);
    final all = districtCache[stateCode] ??= await api.getServiceableDistricts(
      stateCode,
    );
    return all.where((d) => d.available).toList();
  }

  /// Every serviceable city, flat, for the send screen's city strip.
  ///
  /// ── Why this exists rather than a state picker ──
  ///
  /// The serviceability API is a hierarchy — states, then districts — because
  /// that is how coverage is administered. It is not how anyone thinks about
  /// sending a parcel: nobody picks "Tamil Nadu" on the way to picking
  /// "Chennai", and on a network this size the state step is a screen that asks
  /// a question with one useful answer.
  ///
  /// So the hierarchy is flattened here, once, into the list the screen wants.
  /// The districts come back in parallel — five states is five calls, not a
  /// waterfall — and each city carries its state so [selectCity] can set both
  /// codes the booking contract needs.
  Future<List<CityOption>> loadCities({bool refresh = false}) async {
    if (refresh) {
      statesCache = null;
      districtCache.clear();
    }
    final states = await loadStates();
    final lists = await Future.wait([
      for (final state in states) loadDistricts(state.code),
    ]);
    return [
      for (var i = 0; i < states.length; i++)
        for (final district in lists[i])
          CityOption(state: states[i], district: district),
    ];
  }

  /// Picks a city on the draft's only destination — both codes at once, since
  /// the contract wants `stateCode` and `districtCode` together.
  void selectCity(CityOption city, {int index = 0}) {
    selectStateFor(index, city.state);
    selectDistrictFor(index, city.district);
  }

  /// Names of districts in this state that are not open yet, for a quiet
  /// "Coming soon" line. Empty until the districts have been fetched.
  List<String> upcomingDistricts(String stateCode) => (districtCache[stateCode] ?? const [])
      .where((d) => !d.available)
      .map((d) => d.name)
      .toList();

  /// Bumped whenever [slotsCache] is dropped, so a slot list already on screen
  /// re-reads instead of showing what it fetched the first time.
  int slotsEpoch = 0;

  void _invalidateSlots() {
    slotsCache = null;
    slotsEpoch++;
  }

  Future<List<PickupSlot>> loadSlots({bool refresh = false}) async {
    if (refresh) slotsCache = null;
    final slots = slotsCache ??= await api.getPickupSlots(pickup: draftPickup);
    for (final slot in slots) {
      _slotIndex[slot.id] = slot;
    }
    return slots;
  }

  void clearCaches() {
    statesCache = null;
    districtCache.clear();
    _invalidateSlots();
  }
}