Testing Blocs

Drives Blocs from outside with deterministic inputs and asserts the exact state sequence they emit. bloc_test wraps the lifecycle (build a fresh Bloc, dispatch events, await stream, dispose) into a single declarative call. mocktail provides null-safe Repository mocks without code generation. Concurrency tests use a Completer to control timing so an assertion can run between a stale and a fresh request. Builds on flutter-bloc-async-api; the example suite tests RestaurantListBloc from that skill.

Contents

• The blocTest helper
• Mocking repositories with mocktail
• Testing concurrency transformers
• Testing HydratedBloc persistence
• Workflow: Test a Bloc
• Applied to Talabat-clone
• Examples

The blocTest helper

blocTest<B, S>(description, ...) registers a Dart test that builds a fresh Bloc, runs an action, and asserts the emitted states. It is a structured wrapper around expectLater(bloc.stream, emitsInOrder([...])) with five named hooks:

A passing blocTest is proof that for these exact inputs, the Bloc emits exactly this sequence. The Bloc's initial state is implicit and not asserted — expect: describes the deltas.

Mocking repositories with mocktail

mocktail generates mocks at runtime via class MockX extends Mock implements X {}. No build_runner, no generated files. Two patterns matter:

Stubbing a return value:

when(() => repo.fetchNearbyStores(any()))
    .thenAnswer((_) async => const Result.success(<Store>[]));

any() matches any positional argument. For named arguments use any(named: 'foo'). For matching specific values pass the value directly.

Fallback values for unhandled types:

If a method takes a custom object as a parameter, register a fallback once in setUpAll:

setUpAll(() {
  registerFallbackValue(const Location(lat: 0, lng: 0));
});

Without the fallback, any() against a Location parameter throws a StateError at first use.

Testing concurrency transformers

restartable() says "cancel the in-flight handler when a new event arrives". A naive test that dispatches two events back-to-back and asserts the second result will pass even without restartable() — because the events run synchronously fast enough that the first never overlaps with the second.

The reliable test uses a Completer to pin the first handler in flight while the second arrives:

test('refresh cancels the in-flight initial load', () async {
  final initialCompleter = Completer<Result<List<Store>>>();
  final refreshCompleter = Completer<Result<List<Store>>>();
  final calls = <Completer<Result<List<Store>>>>[initialCompleter, refreshCompleter];

  when(() => repo.fetchNearbyStores(any()))
      .thenAnswer((_) => calls.removeAt(0).future);

  final bloc = RestaurantListBloc(repo: repo);
  bloc.add(const RestaurantListLoadRequested(_loc));
  await Future<void>.delayed(Duration.zero); // let _onLoad start
  bloc.add(const RestaurantListRefreshRequested(_loc));
  await Future<void>.delayed(Duration.zero); // let restartable() cancel

  // `_storeA`/`_storeB` are runtime finals (not `const`), so `Result.success([...])`
  // cannot be `const` either.
  initialCompleter.complete(Result.success([_storeA])); // dropped — restartable cancelled this handler's emitter
  refreshCompleter.complete(Result.success([_storeB]));
  await bloc.close();

  // The first emit was Loading, then Loading again from the refresh,
  // then Loaded with the second result. The first Result.success([_storeA])
  // is dropped because the handler that would emit it was cancelled.
  expect(bloc.state, isA<RestaurantListLoaded>());
  expect((bloc.state as RestaurantListLoaded).stores, [_storeB]);
});

The Completer-based timing is what proves restartable() is actually doing its job. Without it the test only proves "events serialize", which they would anyway under sequential() or even concurrent() for this simple case.

Testing HydratedBloc persistence

HydratedBloc reads and writes a global HydratedBloc.storage singleton. Test files that touch HydratedBlocs must register an in-memory Storage fake once per test process, or tests will pollute each other (and the device, if you forget to swap to a fake).

class _InMemoryStorage implements Storage {
  final _map = <String, dynamic>{};

  // `dynamic` matches the upstream `Storage` interface — a real fake can tighten it.
  @override
  dynamic read(String key) => _map[key];
  @override
  Future<void> write(String key, dynamic value) async => _map[key] = value;
  @override
  Future<void> delete(String key) async => _map.remove(key);
  @override
  Future<void> clear() async => _map.clear();
  // The official `Storage` interface declares four required members
  // (read/write/delete/clear). `close()` isn't in the contract today,
  // but a no-op makes the fake forward-compatible if a future version
  // adds it.
  @override
  Future<void> close() async {}
}

setUpAll(() {
  HydratedBloc.storage = _InMemoryStorage();
});

setUp(() async {
  // `clear()` returns a Future. With the in-memory fake the mutation lands
  // synchronously before the first await, but always `await` it so the pattern
  // survives a swap to an async-yielding fake (e.g. a SharedPreferences-backed one).
  await HydratedBloc.storage.clear();
});

Asserting rehydration is then a two-Bloc dance: build, mutate, close, build a second instance, check state. The firstWhere call subscribes to the stream before add enqueues the handler (microtask order), so the persisted write happens before close().

test('CartBloc rehydrates after restart', () async {
  final a = CartBloc(repo: repo);
  a.add(CartItemAdded(_product, 2));
  await a.stream.firstWhere((s) => s is CartLoaded); // subscribes before handler runs
  await a.close();

  // Second instance reads the persisted state synchronously in its constructor.
  final b = CartBloc(repo: repo);
  expect(b.state, isA<CartLoaded>());
  expect((b.state as CartLoaded).items.length, 1);
});

Workflow: Test a Bloc

Task Progress

• Step 1 — Mirror the bloc path. Test file lives at test/ui/features/<feature>/bloc/<feature>_bloc_test.dart.
• Step 2 — Add dev deps. flutter pub add --dev bloc_test mocktail.
• Step 3 — Declare mock classes. class MockStoreRepository extends Mock implements StoreRepository {} at the top of the file.
• Step 4 — setUpAll for fallbacks. Register registerFallbackValue(<custom-type>(...)) for every non-primitive type used in any().
• Step 5 — setUp per test. Instantiate the mock, instantiate the Bloc.
• Step 6 — Write blocTest cases. One per "given X event(s), expect Y state sequence". Always cover: happy path, failure path, and any branch in the handler.
• Step 7 — Cover transformer behavior. For each restartable() / droppable() / sequential() handler, write one explicit test that proves the transformer's contract (use Completer for timing).
• Step 8 — If HydratedBloc. Add the _InMemoryStorage fake in setUpAll, clear it in setUp, and write a rehydration round-trip test.
• Step 9 — Feedback loop. flutter test test/ui/features/<feature>/bloc/<feature>_bloc_test.dart → if a test hangs, the Bloc is probably awaiting a future that never completes (check Completer usage) → if a test reports an unexpected extra state, the previous test's mock leaked (check setUp scoping).

Applied to Talabat-clone

Two tests from the cart suite illustrate the patterns.

CartBloc rehydrates after cold start (PRD_states.md §7).

The cart is Cart: active → converted | abandoned. On cold start, a returning user must see their cart exactly as they left it. The test seeds _InMemoryStorage, builds a fresh CartBloc, and asserts the initial emitted state already has the persisted items — no Loading flicker.

CartBloc rejects items from a second store (PRD_catalog.md §15.5).

The one-cart / one-store rule says adding a product from a different store should transition to CartStoreSwitchPending(pendingProduct) rather than blend the carts. The test seeds a CartLoaded(items: [productFromStoreA]), dispatches CartItemAdded(productFromStoreB, 1), and asserts the next state is CartStoreSwitchPending, not CartLoaded with both items. A follow-up test dispatches CartStoreSwitchConfirmed and asserts the cart is replaced.

These tests are the only safety net between the PRD rules and a future refactor that silently drops one of them.

Examples

A test suite for RestaurantListBloc from flutter-bloc-async-api. Covers success, failure, and the restartable refresh transformer.

test/ui/features/restaurant_list/bloc/restaurant_list_bloc_test.dart

import 'dart:async';

import 'package:bloc_test/bloc_test.dart';
import 'package:flutter_test/flutter_test.dart';
import 'package:mocktail/mocktail.dart';

import 'package:my_app/core/result/failure.dart';
import 'package:my_app/core/result/result.dart';
import 'package:my_app/data/repositories/store_repository.dart';
import 'package:my_app/domain/models/location.dart';
import 'package:my_app/domain/models/store.dart';
import 'package:my_app/ui/features/restaurant_list/bloc/restaurant_list_bloc.dart';
import 'package:my_app/ui/features/restaurant_list/bloc/restaurant_list_event.dart';
import 'package:my_app/ui/features/restaurant_list/bloc/restaurant_list_state.dart';

class MockStoreRepository extends Mock implements StoreRepository {}

const _loc = Location(lat: 25.276987, lng: 55.296249); // Dubai
final _storeA = Store(id: 'a', name: 'A', deliveryMinutes: 20);
final _storeB = Store(id: 'b', name: 'B', deliveryMinutes: 30);

void main() {
  setUpAll(() {
    // `_loc` is `const` — same canonical instance used by both registration and dispatch.
    registerFallbackValue(_loc);
  });

  group('RestaurantListBloc', () {
    late MockStoreRepository repo;

    setUp(() {
      repo = MockStoreRepository();
    });

    blocTest<RestaurantListBloc, RestaurantListState>(
      'emits [Loading, Loaded] when fetch succeeds',
      setUp: () {
        when(() => repo.fetchNearbyStores(any()))
            .thenAnswer((_) async => Result.success([_storeA, _storeB]));
      },
      build: () => RestaurantListBloc(repo: repo),
      act: (b) => b.add(const RestaurantListLoadRequested(_loc)),
      expect: () => [
        const RestaurantListState.loading(),
        RestaurantListState.loaded([_storeA, _storeB]),
      ],
      verify: (_) {
        verify(() => repo.fetchNearbyStores(_loc)).called(1);
      },
    );

    blocTest<RestaurantListBloc, RestaurantListState>(
      'emits [Loading, Empty] when fetch returns no stores',
      setUp: () {
        when(() => repo.fetchNearbyStores(any()))
            .thenAnswer((_) async => const Result.success(<Store>[]));
      },
      build: () => RestaurantListBloc(repo: repo),
      act: (b) => b.add(const RestaurantListLoadRequested(_loc)),
      expect: () => [
        const RestaurantListState.loading(),
        const RestaurantListState.empty(),
      ],
    );

    blocTest<RestaurantListBloc, RestaurantListState>(
      'emits [Loading, Failure] on network error',
      setUp: () {
        when(() => repo.fetchNearbyStores(any()))
            .thenAnswer((_) async => const Result.failure(Failure.network()));
      },
      build: () => RestaurantListBloc(repo: repo),
      act: (b) => b.add(const RestaurantListLoadRequested(_loc)),
      expect: () => [
        const RestaurantListState.loading(),
        const RestaurantListState.failure('No internet connection'),
      ],
    );

    // This test drops out of `blocTest` and uses raw `test(...)` because
    // `blocTest`'s `wait:` parameter is a fixed `Duration` — it cannot interleave
    // `Completer.complete` calls between event dispatches, which is what proves
    // the `restartable()` contract.
    test('refresh cancels the in-flight initial load', () async {
      // Named completers so we can resolve them in a deliberate order.
      final initialCompleter = Completer<Result<List<Store>>>();
      final refreshCompleter = Completer<Result<List<Store>>>();
      final completers = [initialCompleter, refreshCompleter];

      // First call to `fetchNearbyStores` returns initialCompleter.future,
      // the second returns refreshCompleter.future.
      when(() => repo.fetchNearbyStores(any()))
          .thenAnswer((_) => completers.removeAt(0).future);

      final bloc = RestaurantListBloc(repo: repo);
      final emitted = <RestaurantListState>[];
      final sub = bloc.stream.listen(emitted.add);

      // Dispatch the initial load; let _onLoad start awaiting the first future.
      bloc.add(const RestaurantListLoadRequested(_loc));
      await Future<void>.delayed(Duration.zero);

      // Dispatch refresh; restartable() cancels the initial handler's emitter.
      bloc.add(const RestaurantListRefreshRequested(_loc));
      await Future<void>.delayed(Duration.zero);

      // Now resolve the FIRST completer. Its emit must be dropped because the
      // initial handler's emitter is already done.
      initialCompleter.complete(Result.success([_storeA]));
      refreshCompleter.complete(Result.success([_storeB]));

      // One more turn so the refresh handler emits its Loaded.
      await Future<void>.delayed(Duration.zero);
      await bloc.close();
      await sub.cancel();

      // The load-bearing invariant: we never emit `Loaded([_storeA])`.
      // Only one `Loaded` should appear, and it must carry the refresh's result.
      final loadedStates = emitted.whereType<RestaurantListLoaded>().toList();
      expect(loadedStates, hasLength(1),
          reason: 'restartable() should drop the stale Loaded([_storeA])');
      expect(loadedStates.single.stores, [_storeB]);
      expect(bloc.state, isA<RestaurantListLoaded>());
    });
  });
}

For features that extend HydratedBloc (e.g., CartBloc), add the in-memory storage fake from the "Testing HydratedBloc persistence" section above to the top-level setUpAll, and clear it in setUp so tests do not see each other's persisted state.

Source: abdallhMoukdad/flutter-bloc-skills — distributed by TomeVault.