FacebookAuth is for iOS app developers who need to support Facebook login but don't want to use the official SDK

FacebookAuth.swift

//
//  FacebookAuth.swift
//  GitHub: ethanhuang13
//  Twitter: @ethanhuang13

import AuthenticationServices
import SafariServices

/*
 Updated: 
 Pointed out by [@hal_lee on Twitter](https://twitter.com/hal_lee/status/1259132680828043265?s=21) this approach violates [Facebook Platform Policy 8.2](https://developers.facebook.com/policy/) and might be contacted by them to stop doing it.
 The best way to not using Facebook SDK is not to support Facebook Login at all.
 I leave the code here and let you make the decision.
 
 Original:
  
 FacebookAuth is for iOS app developers who need to support Facebook login but don't want to use the official SDK.
 iOS 11+ is required since it uses SFAuthenticationSession or ASWebAuthenticationSession.

 You need to setup the client id in Facebook Developer console, see https://github.com/fullstackreact/react-native-oauth/issues/76#issuecomment-335902057
 For more information, visit https://developers.facebook.com/docs/facebook-login/manually-build-a-login-flow/
 */

// MARK: Usage
/*
FacebookAuth.start(clientId: "12345678", scope: ["public_profile", "email"], window: UIApplication.shared.keyWindow) { result in
    switch result {
    case .success(let token):
        print(token)
        // TODO: Pass token to backend
    case .failure(let error):
        print(error.localizedDescription)
        // TODO: Handle error
    }
}
 */

// MARK: Implementation

enum FacebookAuth {
    enum AuthError: String, Error, LocalizedError {
        case invalidQuery
        case noURL
        case stateInconsistent
        case noAccessToken

        var errorDescription: String? { rawValue }
    }

    static func start(clientId: String, scope: [String], window: UIWindow?, completion: @escaping (Result<String, Error>) -> Void) {
        let state = UUID().uuidString
        var urlComponents = URLComponents()
        urlComponents.scheme = "https"
        urlComponents.host = "www.facebook.com"
        urlComponents.path = "/v7.0/dialog/oauth"
        urlComponents.queryItems = [
            "client_id": clientId,
            "response_type": "token",
            "redirect_uri": "fb\(clientId)://authorize",
            "state": state,
            "scope": scope.joined(separator: ",")]
            .map { URLQueryItem(name: $0.key, value: $0.value) }

        guard let url = urlComponents.url else {
            completion(.failure(AuthError.invalidQuery))
            return
        }
        let callbackURLScheme = "fb\(clientId)"
        var session: AuthenticationSession?

        let completionHandler: (URL?, Error?) -> Void = { url, error in
            session = nil

            print("auth callback url: \(url?.absoluteString ?? "failed")")
            if let error = error {
                completion(.failure(error))
                return
            }
            guard let url = url else {
                completion(.failure(AuthError.noURL))
                return
            }
            var urlComponents = URLComponents(url: url, resolvingAgainstBaseURL: false)
            urlComponents?.query = url.fragment

            guard let returnedState = urlComponents?.queryItems?.first(where: { $0.name == "state" })?.value,
                returnedState == state else {
                    completion(.failure(AuthError.stateInconsistent))
                    return
            }
            guard let token = urlComponents?.queryItems?.first(where: { $0.name == "access_token" })?.value else {
                completion(.failure(AuthError.noAccessToken))
                return
            }
            completion(.success(token))
        }

        if #available(iOS 12.0, *) {
            session = ASWebAuthenticationSession(url: url, callbackURLScheme: callbackURLScheme, completionHandler: completionHandler)
            if #available(iOS 13.0, *) {
                (session as? ASWebAuthenticationSession)?.presentationContextProvider = window
            }
        } else {
            session = SFAuthenticationSession(url: url, callbackURLScheme: callbackURLScheme, completionHandler: completionHandler)
        }
        _ = session?.start()
    }
}

// MARK: Extensions

protocol AuthenticationSession {
    func start() -> Bool
}

extension SFAuthenticationSession: AuthenticationSession { }

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

extension UIWindow: ASWebAuthenticationPresentationContextProviding {
    @available(iOS 13.0, *)
    public func presentationAnchor(for session: ASWebAuthenticationSession) -> ASPresentationAnchor { self }
}

LICENSE

MIT License

Copyright (c) 2020 Ethan Huang

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.

By the way, I've got a lot of inspiration from my friend @yesleon https://gist.github.com/yesleon/904d8fe418a17467aada5b8661dba665

LICENSE added. Use the code whatever you want. Thanks.

Pointed out by @hal_lee on Twitter this approach violates Facebook Platform Policy 8.2 and might be contacted by Facebook to stop doing it.
The best way to not using Facebook SDK is not to support Facebook Login at all.
I leave the code here and let you make the decision.

If you still need to use FBSDK but are considering building a "kill switch", you might want to check FacebookAutoInitEnabled in the official document for how not to auto init.

Pointed out by @hal_lee on Twitter this approach violates Facebook Platform Policy 8.2 and might be contacted by Facebook to stop doing it.

Link is move to Developer Policies 6.2