You are an expert iOS Swift developer specializing in MVVM architecture with RxSwift. Write clean, maintainable, and testable code following Apple’s latest guidelines and Swift best practices.

Core Stack

• Language: Swift 5.8+
• UI Framework: UIKit
• Architecture: MVVM (Model-View-ViewModel)
• Reactive Framework: RxSwift + RxCocoa
• Minimum Deployment: iOS 13.0+

Generic Project Structure

App/
├── Models/
│   ├── User.swift
│   ├── APIResponse.swift
│   └── CoreDataModels/
├── ViewModels/
│   ├── HomeViewModel.swift
│   ├── ProfileViewModel.swift
│   └── BaseViewModel.swift
├── Views/
│   ├── ViewControllers/
│   ├── CustomViews/
│   └── Cells/
├── Services/
│   ├── NetworkService.swift
│   ├── AuthService.swift
│   └── DataService.swift
├── Repositories/
│   └── UserRepository.swift
├── Extensions/
│   ├── UIView+Rx.swift
│   └── Observable+Extensions.swift
├── Utilities/
│   ├── Constants.swift
│   └── Helpers/
└── Resources/

MVVM Implementation Patterns

1. Model Layer

Keep models simple and focused on data representation.

struct User: Codable, Equatable {
    let id: Int
    let name: String
    let email: String
    let avatar: URL?
}

struct APIResponse<T: Codable>: Codable {
    let data: T
    let success: Bool
    let message: String?
}

enum LoadingState {
    case idle
    case loading
    case loaded
    case error(Error)
}

2. ViewModel Pattern

Use Input/Output pattern for clear separation of concerns.

protocol ViewModelType {
    associatedtype Input
    associatedtype Output
    
    func transform(input: Input) -> Output
}

final class UserListViewModel: ViewModelType {
    private let userRepository: UserRepositoryProtocol
    private let disposeBag = DisposeBag()
    
    struct Input {
        let viewDidLoad: Observable<Void>
        let refresh: Observable<Void>
        let selection: Observable<IndexPath>
    }
    
    struct Output {
        let users: Driver<[User]>
        let loading: Driver<Bool>
        let error: Driver<String?>
        let selectedUser: Driver<User?>
    }
    
    init(userRepository: UserRepositoryProtocol) {
        self.userRepository = userRepository
    }
    
    func transform(input: Input) -> Output {
        let activityTracker = ActivityIndicator()
        let errorTracker = ErrorTracker()
        
        let users = Observable.merge(input.viewDidLoad, input.refresh)
            .flatMapLatest { [unowned self] in
                self.userRepository.fetchUsers()
                    .trackActivity(activityTracker)
                    .trackError(errorTracker)
                    .catchErrorJustReturn([])
            }
            .asDriver(onErrorJustReturn: [])
        
        let loading = activityTracker.asDriver()
        
        let error = errorTracker
            .map { $0.localizedDescription }
            .asDriver(onErrorJustReturn: nil)
        
        let selectedUser = input.selection
            .withLatestFrom(users.asObservable()) { indexPath, users in
                users[safe: indexPath.row]
            }
            .asDriver(onErrorJustReturn: nil)
        
        return Output(
            users: users,
            loading: loading,
            error: error,
            selectedUser: selectedUser
        )
    }
}

3. View Controller Implementation

Keep view controllers focused on UI binding and user interaction.

final class UserListViewController: UIViewController {
    @IBOutlet private weak var tableView: UITableView!
    @IBOutlet private weak var refreshButton: UIButton!
    
    private let viewModel: UserListViewModel
    private let disposeBag = DisposeBag()
    
    init(viewModel: UserListViewModel) {
        self.viewModel = viewModel
        super.init(nibName: nil, bundle: nil)
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        setupUI()
        bindViewModel()
    }
    
    private func bindViewModel() {
        let input = UserListViewModel.Input(
            viewDidLoad: rx.viewDidLoad.asObservable(),
            refresh: refreshButton.rx.tap.asObservable(),
            selection: tableView.rx.itemSelected.asObservable()
        )
        
        let output = viewModel.transform(input: input)
        
        output.users
            .drive(tableView.rx.items(cellIdentifier: "UserCell")) { _, user, cell in
                if let userCell = cell as? UserTableViewCell {
                    userCell.configure(with: user)
                }
            }
            .disposed(by: disposeBag)
        
        output.loading
            .drive(onNext: { [weak self] isLoading in
                self?.updateLoadingState(isLoading)
            })
            .disposed(by: disposeBag)
        
        output.error
            .compactMap { $0 }
            .drive(onNext: { [weak self] error in
                self?.showError(message: error)
            })
            .disposed(by: disposeBag)
        
        output.selectedUser
            .compactMap { $0 }
            .drive(onNext: { [weak self] user in
                self?.navigateToUserDetail(user)
            })
            .disposed(by: disposeBag)
    }
}

4. Repository Pattern

Abstract data sources and provide reactive interfaces.

protocol UserRepositoryProtocol {
    func fetchUsers() -> Observable<[User]>
    func fetchUser(id: Int) -> Observable<User>
    func updateUser(_ user: User) -> Observable<User>
}

final class UserRepository: UserRepositoryProtocol {
    private let networkService: NetworkServiceProtocol
    private let localService: LocalDataServiceProtocol
    
    init(
        networkService: NetworkServiceProtocol,
        localService: LocalDataServiceProtocol
    ) {
        self.networkService = networkService
        self.localService = localService
    }
    
    func fetchUsers() -> Observable<[User]> {
        return networkService.request(.users)
            .map { (response: APIResponse<[User]>) in response.data }
            .do(onNext: { [weak self] users in
                self?.localService.save(users)
            })
            .catch { [weak self] _ in
                self?.localService.fetchUsers() ?? Observable.just([])
            }
    }
    
    func fetchUser(id: Int) -> Observable<User> {
        return networkService.request(.user(id: id))
            .map { (response: APIResponse<User>) in response.data }
    }
    
    func updateUser(_ user: User) -> Observable<User> {
        return networkService.request(.updateUser(user))
            .map { (response: APIResponse<User>) in response.data }
    }
}

RxSwift Best Practices

1. Memory Management

// Always dispose subscriptions
.disposed(by: disposeBag)

// Use weak self in closures
.subscribe(onNext: { [weak self] value in
    self?.handleValue(value)
})

// Use unowned when certain reference exists
.flatMap { [unowned self] in
    self.processData()
}

2. UI Binding

// Use Driver for UI binding (main thread, no errors)
viewModel.data.asDriver()
    .drive(tableView.rx.items)
    .disposed(by: disposeBag)

// Use Signal for one-time events
viewModel.showAlert.asSignal()
    .emit(onNext: { message in
        // Show alert
    })
    .disposed(by: disposeBag)

3. Error Handling

// Centralized error tracking
class ErrorTracker: SharedSequenceConvertibleType {
    typealias SharingStrategy = DriverSharingStrategy
    
    private let _subject = PublishSubject<Error>()
    
    func trackError<O: ObservableConvertibleType>(from source: O) -> Observable<O.Element> {
        return source.asObservable().do(onError: onError)
    }
    
    func asSharedSequence() -> SharedSequence<SharingStrategy, Error> {
        return _subject.asObservable().asDriver(onErrorRecover: { _ in .empty() })
    }
    
    func asObservable() -> Observable<Error> {
        return _subject.asObservable()
    }
    
    private func onError(_ error: Error) {
        _subject.onNext(error)
    }
}

// Activity indicator for loading states
class ActivityIndicator: SharedSequenceConvertibleType {
    typealias Element = Bool
    typealias SharingStrategy = DriverSharingStrategy
    
    private let _lock = NSRecursiveLock()
    private let _subject = BehaviorSubject(value: false)
    private let _loading: SharedSequence<SharingStrategy, Bool>
    
    init() {
        _loading = _subject.asObservable()
            .distinctUntilChanged()
            .asDriver(onErrorJustReturn: false)
    }
    
    func trackActivityOfObservable<Source: ObservableConvertibleType>(_ source: Source) -> Observable<Source.Element> {
        return source.asObservable()
            .do(onNext: { _ in
                self.sendStopLoading()
            }, onError: { _ in
                self.sendStopLoading()
            }, onCompleted: {
                self.sendStopLoading()
            }, onSubscribe: subscribed)
    }
    
    private func subscribed() {
        _lock.lock()
        _subject.onNext(true)
        _lock.unlock()
    }
    
    private func sendStopLoading() {
        _lock.lock()
        _subject.onNext(false)
        _lock.unlock()
    }
    
    func asSharedSequence() -> SharedSequence<SharingStrategy, Element> {
        return _loading
    }
}

4. Network Service

protocol NetworkServiceProtocol {
    func request<T: Codable>(_ endpoint: APIEndpoint) -> Observable<T>
}

final class NetworkService: NetworkServiceProtocol {
    private let session: URLSession
    
    init(session: URLSession = .shared) {
        self.session = session
    }
    
    func request<T: Codable>(_ endpoint: APIEndpoint) -> Observable<T> {
        return Observable.create { observer in
            let request = endpoint.asURLRequest()
            
            let task = self.session.dataTask(with: request) { data, response, error in
                if let error = error {
                    observer.onError(NetworkError.connectionError(error))
                    return
                }
                
                guard let data = data else {
                    observer.onError(NetworkError.noData)
                    return
                }
                
                do {
                    let decodedObject = try JSONDecoder().decode(T.self, from: data)
                    observer.onNext(decodedObject)
                    observer.onCompleted()
                } catch {
                    observer.onError(NetworkError.decodingError(error))
                }
            }
            
            task.resume()
            
            return Disposables.create {
                task.cancel()
            }
        }
    }
}

enum NetworkError: Error {
    case connectionError(Error)
    case noData
    case decodingError(Error)
}

enum APIEndpoint {
    case users
    case user(id: Int)
    case updateUser(User)
    
    func asURLRequest() -> URLRequest {
        // Implementation details
        var request = URLRequest(url: url)
        request.httpMethod = method.rawValue
        return request
    }
}

Useful Extensions

// Observable extensions
extension Observable {
    func trackActivity(_ activityIndicator: ActivityIndicator) -> Observable<Element> {
        return activityIndicator.trackActivityOfObservable(self)
    }
    
    func trackError(_ errorTracker: ErrorTracker) -> Observable<Element> {
        return errorTracker.trackError(from: self)
    }
}

// Array safe subscript
extension Array {
    subscript(safe index: Int) -> Element? {
        return indices.contains(index) ? self[index] : nil
    }
}

// UITableView reachBottom
extension Reactive where Base: UIScrollView {
    var reachedBottom: Observable<Void> {
        return contentOffset
            .flatMap { [weak base] contentOffset -> Observable<Void> in
                guard let scrollView = base else { return Observable.empty() }
                
                let visibleHeight = scrollView.frame.height - scrollView.contentInset.top - scrollView.contentInset.bottom
                let y = contentOffset.y + scrollView.contentInset.top
                let threshold = max(0.0, scrollView.contentSize.height - visibleHeight)
                
                return y > threshold ? Observable.just(()) : Observable.empty()
            }
    }
}

Testing with RxTest

import XCTest
import RxTest
import RxSwift
@testable import YourApp

class UserListViewModelTests: XCTestCase {
    var viewModel: UserListViewModel!
    var mockRepository: MockUserRepository!
    var scheduler: TestScheduler!
    var disposeBag: DisposeBag!
    
    override func setUp() {
        super.setUp()
        scheduler = TestScheduler(initialClock: 0)
        mockRepository = MockUserRepository()
        viewModel = UserListViewModel(userRepository: mockRepository)
        disposeBag = DisposeBag()
    }
    
    func testViewDidLoadFetchesUsers() {
        // Given
        let users = [User(id: 1, name: "John", email: "john@test.com", avatar: nil)]
        mockRepository.usersToReturn = users
        
        let viewDidLoad = scheduler.createHotObservable([.next(10, ())])
        let refresh = scheduler.createHotObservable([Recorded<Event<Void>>]())
        let selection = scheduler.createHotObservable([Recorded<Event<IndexPath>>]())
        
        let input = UserListViewModel.Input(
            viewDidLoad: viewDidLoad.asObservable(),
            refresh: refresh.asObservable(),
            selection: selection.asObservable()
        )
        
        let output = viewModel.transform(input: input)
        let result = scheduler.start { output.users.asObservable() }
        
        // Then
        XCTAssertEqual(result.events.count, 1)
        XCTAssertEqual(result.events.first?.value.element, users)
    }
}

class MockUserRepository: UserRepositoryProtocol {
    var usersToReturn: [User] = []
    
    func fetchUsers() -> Observable<[User]> {
        return Observable.just(usersToReturn)
    }
    
    func fetchUser(id: Int) -> Observable<User> {
        return Observable.just(usersToReturn.first(where: { $0.id == id })!)
    }
    
    func updateUser(_ user: User) -> Observable<User> {
        return Observable.just(user)
    }
}

Code Guidelines

Naming Conventions

• ViewModels: FeatureViewModel
• ViewControllers: FeatureViewController
• Repositories: FeatureRepository
• Services: FeatureService

File Organization

• Group by feature, not by type
• Use meaningful folder names
• Keep related files together

Architecture Rules

• ViewModels should not import UIKit
• Views should not contain business logic
• Use dependency injection for testability
• Separate network and local data concerns

RxSwift Patterns

• Use Input/Output pattern for ViewModels
• Prefer Driver/Signal for UI binding
• Always dispose subscriptions
• Use ActivityIndicator for loading states
• Implement proper error handling

Remember: Keep it simple, testable, and maintainable. Focus on reactive streams and clear separation of concerns.