Dart Matcher Best Practices

When to use this skill

Use this skill when:

• Writing assertions using expect and package:matcher.
• Migrating legacy manual checks to cleaner matchers.
• Debugging confusing test failures.

Core Matchers

1. Collections (hasLength, contains, isEmpty)

• hasLength(n):

  • Prefer expect(list, hasLength(n)) over expect(list.length, n).
  • Gives better error messages on failure (shows actual list content).

• isEmpty / isNotEmpty:

  • Prefer expect(list, isEmpty) over expect(list.isEmpty, true).
  • Prefer expect(list, isNotEmpty) over expect(list.isNotEmpty, true).

• contains(item):

  • Verify existence without manual iteration.

• unorderedEquals(items):

  • Verify contents regardless of order.

2. Type Checks (isA<T> and TypeMatcher<T>)

• isA<T>():

  • Prefer for inline assertions: expect(obj, isA<Type>()).
  • More concise and readable than TypeMatcher<Type>().
  • Allows chaining constraints using .having().

• TypeMatcher<T>:

  • Prefer when defining top-level reusable matchers.
  • Use const: const isMyType = TypeMatcher<MyType>();
  • Chaining .having() works here too, but the resulting matcher is not const.

3. Object Properties (having)

Use .having() on isA<T>() or other TypeMatchers to check properties.

• Descriptive Names: Use meaningful parameter names in the closure (e.g., (e) => e.message) instead of generic ones like p0 to improve readability.

expect(person, isA<Person>()
    .having((p) => p.name, 'name', 'Alice')
    .having((p) => p.age, 'age', greaterThan(18)));

This provides detailed failure messages indicating exactly which property failed.

4. Async Assertions

• completion(matcher):

  • Wait for a future to complete and check its value.
  • Prefer await expectLater(...) to ensure the future completes before the test continues.
  • await expectLater(future, completion(equals(42))).

• throwsA(matcher):

  • Check that a future or function throws an exception.
  • await expectLater(future, throwsA(isA<StateError>())).
  • expect(() => function(), throwsA(isA<ArgumentError>())) (synchronous function throwing is fine with expect).

5. Using expectLater

Use await expectLater(...) when testing async behavior to ensure proper sequencing.

// GOOD: Waits for future to complete before checking side effects
await expectLater(future, completion(equals(42)));
expect(sideEffectState, equals('done'));

// BAD: Side effect check might run before future completes
expect(future, completion(equals(42)));
expect(sideEffectState, equals('done')); // Race condition!

Principles

1. Readable Failures: Choose matchers that produce clear error messages.
2. Avoid Manual Logic: Don't use if statements or for loops for assertions; let matchers handle it.
3. Specific Matchers: Use the most specific matcher available (e.g., containsPair for maps instead of checking keys manually).

Related Skills

• dart-test-fundamentals: Core concepts for structuring tests, lifecycles, and configuration.
• dart-checks-migration: Use this skill if you are migrating tests from package:matcher to modern package:checks.