Reactive Programming Swift: Combine and RxSwift

Reactive Programming in Swift (iOS/macOS): Combine and RxSwift

Reactive programming is a paradigm based on working with asynchronous data streams and propagating changes. In the Apple ecosystem for iOS and macOS, the main tools are the native Combine framework (since iOS 13/macOS 10.15) and the third-party library RxSwift. Both allow writing declarative, concise, and easily testable code, freeing the developer from "callback hell" and complex state management.

Basic Concepts

Publisher — a data source that can send values, completion, or an error.
Subscriber — receives data from the Publisher and reacts to it.
Operator — a stream transformation function (map, filter, combineLatest, debounce, etc.).
Subject — a hybrid that can both receive and send events (PassthroughSubject, CurrentValueSubject).
Scheduler — determines on which thread the code executes.

Combine: Native Apple Framework

Combine is built into Foundation and works with Publisher and Subscriber. Example of use for handling a text field:

import Combineimport UIKit

class ViewController: UIViewController {    @IBOutlet weak var textField: UITextField!    @IBOutlet weak var label: UILabel!    private var cancellables = Set<AnyCancellable>()

    override func viewDidLoad() {        super.viewDidLoad()                // Create a Publisher from NotificationCenter        let publisher = NotificationCenter.default            .publisher(for: UITextField.textDidChangeNotification, object: textField)            .compactMap { ($0.object as? UITextField)?.text }            .debounce(for: .milliseconds(500), scheduler: RunLoop.main)            .removeDuplicates()                // Subscribe        publisher            .sink { [weak self] value in                self?.label.text = "You entered: \\(value)"            }            .store(in: &cancellables)    }}

In the example, debounce is used to delay the reaction to input and removeDuplicates to ignore duplicate values.

RxSwift: Powerful Alternative

RxSwift is a port of ReactiveX for Swift. It offers a wider set of operators and supports older iOS versions. The same example with RxSwift:

import RxSwiftimport RxCocoa

class ViewController: UIViewController {    @IBOutlet weak var textField: UITextField!    @IBOutlet weak var label: UILabel!    private let disposeBag = DisposeBag()

    override func viewDidLoad() {        super.viewDidLoad()                textField.rx.text            .orEmpty            .debounce(.milliseconds(500), scheduler: MainScheduler.instance)            .distinctUntilChanged()            .subscribe(onNext: { [weak self] text in                self?.label.text = "You entered: \\(text)"            })            .disposed(by: disposeBag)    }}

Note the use of rx.text (via RxCocoa) and distinctUntilChanged() — the equivalent of removeDuplicates in Combine.