OIDExternalUserAgentASWebAuthenticationSession.swift.swift

import UIKit
import SafariServices
import AuthenticationServices
import AppAuth
import Reachability

class OIDExternalUserAgentASWebAuthenticationSession: NSObject, OIDExternalUserAgent {
    private let presentingViewController: UIViewController
    private var externalUserAgentFlowInProgress: Bool = false
    private var authenticationViewController: ASWebAuthenticationSession?
    
    private weak var session: OIDExternalUserAgentSession?
    
    init(with presentingViewController: UIViewController) {
        self.presentingViewController = presentingViewController
        super.init()
    }
    
    func present(_ request: OIDExternalUserAgentRequest, session: OIDExternalUserAgentSession) -> Bool {
        if externalUserAgentFlowInProgress {
            return false
        }
        guard let requestURL = request.externalUserAgentRequestURL() else {
            return false
        }
        self.externalUserAgentFlowInProgress = true
        self.session = session
       
        var openedUserAgent = false
        
        // ASWebAuthenticationSession doesn't work with guided access (Search web for "rdar://40809553")
        // Make sure that the device is not in Guided Access mode "(Settings -> General -> Accessibility -> Enable Guided Access)"
        if UIAccessibility.isGuidedAccessEnabled == false {
            let redirectScheme = request.redirectScheme()
            let authenticationViewController = ASWebAuthenticationSession(url: requestURL, callbackURLScheme: redirectScheme) { (callbackURL, error) in
                self.authenticationViewController = nil
                if let url = callbackURL {
                    self.session?.resumeExternalUserAgentFlow(with: url)
                } else {
                    let webAuthenticationError = OIDErrorUtilities.error(with: OIDErrorCode.userCanceledAuthorizationFlow,
                                                              underlyingError: error,
                                                              description: nil)
                    self.session?.failExternalUserAgentFlowWithError(webAuthenticationError)
                }
            }
            authenticationViewController.presentationContextProvider = self
            /// ** Key Line of code  -> `.prefersEphemeralWebBrowserSession`** allows for private browsing
            authenticationViewController.prefersEphemeralWebBrowserSession = true
            self.authenticationViewController = authenticationViewController
            openedUserAgent = authenticationViewController.start()
        } else {
            let webAuthenticationError = OIDErrorUtilities.error(with: OIDErrorCode.safariOpenError,
                                                      underlyingError: AuthenticationManager.AuthenticationError.appInGuidedAccessMode,
                                                      description: nil)
            self.session?.failExternalUserAgentFlowWithError(webAuthenticationError)
        }
        return openedUserAgent
    }
    
    func dismiss(animated: Bool, completion: @escaping () -> Void) {
        // Ignore this call if there is no authorization flow in progress.
        if externalUserAgentFlowInProgress == false {
            completion()
        }
        cleanUp()
        if authenticationViewController != nil {
            authenticationViewController?.cancel()
            completion()
        } else {
            completion()
        }
        return
    }
}

extension OIDExternalUserAgentASWebAuthenticationSession {
    /// Sets class variables to nil. Note 'weak references i.e. session are set to nil to avoid accidentally using them while not in an authorization flow.
    func cleanUp() {
        session = nil
        authenticationViewController = nil
        externalUserAgentFlowInProgress = false
    }
}

extension OIDExternalUserAgentASWebAuthenticationSession: ASWebAuthenticationPresentationContextProviding {
    func presentationAnchor(for session: ASWebAuthenticationSession) -> ASPresentationAnchor {
        return presentingViewController.view.window!
    }
}

Call Like this
let sceneDelegate = viewController.view.window?.windowScene?.delegate as? SceneDelegate
let externalAgent = OIDExternalUserAgentASWebAuthSession(with: viewController)
sceneDelegate?.currentAuthorizationFlow = OIDAuthState.authState(byPresenting: request, externalUserAgent: externalAgent, callback: { authState, error in
handleAuthState(authState, error, completion)
})

"prefersEphemeralWebBrowserSession" is only available on iOS 13 and later. Is there any other solution so it works on lower iOS versions?

Just add if #available(iOS 13, *) {...} around the line setting prefersEphemeralWebBrowserSession.

i mean is there any solution to make permission dialog not appearing on lower versions than iOS 13.

Depending on the scenario you could use SFSafariViewController as an external user agent, check the discussion here: openid/AppAuth-iOS#402