Step 1 - Before (hidden dependency)

class AuthService {
  Future<bool> login(String email, String password) async {
    final client = HttpApiClient();
    final raw = await client.post('/login', body: {'email': email, 'password': password});
    return raw.status == 200;
  }
}

• Hard to fake HttpApiClient in a unit test without network hooks.

Step 2 - After (injected)

class AuthService {
  final ApiClient _client;

  AuthService(this._client);

  Future<bool> login(String email, String password) async {
    final raw = await _client.post('/login', body: {'email': email, 'password': password});
    return raw.status == 200;
  }
}

abstract class ApiClient {
  Future<HttpResponse> post(String path, {Map<String, String>? body});
}

class HttpResponse {
  final int status;
  HttpResponse(this.status);
}

• Tests pass a FakeApiClient that returns canned HttpResponse values.

Good habits

• Prefer final fields set once in the constructor.
• Avoid optional “inject or construct internally” parameters; pick one style per type.

Practice tasks

• Convert one static DateTime.now() usage to an injectable Clock interface with DateTime now() for tests.
• List constructors in a feature and mark which parameters are infrastructure vs pure config.