https://www.hackingwithswift.com/read/9/5/easy-gcd-using-performselectorinbackground

The end of project 9 separates the fetchJSON code into an objc function and you call it in viewDidLoad with:

override func viewDidLoad() {
  performSelector(inBackground: #selector(fetchJSON), with: nil)
}

    @objc func fetchJSON() {
        let urlSting: String
         if navigationController?.tabBarItem.tag == 0 { 
            urlSting = "https://www.hackingwithswift.com/samples/petitions-1.json"
        } else {
            urlSting = "https://api.whitehouse.gov/v1/petitions.json?signatureCountFloor=10000&limit=100"
        }

        if let url = URL(string: urlSting)  {
            if let data = try? Data(contentsOf: url){
                parse(json: data)
                return
            }
        }
        performSelector(onMainThread: #selector(showError), with: nil, waitUntilDone: false)
    }

This throws the warning: "-[UINavigationController tabBarItem] must be used from main thread only" on

navigationController?.tabBarItem.tag == 0

My quick solution was:

override func viewDidLoad() {

      let urlString: String
      if navigationController?.tabBarItem.tag == 0 {
           urlString = "https://www.hackingwithswift.com/samples/petitions-1.json"
        } else {
            urlString = "https://api.whitehouse.gov/v1/petitions.json?signatureCountFloor=10000&limit=100"
        }

        DispatchQueue.global(qos: .userInteractive).async{ [weak self] in
            if let url = URL(string: urlString){
                if let data = try? Data(contentsOf: url){
                    self?.parse(json: data)
                    return
                }
            }
            DispatchQueue.main.async { [weak self] in
                self?.showError()
            }
        }
}