Dart Matcher Best Practices Agent Skill

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.

Discovery

To find candidates for improving matcher usage, search for suboptimal patterns:

Suboptimal Length Checks

Search for length checks that should use hasLength:

• Regex: expect\([^,]+.length,\s*

Suboptimal Boolean Checks

Search for checks on boolean properties that have specific matchers:

• Regex: expect\([^,]+.isEmpty,\s*(true|equals\(true\))
• Regex: expect\([^,]+.isNotEmpty,\s*(true|equals\(true\))
• Regex: expect\([^,]+.contains\(.*\),\s*(true|equals\(true\))

Suboptimal Map Lookups

Search for manual map lookups instead of containsPair:

• Regex: expect\([^,]+\[.*\],\s*

Core Matchers

1. Collections (hasLength, contains, isEmpty, unorderedEquals, containsPair)

• 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.
• Prefer over expect(list.contains(item), true).

• unorderedEquals(items):
• Verify contents regardless of order.
• Prefer over expect(list, containsAll(items)).

• containsPair(key, value):
• Verify a map contains a specific key-value pair.
• Prefer over checking expect(map[key], value) or

expect(map.containsKey(key), true).

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

• Readable Failures: Choose matchers that produce clear error messages.
• Avoid Manual Logic: Don't use if statements or for loops for

assertions; let matchers handle it.

• 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-test-fundamentals]: https://github.com/kevmoo/dash_skills/blob/main/skills/dart-test-fundamentals/SKILL.md