iOS Authentication Session for iOS 11/12

auth.swift

import Foundation
import SafariServices
import AuthenticationServices

private protocol AuthenticationSessionProvider {
    func start() -> Bool
    func cancel()
}

extension SFAuthenticationSession: AuthenticationSessionProvider {}

@available(iOS 12.0, *)
extension ASWebAuthenticationSession: AuthenticationSessionProvider {}

class AuthenticationSession: NSObject {

    private var session: AuthenticationSessionProvider?

    private let completion: ((URL?, Error?) -> Void)

    init(url: URL, callbackSchemeURL: String?, completion: @escaping (URL?, Error?) -> Void) {

        if #available(iOS 12.0, *) {
            session = ASWebAuthenticationSession(url: url, callbackURLScheme: callbackSchemeURL, completionHandler: completion)
        } else {
            session = SFAuthenticationSession(url: url, callbackURLScheme: callbackSchemeURL, completionHandler: completion)
        }

        self.completion = completion
    }

    func start() -> Bool {
        return session?.start() ?? false
    }

    func cancel() {
        session?.cancel()
    }
}