Dependency Injection with Swinject in iOS app

Setting up Dependency Injection with Swinject in iOS

Swinject — one of most mature DI containers for Swift. Widely used before SwiftUI era, continues applying in UIKit projects and mixed codebases where SwiftUI exists alongside UIKit. Pure SwiftUI project — look at Factory or native @Environment. For UIKit architectures with MVVM, VIPER or Clean Architecture, Swinject remains relevant choice.

Basic Container Setup

Assembly point for entire dependency graph — Container. Typical organization: AppAssembly protocol + separate Assembly classes per module:

import Swinject

class NetworkAssembly: Assembly {
  func assemble(container: Container) {
    container.register(URLSession.self) { _ in
      URLSession(configuration: .default)
    }.inObjectScope(.container)  // singleton within container

    container.register(APIClient.self) { r in
      DefaultAPIClient(session: r.resolve(URLSession.self)!)
    }.inObjectScope(.container)
  }
}

class AuthAssembly: Assembly {
  func assemble(container: Container) {
    container.register(AuthRepository.self) { r in
      DefaultAuthRepository(
        apiClient: r.resolve(APIClient.self)!,
        keychain: r.resolve(KeychainService.self)!
      )
    }
    container.register(AuthViewModel.self) { r in
      AuthViewModel(repository: r.resolve(AuthRepository.self)!)
    }
  }
}

Initialize in AppDelegate or SceneDelegate:

let assembler = Assembler([
  NetworkAssembly(),
  KeychainAssembly(),
  AuthAssembly(),
  ProfileAssembly()
])
let container = assembler.resolver

Where It Most Often Breaks

Force unwrap on resolve. r.resolve(SomeService.self)! — standard Swinject pattern, but on missed registration this crashes at runtime. Alternative: use Container.loggingBehavior = .verbose in debug builds — then unregistered dependencies logged before crash. For critical dependencies use safeResolve with check in applicationDidFinishLaunching:

func validateRegistrations(_ container: Container) {
  assert(container.resolve(APIClient.self) != nil, "APIClient not registered")
  assert(container.resolve(AuthRepository.self) != nil, "AuthRepository not registered")
}

ObjectScope and memory leaks. .container (singleton) keeps object for entire container lifetime. For ViewModel in UIKit this problem: if ViewModel registered in .container and holds strong reference to ViewController — leak. Register ViewModel in .transient (new object per resolve) or .graph (single object per resolve tree).

Circular dependencies. If AuthViewModel depends on Router, and Router depends on AuthViewModel — Swinject recurses infinitely on resolve. Solved via initCompleted callback to break cycle:

container.register(AuthViewModel.self) { _ in AuthViewModel() }
  .initCompleted { r, vm in
    vm.router = r.resolve(Router.self)
  }

UIKit Navigation Integration

Swinject works well with Coordinator pattern. Coordinator gets resolver and resolves dependencies on screen creation:

class AuthCoordinator {
  private let resolver: Resolver
  init(resolver: Resolver) { self.resolver = resolver }

  func showLogin() {
    let vm = resolver.resolve(AuthViewModel.self)!
    let vc = LoginViewController(viewModel: vm)
    navigationController.pushViewController(vc, animated: true)
  }
}

Work Included

• Container and Assembler setup with modular Assembly split
• All layers registration: network, repositories, ViewModel, services
• Proper ObjectScope for each type
• Coordinator or Router integration
• Registration validation in debug mode
• Dependency graph documentation

Timeline

2–3 days for typical project with 30–50 registered types. Cost depends on current architecture and refactoring scope.