Expands a URL by doing a HEAD request against its URL and returning the redirected URL, or the original if the URL wasn't shortened. You can call `expand(url:)` multiple times and the queue guarantees that only 1 request will be triggered at a time, in calling order.

URLExpander.swift

import Foundation

final class URLExpander: NSObject, URLSessionTaskDelegate {

    private lazy var session: URLSession = {
        let config = URLSessionConfiguration.default
        return URLSession(configuration: config, delegate: self, delegateQueue: queue)
    }()

    private let queue: OperationQueue
    private var handlers: [URL: (Result<URL, Error>) -> Void] = [:]

    override init() {
        queue = OperationQueue()
        queue.maxConcurrentOperationCount = 1
    }

    func expand(url: URL, completion: @escaping (Result<URL, Error>) -> Void) {
        handlers[url] = completion
        var request = URLRequest(url: url)
        request.httpMethod = "HEAD"
        request.cachePolicy = .returnCacheDataElseLoad
        request.allowsCellularAccess = true
        session.dataTask(with: request).resume()
    }

    func urlSession(_ session: URLSession, task: URLSessionTask, willPerformHTTPRedirection response: HTTPURLResponse, newRequest request: URLRequest, completionHandler: @escaping (URLRequest?) -> Void) {
        defer {
            completionHandler(nil)
        }

        guard let shortened = task.currentRequest?.url,
            let handler = handlers[shortened],
            let expanded = request.url else {
                return
        }

        handler(.success(expanded))
        handlers[shortened] = nil
    }

    func urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: Error?) {
        guard let shortened = task.currentRequest?.url,
            let handler = handlers[shortened] else {
                return
        }

        if let error = error {
            handler(.failure(error))
        } else {
            // the url wasn't shortened, we can just return it
            handler(.success(shortened))
        }

        handlers[shortened] = nil
    }

}