Inversi Dependensi dengan Abstraksi Protokol

Prinsip Pembalikan Dependensi

// WITHOUT DIP:
class OrderService { let db = SQLiteDatabase() }  // depends on concrete
// WITH DIP:
class OrderService { let db: Database }  // depends on abstraction

Mendefinisikan Abstraksi

protocol Database {
  func save<T: Encodable>(_ value: T, key: String) throws
  func load<T: Decodable>(key: String) throws -> T
}

Implementasi Konkret

class SQLiteDatabase: Database { ... }
class InMemoryDatabase: Database { ... }
class KeychainDatabase: Database { ... }

Injeksi Konstruktor

class ProfileService {
  private let db: Database
  init(db: Database) { self.db = db }
}

Mengapa Tidak Menggunakan Singleton untuk Semua Hal?

// Hard to test:
class Service { func run() { MySingleton.shared.doWork() } }
// Easy to test:
class Service { let dep: MyProtocol; func run() { dep.doWork() } }

Abstraksi Protokol di Swift

protocol Cacheable {
  associatedtype Key: Hashable
  associatedtype Value
  func get(_ key: Key) -> Value?
  func set(_ key: Key, value: Value)
}

Granularitas Abstraksi

// Fat (avoid):
protocol DataStore {
  func save(...); func load(...); func delete(...); func query(...); func migrate()
}
// Narrow (prefer):
protocol Readable { func load<T>(...) throws -> T }
protocol Writable { func save<T>(...) throws }

DIP dengan Combine

protocol UserStream {
  func userUpdates() -> AnyPublisher<User, Never>
}

DIP dengan async/await

protocol Analytics {
  func track(event: String) async
}
struct FirebaseAnalytics: Analytics {
  func track(event: String) async { /* Firebase call */ }
}

DIP di Tingkat Aplikasi

@main
struct MyApp: App {
  let db: Database = SQLiteDatabase()
  let repo: UserRepository = RemoteUserRepository(db: db)
  let useCase = FetchUserUseCase(repo: repo)
  var body: some Scene {
    WindowGroup { ContentView(useCase: useCase) }
  }
}

Pengujian dengan DIP

let mockDB = InMemoryDatabase()
let service = ProfileService(db: mockDB)
// Exercise service
XCTAssertEqual(try mockDB.load(key: "profile"), expectedProfile)

Pemeriksaan Singkat

Ringkasan Pelajaran