Flutter Unit Testing

Flutter Unit Testing That Actually Catches Bugs

Core Principle

Tests must verify REAL behavior, not mock behavior. If your test can't catch a real bug, delete it—it gives false confidence.

The 5 Laws of Bug-Catching Unit Tests

1. Test BEHAVIOR, not implementation
2. Test BOUNDARIES, not just happy path
3. Test ERROR paths explicitly
4. Mock at the LOWEST possible level
5. Every test must have a REASON — "what bug does this catch?"

Flutter/Dart Testing Patterns

Pattern 1: Pure Function Tests (Highest Value)

Extract logic into pure functions and test directly — zero mocking needed.

// ✅ GOOD: Pure function, directly testable
@visibleForTesting
WasteCategory mapWasteCategory(String raw) {
  final text = raw.toLowerCase();
  if (text.contains('hazard') || text.contains('有害')) return WasteCategory.hazardous;
  if (text.contains('recycl') || text.contains('回收')) return WasteCategory.recyclable;
  return WasteCategory.unknown;
}

test('maps Chinese hazardous terms correctly', () {
  expect(mapWasteCategory('有害廢棄物'), WasteCategory.hazardous);
  expect(mapWasteCategory('電池'), WasteCategory.hazardous);
});

test('handles mixed case and whitespace', () {
  expect(mapWasteCategory('  RECYCLABLE  '), WasteCategory.recyclable);
});

test('returns unknown for unrecognized input', () {
  expect(mapWasteCategory('asdf'), WasteCategory.unknown);
  expect(mapWasteCategory(''), WasteCategory.unknown);
});

Bug this catches: Incorrect category mapping → wrong disposal advice to user.

Pattern 2: Riverpod Provider Tests

// Use ProviderContainer for isolated provider testing
test('favorites toggle adds then removes', () {
  final container = ProviderContainer.test(
    overrides: [
      // Override SharedPreferences or repository
      sharedPreferencesProvider.overrideWithValue(FakeSharedPreferences()),
    ],
  );

  final notifier = container.read(favoritesProvider.notifier);

  // Act: toggle on
  notifier.toggle('stop-123');
  expect(container.read(favoritesProvider), contains('stop-123'));

  // Act: toggle off
  notifier.toggle('stop-123');
  expect(container.read(favoritesProvider), isNot(contains('stop-123')));
});

Key rules:

• Use ProviderContainer.test() (auto-disposes)
• Override only external dependencies (DB, network)
• Test state transitions, not internal implementation
• Verify provider interactions through state changes

Pattern 3: Boundary & Edge Case Testing

group('boundary cases that catch real bugs', () {
  test('empty list does not crash', () {
    final result = findNearestStop([]);
    expect(result, isNull);
  });

  test('single item list returns that item', () {
    final result = findNearestStop([onlyStop]);
    expect(result, equals(onlyStop));
  });

  test('exactly at boundary distance', () {
    // 3000m is the cutoff — test AT the boundary
    final result = findWithinRadius(stops, 3000);
    expect(result, contains(stopAt3000m)); // or not? clarify spec
  });

  test('negative values handled gracefully', () {
    expect(() => calculateDistance(-91, 0, 0, 0), throwsArgumentError);
  });

  test('unicode and special characters in input', () {
    final result = parseScheduleAddress('忠孝路28號（A03路線）');
    expect(result.address, '忠孝路28號');
  });
});

Pattern 4: Error Path Testing

test('network timeout returns friendly error message', () {
  final result = friendlyError(SocketException('timeout'));
  expect(result, contains('網路連線異常'));
});

test('rate limit returns retry message', () {
  final result = friendlyError(Exception('429 too many requests'));
  expect(result, contains('繁忙'));
});

test('unknown error does not expose internals', () {
  final result = friendlyError(Exception('NullPointerException at line 42'));
  expect(result, isNot(contains('NullPointerException')));
  expect(result, contains('暫時不可用'));
});

Bug this catches: Error messages leaking stack traces to users.

Anti-Patterns to AVOID

❌ Testing Mock Behavior

// BAD: You're testing that Mockito works, not your code
verify(mockRepo.fetchData()).called(1);
// This tells you nothing about whether fetchData returns correct data

❌ No Assertion After Action

// BAD: Test passes even if function is broken
test('does something', () {
  myFunction();
  // No expect() — what are you testing?
});

❌ Testing Implementation Details

// BAD: Breaks when you refactor, even if behavior is unchanged
test('uses cache', () {
  provider.getData();
  verify(mockCache.get('key')).called(1);
  // Tests HOW, not WHAT
});

// GOOD: Test the result, not the path
test('returns cached data on second call', () {
  final first = provider.getData();
  final second = provider.getData();
  expect(first, equals(second));
  // Tests WHAT: same result, regardless of HOW
});

❌ Fragile Time-Dependent Tests

// BAD: Fails at midnight or across time zones
test('shows today schedule', () {
  expect(getSchedule(), contains(DateTime.now().toString()));
});

// GOOD: Inject time dependency
test('shows schedule for given day', () {
  final schedule = getScheduleForDay(DateTime(2026, 3, 13));
  expect(schedule, isNotEmpty);
});

Bug-Finding Test Categories (Priority Order)

1. Data transformation bugs — Parse/format/convert functions
2. State transition bugs — Provider state changes in wrong order
3. Boundary bugs — Off-by-one, empty collections, max values
4. Error handling bugs — Uncaught exceptions, wrong error messages
5. Null safety bugs — Unexpected null values in optional fields
6. Concurrency bugs — Race conditions in async providers

Test Naming Convention

test('[WHAT] [WHEN] [THEN]')

// Examples:
test('bearing label returns 東北 when target is northeast')
test('parse classification returns null when JSON is invalid')
test('favorites toggle removes item when already favorited')

Verification Checklist

Before claiming tests are complete:

• Every public function has at least one test
• Happy path tested
• At least 2 edge cases per function (empty, null, boundary)
• Error paths tested (what happens when things fail?)
• No tests that ONLY verify mock behavior
• Test names describe WHAT behavior is being verified
• All tests pass: fvm flutter test

Related skills

• unit-testing-dart — DISAMBIGUATION: unit-testing-dart covers Dart test patterns and quality best practices. flutter-unit-testing is the TDD workflow for Flutter. Use flutter-unit-testing for TDD cycles; use unit-testing-dart for test quality audits.
• test-driven-development — use alongside flutter-unit-testing. TDD provides the red→green→refactor cycle; flutter-unit-testing provides the Flutter-specific test patterns.
• testing-anti-patterns — run after unit tests pass to catch mocking errors.