Protocol-Delegate.md

Protocol Delegate DataSource Pattern

You start off by defining a protocol (WeatherServiceDelegate) and datasource (WeatherServiceDataSource) for your ViewController to implement.

protocol WeatherServiceDelegate: AnyObject {
    func didFetchWeather(_ weather: Weather)
}

protocol WeatherServiceDataSource: AnyObject {
    var city: String? { get }
}

class WeatherService {

    weak var delegate: WeatherServiceDelegate?
    weak var dataSource: WeatherServiceDataSource?

    func fetchWeather() {
        guard let dataSource = dataSource, let city = dataSource.city else {
            assertionFailure("DataSource not set")
            return
        }
        let weather = Weather(city: city, temperature: "21 °C", imageName: "sunset.fill")
        delegate?.didFetchWeather(weather)
    }
}

The viewController then registers itself as the delegate and datasource.

        weatherService.delegate = self
        weatherService.dataSource = self

It must then implement the corresponding protocols. Now when feather weather button is pressed, the WeatherService can call back to the viewController via it's delegate and dataSource.

import UIKit

extension ProtocolDelegateViewController: WeatherServiceDelegate {
    func didFetchWeather(_ weather: Weather) {
        cityLabel.text = weather.city
        temperatureLabel.text = weather.temperature

        let configuration = UIImage.SymbolConfiguration(scale: .large)
        let image = UIImage(systemName: weather.imageName, withConfiguration: configuration)
        imageView.image = image
    }
}

extension ProtocolDelegateViewController: WeatherServiceDataSource {
    var city: String? {
        let _city: String? = "San Francisco"
        return _city
    }
}

class ProtocolDelegateViewController: UIViewController {
    let weatherService = WeatherService() // 1

    override func viewDidLoad() {
        super.viewDidLoad()
        setupViews()

        weatherService.delegate = self // 2
        weatherService.dataSource = self
    }

    @objc func weatherPressed() {
        weatherService.fetchWeather() // 3
    }
    
    // MARK: - Controls

    let weatherButton: UIButton = {
        let button = makeButton(withText: "Fetch Weather")
        button.addTarget(self, action: #selector(weatherPressed), for: .primaryActionTriggered)
        return button
    }()

    var imageView: UIImageView = {
        let configuration = UIImage.SymbolConfiguration(scale: .large)
        let image = UIImage(systemName: "zzz", withConfiguration: configuration)
        let imageView = UIImageView(image: image)
        imageView.translatesAutoresizingMaskIntoConstraints = false
        imageView.setContentHuggingPriority(UILayoutPriority(rawValue: 999), for: .horizontal)

        return imageView
    }()

    let cityLabel: UILabel = {
        let label = makeLabel(withTitle: "City")
        return label
    }()

    let temperatureLabel: UILabel = {
        let label = makeLabel(withTitle: "°C")
        return label
    }()

    func setupViews() {
        view.backgroundColor = .white
        navigationItem.title = "Protocol Delegate"

        view.addSubview(weatherButton)

        weatherButton.centerXAnchor.constraint(equalTo: view.centerXAnchor).isActive = true
        weatherButton.topAnchor.constraint(equalToSystemSpacingBelow: view.safeAreaLayoutGuide.topAnchor, multiplier: 3).isActive = true

        let stackView = makeRowStackView()

        view.addSubview(stackView)

        stackView.addArrangedSubview(cityLabel)
        stackView.addArrangedSubview(imageView)
        stackView.addArrangedSubview(temperatureLabel)

        stackView.centerXAnchor.constraint(equalTo: view.centerXAnchor).isActive = true
        stackView.topAnchor.constraint(equalToSystemSpacingBelow: weatherButton.bottomAnchor, multiplier: 3).isActive = true
        stackView.leadingAnchor.constraint(equalToSystemSpacingAfter: view.leadingAnchor, multiplier: 3).isActive = true
        view.trailingAnchor.constraint(equalToSystemSpacingAfter: stackView.trailingAnchor, multiplier: 3).isActive = true

        cityLabel.widthAnchor.constraint(equalTo: temperatureLabel.widthAnchor).isActive = true
    }
}

Video

• Protocol Delegate Pattern Explained