Delegate: что это, паттерн делегирования и как работает на iOS

// Протокол Delegate
protocol DownloadManagerDelegate: AnyObject {
    func downloadManager(_ manager: DownloadManager,
                          didFinishWith data: Data)
    func downloadManager(_ manager: DownloadManager,
                          didFailWith error: Error)

    @objc optional func downloadManager(_ manager: DownloadManager,
                                didUpdateProgress progress: Float)
}

// Класс, использующий Delegate
class DownloadManager {
    weak var delegate: DownloadManagerDelegate?

    func startDownload(from url: URL) {
        URLSession.shared.dataTask(with: url) { [weak self] data, _, error in
            guard let self else { return }
            if let error = error {
                self.delegate?.downloadManager(self, didFailWith: error)
            } else if let data = data {
                self.delegate?.downloadManager(self, didFinishWith: data)
            }
        }.resume()
    }
}

class ViewController: DownloadManagerDelegate {
    let manager = DownloadManager()

    override func viewDidLoad() {
        super.viewDidLoad()
        manager.delegate = self // weak — нет retain cycle
        manager.startDownload(from: url)
    }

    func downloadManager(_ manager: DownloadManager, didFinishWith data: Data) {
        processData(data)
    }

    func downloadManager(_ manager: DownloadManager, didFailWith error: Error) {
        showError(error)
    }
}

class NetworkService: NSObject, URLSessionDataDelegate {
    private lazy var session = URLSession(
        configuration: .default,
        delegate: self,
        delegateQueue: OperationQueue()
    )
    private var receivedData = Data()

    func urlSession(_ session: URLSession,
                    dataTask: URLSessionDataTask,
                    didReceive data: Data) {
        receivedData.append(data)
        let progress = Float(receivedData.count) / Float(expectedSize)
        DispatchQueue.main.async {
            self.progressHandler?(progress)
        }
    }

    func urlSession(_ session: URLSession,
                    task: URLSessionTask,
                    didCompleteWithError error: Error?) {
        if let error = error {
            delegate?.networkService(self, didFailWith: error)
        } else {
            delegate?.networkService(self, didReceive: receivedData)
        }
    }
}

class SafeViewController: UIViewController {
    private var session: URLSession?
    private var service: NetworkService?

    override func viewDidLoad() {
        super.viewDidLoad()
        service = NetworkService()
        service?.delegate = self
    }

    deinit {
        // Обнуление delegate в deinit — best practice
        service?.delegate = nil
        session?.invalidateAndCancel()
    }
}

// URLSession с weak delegate через NSObject
class WeakDelegateSession: NSObject {
    private weak var delegate: URLSessionDelegate?

    func createSession() -> URLSession {
        let queue = OperationQueue()
        queue.maxConcurrentOperationCount = 1
        return URLSession(
            configuration: .default,
            delegate: self,
            delegateQueue: queue
        )
    }
}