Facebook login alternative to avoid issues with IDFA

FacebookSignInHelper.swift

import Foundation
import AuthenticationServices
import SafariServices
import FacebookCore

// This helper is a workaroung for the issue in https://github.com/facebook/facebook-ios-sdk/issues/2387
// Starting with new FBSDK 17.0.0, the login method is not working as expected when the user has not given IDFA consent
// The workaround is to use ASWebAuthenticationSession to open the SafariViewController to perform the login manually
public class FacebookSignInHelper: NSObject, ASWebAuthenticationPresentationContextProviding {
    private static var shared: FacebookSignInHelper?
    private let appID: String
    private let redirectURI: String
    private var completionHandler: ((Result<(accessToken: String, grantedPermissions: [String]), Error>) -> Void)?
    private var permissions: [String]
    
    public static func login(
        permissions: [String] = ["public_profile", "email"],
        from viewController: UIViewController,
        completion: @escaping (Result<(accessToken: String, grantedPermissions: [String]), Error>) -> Void
    ) {        
        guard shared == nil else {
            completion(.failure(NSError(domain: "FacebookLogin", code: -1, userInfo: [NSLocalizedDescriptionKey: "Already logging in"])))
            return
        }
        
        let appID = FacebookCore.Settings.shared.appID ?? ""
        let redirectURI = "fb\(appID)"
        shared = FacebookSignInHelper(appID: appID, redirectURI: redirectURI, permissions: permissions)
        shared?.login(from: viewController) { result in
            shared = nil
            completion(result)
        }
    }
    
    private init(appID: String, redirectURI: String, permissions: [String]) {
        self.appID = appID
        self.redirectURI = redirectURI
        self.permissions = permissions
        super.init()
    }
    
    private func login(from viewController: UIViewController, completion: @escaping (Result<(accessToken: String, grantedPermissions: [String]), Error>) -> Void) {
        self.completionHandler = completion
        guard let authURL = constructFacebookAuthURL() else {
            completion(.failure(NSError(domain: "FacebookLogin", code: -1, userInfo: [NSLocalizedDescriptionKey: "Failed to construct Facebook auth URL"])))
            return
        }
        let session = ASWebAuthenticationSession(url: authURL, callbackURLScheme: redirectURI) { callbackURL, error in
            self.handleAuthResult(callbackURL: callbackURL, error: error)
        }
        session.presentationContextProvider = self
        session.start()
    }
    
    private func constructFacebookAuthURL() -> URL? {
        guard var components = URLComponents(string: "https://www.facebook.com/v21.0/dialog/oauth") else {
            return nil
        }
        components.queryItems = [
            URLQueryItem(name: "client_id", value: appID),
            URLQueryItem(name: "display", value: "touch"),
            URLQueryItem(name: "return_scopes", value: "true"),
            URLQueryItem(name: "redirect_uri", value: "\(redirectURI)://authorize"),
            URLQueryItem(name: "response_type", value: "token"),
            URLQueryItem(name: "scope", value: permissions.joined(separator: ",")),
            URLQueryItem(name: "auth_type", value: "rerequest")
        ]
        return components.url
    }
    
    private func handleAuthResult(callbackURL: URL?, error: Error?) {
        if let error = error {
            completionHandler?(.failure(error))
            return
        }
        
        guard let callbackURL = callbackURL,
              let fragment = callbackURL.fragment else {
            completionHandler?(.failure(NSError(domain: "FacebookLogin", code: -1, userInfo: [NSLocalizedDescriptionKey: "Invalid callback URL"])))
            return
        }
        
        let params = fragment.components(separatedBy: "&")
            .map { $0.components(separatedBy: "=") }
            .reduce(into: [String: String]()) { result, param in
                if param.count == 2 {
                    result[param[0]] = param[1].removingPercentEncoding
                }
            }
        if let accessToken = params["access_token"], let grantedScopes = params["granted_scopes"]?.split(separator: ",").map(String.init) {
            completionHandler?(.success((accessToken: accessToken, grantedPermissions: grantedScopes)))
        } else if let error = params["error"] {
            completionHandler?(.failure(NSError(domain: "FacebookLogin", code: -1, userInfo: [NSLocalizedDescriptionKey: error])))
        } else {
            completionHandler?(.failure(NSError(domain: "FacebookLogin", code: -1, userInfo: [NSLocalizedDescriptionKey: "Unknown error occurred"])))
        }
    }
    
    public func presentationAnchor(for session: ASWebAuthenticationSession) -> ASPresentationAnchor {
        UIApplication.shared.windows[0]
    }
}