Archive for November 2017

Background thread URL download in Swift 4

I was recently working on a project in swift, and was not able to quickly find a way to use a background thread to do a URL download in Swift 4. There were plenty of snippets and tutorials out there, but they were all for older versions of Swift. (BTW Swift, you have to stop it with these breaking changes.)

So here is what I cobbled together to download from the OpenAQ web site, which has an open API to provide air quality data.

The Swift class that does the downloading and processing of the data from the site is this class, which I called OpenAQ.swift:

//  OpenAQ.swift
//  openaq
//  Created by Brian Prescott on 11/16/17.
//  Copyright © 2017 Brian Prescott. All rights reserved.
import UIKit
class OpenAQResult: NSObject {
    var city: String = ""
    var count: Int = 0
    var country: String = ""
    var locations: Int = 0
protocol OpenAQDelegate: class {
    func didFinish(sender: Any)
    func didError(sender: Any)
class OpenAQ: NSObject {
    weak var delegate:OpenAQDelegate?
    var returnValue: [OpenAQResult] = []
    func readDataBackground() -> Void {
        let urlString = ""
        guard let url = URL(string: urlString) else { return }
        URLSession.shared.dataTask(with: url) { (data, res, err) in
            guard let data = data else {
            do {
                let json = try JSONSerialization.jsonObject(with: data, options: []) as! [String: Any]
                let resultsArray = json["results"] as! NSArray
                var retval: [OpenAQResult] = []
                for x in resultsArray {
                    let dict = x as! NSDictionary
                    let res = OpenAQResult()
           = dict.value(forKey: "city") as! String
                    res.count = dict.value(forKey: "count") as! Int
           = dict.value(forKey: "country") as! String
                    res.locations = dict.value(forKey: "locations") as! Int
                    if (res.count >= 10000) {
                self.returnValue = retval.sorted(by: { $0.count > $1.count })
                self.delegate?.didFinish(sender: self)
            } catch {
                print("An error occurred")

So then to use this class, here is a template of what your main view controller (or wherever you want to use it) would look like:

class MasterViewController: UITableViewController {
    var detailViewController: DetailViewController? = nil
    var objects = [OpenAQResult]()
    var openAQ: OpenAQ? = nil
    override func viewDidLoad() {
        openAQ = OpenAQ()
        openAQ?.delegate = self
        // do the rest of your initialization
    // the rest of your class would go here
extension MasterViewController: OpenAQDelegate {
    func didError(sender: Any) {
        print ("An error occurred!!!")
    func didFinish(sender: Any) {
        objects = (sender as! OpenAQ).returnValue
        if (objects.count == 0) {
            let alert = UIAlertController(title: "Error", message: "The data could not be loaded. Please establish an internet connection and press the Refresh button above.", preferredStyle: UIAlertControllerStyle.alert)
            alert.addAction(UIAlertAction(title: "OK", style: UIAlertActionStyle.default, handler: nil))
            self.present(alert, animated: true, completion: nil)
        DispatchQueue.main.async {
            // do something here
            // for example, reload a table view

In the code above, when the didFinish method is called, the objects instance variable is retrieved from the openAQ variable, and you can then do something with the data you just retrieved.

BTW, Happy Thanksgiving to all my U.S. readers. It has been a very trying year personally and all around, but we still have so much to be thankful for.