//
//  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 }
}