Github based auth2 class

auth2.swift

class Authentication: NSObject, KnowsRootViewController, SFSafariViewControllerDelegate {
    
    static let shared = Authentication()
    
    private static let clientId: String         = "666"
    private static let clientSecret: String     = "777777"
    private static let loginURL                 = URL(string: "https://github.com/login/oauth/authorize?scope=user:email&client_id=\(Authentication.clientId)")!
    private static let logoutURL                = URL(string: "https://github.com/logout")!
    
    private let kToken: String          = "ACCESS_TOKEN"
    private let tokenURL                = "https://github.com/login/oauth/access_token"
    private let logoutURL               = "https://github.com/logout"
    private weak var safariVC: SFSafariViewController?
    
    private var dataTask: URLSessionDataTask?
    // TODO: We should store it locally somewhere safe such as keychain
    private(set) var accessToken: String? {
        
        set {
            
            UserDefaults.standard.set(newValue, forKey: self.kToken)
            UserDefaults.standard.synchronize()
        }
        
        get {
            
            assert(Thread.isMainThread)
            if isSigningIn { return nil }
            
            let result = UserDefaults.standard.string(forKey: self.kToken)

            if result == nil {
                self.login()
                isSigningIn = true
            }
            
            return result
        }
    }
    
    private var isSigningIn: Bool = false

    private override init() {}
    
    @objc func didCancelAuth() {
        
        closeLoginView()
    }
    
    private func closeLoginView() {
        
        DispatchQueue.main.async { [weak self] _ in
            self?.safariVC?.dismiss(animated: true, completion: nil)
            self?.isSigningIn = false
        }
    }
    
    private func presentSafariWithURL(url: URL) {
        
        assert(Thread.isMainThread)
        
        let vc = SFSafariViewController(url: url)
        safariVC = vc
        safariVC?.delegate = self
        
        let cancelButton = UIBarButtonItem(barButtonSystemItem: .cancel,
                                           target: nil,
                                           action: #selector(Authentication.didCancelAuth))
        
        vc.navigationItem.rightBarButtonItem = cancelButton
        
        rootViewController.present(vc,
                                   animated: true,
                                   completion: nil)
    }
    
    private func login() {
        
        presentSafariWithURL(url: Authentication.loginURL)
    }
    
    // MARK: API
    
    func logout(completion: @escaping (Bool) -> Void) {
        
        assert(Thread.isMainThread)

        presentSafariWithURL(url: URL(string: logoutURL)!)
        
        accessToken = nil
    }
    
    func getTokenWith(url: URL, completion: @escaping (Bool) -> Void) {
        
        let code: String
        if let validCode = url.queryParameters?["code"] {
            code = validCode
        } else {
            completion(false)
            return
        }
        
        let url = URL(string: tokenURL)!
        var request = URLRequest(url: url,
                                 cachePolicy: .reloadIgnoringCacheData,
                                 timeoutInterval: 5000)
        request.httpMethod = "POST"
        let postString = "client_id=\(Authentication.clientId)&client_secret=\(Authentication.clientSecret)&code=\(code)&accept=json"
        request.httpBody = postString.data(using: .utf8)
        
        let session = URLSession(configuration: .default,
                                 delegate: nil,
                                 delegateQueue: nil)
        
        if dataTask?.state == .running ||
            dataTask?.state == .suspended {
            dataTask?.cancel()
        }
        
        dataTask =  session.dataTask(with: request) { [weak self] (data, response, error) in
            
            // TODO: Manage all the error cases
            guard let data = data else {
                return
            }
            
            self?.accessToken = String(data: data, encoding: .utf8)?.components(separatedBy: "=")[1].components(separatedBy: "&")[0]
            self?.closeLoginView()
            completion(true)
        }
        
        dataTask?.resume()
    }
    
    func safariViewControllerDidFinish(_ controller: SFSafariViewController) {
        
        isSigningIn = false
    }
}