DownloadSequentially.swift

// Prompted by an article here: https://fluffy.es/download-files-sequentially/
// ... an alternative, lighter-weight approach to ensuring asynchronous downloads happen sequentially

let urls = [
    URL(string: "https://github.com/fluffyes/AppStoreCard/archive/master.zip")!,
    URL(string: "https://github.com/fluffyes/currentLocation/archive/master.zip")!,
    URL(string: "https://github.com/fluffyes/DispatchQueue/archive/master.zip")!,
    URL(string: "https://github.com/fluffyes/dynamicFont/archive/master.zip")!,
    URL(string: "https://github.com/fluffyes/telegrammy/archive/master.zip")!
]

func downloadSequentially(_ urls: [URL], completion: @escaping (URL?, URLResponse?, Error?) -> Void) {
    // Queues are automatically retained until all operations are completed
    // (see https://developer.apple.com/documentation/foundation/operationqueue)
    let queue = OperationQueue()
    queue.maxConcurrentOperationCount = 1

    for url in urls {
        queue.addOperation {
            let semaphore = DispatchSemaphore(value: 0)

            URLSession.shared.downloadTask(with: url, completionHandler: { (tempURL, response, error) in
                completion(tempURL, response, error)
                semaphore.signal()
            }).resume()

            semaphore.wait()
        }
    }
}

downloadSequentially(urls) { (tempUrl, response, error) -> Void in
    print("Downloaded to: \(tempUrl)")
}