Effective Dart/Flutter Unit Testing

Core Principles

1. Test BEHAVIOR, Not Implementation

// ❌ BAD: Tests implementation details
test('internal counter increments', () {
  expect(service._counter, 0); // Accessing private state
  service.doWork();
  expect(service._counter, 1);
});

// ✅ GOOD: Tests observable behavior
test('doWork produces correct output', () {
  final result = service.doWork();
  expect(result, expectedOutput);
});