Contents

• Why Migrate to package:checks
• Dependency Management
• Assertion Mapping and Syntax Guidelines
• Advanced Asynchronous Assertions
• Workflow: Executing Syntax Migrations
• Examples

Why Migrate to package:checks

package:checks is a modern assertion library maintained by the Dart team that replaces package:matcher. It offers several benefits over traditional matchers:

• Fluent API: Chain assertions naturally using Dart methods instead of nested helper functions.
• Type Safety: Benefit from compiler checks on expected properties, preventing type mismatches in tests.
• Descriptive Diagnostics: Read extremely clear failure messages showing exactly what failed and why (e.g. displaying collection differences).

Dependency Management

To add the checks library to your package:

1. Add checks as a dev_dependency using the terminal:

   dart pub add dev:checks
   

2. Import the main package entrypoint in all migrating test files:

   import 'package:checks/checks.dart';
   

3. Remove explicit package:matcher dependencies from pubspec.yaml (note that package:test transitively imports matchers, which is acceptable).

Assertion Mapping and Syntax Guidelines

Migrate legacy assertions using the following mapping guidelines:

Chaining Multiple Checks

Use Dart’s cascade operator (..) to assert multiple properties on a single subject without writing separate expectation statements:

check(user)
  ..has((u) => u.name, 'name').equals('Alice')
  ..has((u) => u.age, 'age').isGreaterThan(20);

Advanced Asynchronous Assertions

1. Futures

To verify Future resolutions, always await the base check statement:

// Verify completion value:
await check(fetchScore()).completes((score) => score.equals(100));

// Verify throws error:
await check(failingTask()).throws<HttpException>().has((e) => e.statusCode, 'statusCode').equals(500);

2. Streams

To assert multiple sequential items emitted by a Stream, wrap the target stream in a StreamQueue from package:async:

import 'package:async/async.dart';

final queue = StreamQueue(eventStream);
await check(queue).emits((event) => event.equals('first_event'));
await check(queue).emits((event) => event.equals('second_event'));

Workflow: Executing Syntax Migrations

Follow this checklist when migrating your test suite:

• [ ] Dependency Setup: Install dev:checks and run dart pub get.
• [ ] Find target files: Locate test files containing traditional expect() calls.
• [ ] Update imports: Replace matcher imports or add import 'package:checks/checks.dart'.
• [ ] Convert simple cases: Map basic equalities and type matches from expect to check.
• [ ] Optimize with cascades: Group multiple assertions targeting the same object into cascades.
• [ ] Update async calls: Rewrite asynchronous Future/Stream checks to use await and completes() closures.
• [ ] Validate compiler checks: Run dart analyze to resolve any type mismatch warnings.
• [ ] Execute tests: Run dart test to verify that all migrated assertions execute successfully.

Examples

Migration Comparison

This example demonstrates migrating a complete, high-fidelity user profile test case from legacy matcher syntax to modern checks syntax.

// --- Legacy Matcher Assertion Style ---
import 'package:test/test.dart';

class Account {
  final String email;
  final List<String> tags;
  Account(this.email, this.tags);
}

void badTest() {
  final account = Account('alice@test.com', ['admin', 'premium']);

  expect(account.email, equals('alice@test.com'));
  expect(account.tags, contains('admin'));
  expect(account.tags.length, equals(2));
}

// --- Modern Checks Assertion Style ---
import 'package:checks/checks.dart';

void goodTest() {
  final account = Account('alice@test.com', ['admin', 'premium']);

  // Chain validations cleanly using cascades and property extractors
  check(account)
    ..has((a) => a.email, 'email').equals('alice@test.com')
    ..has((a) => a.tags, 'tags').contains('admin')
    ..has((a) => a.tags.length, 'tags length').equals(2);
}

Testing Exceptions and Future Completions

import 'package:checks/checks.dart';
import 'package:test/test.dart';

Future<String> fetchRole(bool fail) async {
  await Future.delayed(const Duration(milliseconds: 10));
  if (fail) throw StateError('Fetch failed');
  return 'Manager';
}

void main() {
  group('Asynchronous Tests', () {
    test('verifies successful completion', () async {
      await check(fetchRole(false)).completes(
        (role) => role.equals('Manager'),
      );
    });

    test('verifies throwing specific state errors', () async {
      await check(fetchRole(true)).throws<StateError>().has(
        (e) => e.message, 'message',
      ).equals('Fetch failed');
    });
  });
}