OAuth-ViewController.swift

import UIKit
import OctoKit
import AuthenticationServices

extension URL {
    /// Returns a new URL by adding the query items, or nil if the URL doesn't support it.
    /// URL must conform to RFC 3986.
    func appending(_ queryItems: [URLQueryItem]) -> URL? {
        guard var urlComponents = URLComponents(url: self, resolvingAgainstBaseURL: true) else {
            // URL is not conforming to RFC 3986 (maybe it is only conforming to RFC 1808, RFC 1738, and RFC 2732)
            return nil
        }
        // append the query items to the existing ones
        urlComponents.queryItems = (urlComponents.queryItems ?? []) + queryItems

        // return the url from new url components
        return urlComponents.url
    }
}

class ViewController: UIViewController, ASWebAuthenticationPresentationContextProviding {
    var tokenConfig : TokenConfiguration? = nil
    let config = OAuthConfiguration.init(token: "", secret: "", scopes: ["repo", "read:org"])
    var webAuthSession: ASWebAuthenticationSession?

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.
        
        let callbackUrlScheme = "octoissue"
        var url = config.authenticate()?.appending([URLQueryItem(name: "redirect_uri", value: "octoissue://tracker")])

        webAuthSession = ASWebAuthenticationSession.init(url: url!, callbackURLScheme: callbackUrlScheme, completionHandler: { (callBack:URL?, error:Error?) in

            // handle auth response
            guard error == nil, let successURL = callBack else {
                return
            }

            self.config.handleOpenURL(url: successURL) { config in
                self.loadCurrentUser(config: config) // purely optional of course
            }
        })

         // Kick it off
         webAuthSession?.presentationContextProvider = self
         webAuthSession?.start()
    }

    func presentationAnchor(for session: ASWebAuthenticationSession) -> ASPresentationAnchor {
        return self.view.window ?? ASPresentationAnchor()
    }

    func loadCurrentUser(config: TokenConfiguration) {
      Octokit(config).me() { response in
        switch response {
        case .success(let user):
            print(user.login, user.email, user.name)
        case .failure(let error):
          print(error)
        }
      }
    }

}