A wrapper around ASWebAuthenticationSession and SFAuthenticationSession.

UIWindow+AuthenticationSession.swift

// 
// Usage: 
// 
// Facebook:
// 
// 1. Follow this guide (WITHOUT setting up a URL scheme in Xcode): 
// https://github.com/fullstackreact/react-native-oauth/issues/76#issuecomment-335902057
// 
// 2. call startFacebookAuthenticationSession(appID:completionHandler:) on a window:
// 
// view.window?.startFacebookAuthenticationSession(appID: "{app-id}") {
//     switch $0 {
//     case .success(let token):
//         print(token)
//     case .failure(let error):
//         print(error)
//     }
// }
// 
// 
// Dropbox: 
// 
// view.window?.startDropboxAuthenticationSession(
//     appKey: "{app-id}",
//     callbackURI: "{redirect-uri}"
// ) {
//     switch $0 {
//     case .success(let token):
//         print(token)
//     case .failure(let error):
//         print(error)
//     }
// }


import UIKit
import AuthenticationServices
import SafariServices



@available(iOS 11.0, *)
extension UIWindow: ASWebAuthenticationPresentationContextProviding {
    
    // MARK: - Core methods
    
    @available(iOS 13.0, *)
    public func presentationAnchor(for session: ASWebAuthenticationSession) -> ASPresentationAnchor {
        self
    }
    
    // Unified interface for ASWebAuthenticationSession and SFAuthenticationSession.
    func startAuthenticationSession(
        url: URL,
        callbackURLScheme: String?,
        completionHandler: @escaping (URL?, Error?) -> Void
    ) {
        
        if #available(iOS 12.0, *) {
            
            var session: ASWebAuthenticationSession?
            session = ASWebAuthenticationSession(url: url, callbackURLScheme: callbackURLScheme) { url, error in
                completionHandler(url, error)
                session = nil
            }
            if #available(iOS 13.0, *) {
                session?.presentationContextProvider = self
            }
            session?.start()
        } else {
            
            var session: SFAuthenticationSession?
            session = SFAuthenticationSession(url: url, callbackURLScheme: callbackURLScheme) { url, error in
                completionHandler(url, error)
                session = nil
            }
            session?.start()
        }
    }
    
    func startImplicitGrantFlowAuthenticationSession(
        endpoint: URL,
        clientID: String,
        callbackURI: String,
        completionHandler: @escaping (Result<String, Error>) -> Void
    ) {
        
        let state = UUID().uuidString
        var urlComponents = URLComponents(url: endpoint, resolvingAgainstBaseURL: true)!
        urlComponents.queryItems = [
            .init(name: "client_id", value: clientID),
            .init(name: "response_type", value: "token"),
            .init(name: "redirect_uri", value: callbackURI),
            .init(name: "state", value: state)
        ]
        let callbackURLScheme = String(callbackURI.prefix(while: { $0 != ":" }))
        
        startAuthenticationSession(url: urlComponents.url!, callbackURLScheme: callbackURLScheme) { url, error in
            if let error = error {
                completionHandler(.failure(error))
            } else if let url = url {
                var urlComponents = URLComponents(string: url.absoluteString)
                let query = urlComponents?.fragment
                urlComponents?.query = query
                let queryItems = urlComponents?.queryItems
                guard queryItems?.first(where: { $0.name == "state" })?.value == state else { fatalError() }
                let token = queryItems?.first(where: { $0.name == "access_token" })?.value
                completionHandler(.success(token!))
            }
        }
    }
    
    // MARK: - Convenience methods
    
    func startDropboxAuthenticationSession(
        appKey: String,
        callbackURI: String,
        completionHandler: @escaping (Result<String, Error>) -> Void
    ) {
        
        startImplicitGrantFlowAuthenticationSession(
            endpoint: URL(string: "https://www.dropbox.com/oauth2/authorize")!,
            clientID: appKey,
            callbackURI: callbackURI,
            completionHandler: completionHandler
        )
    }
    
    func startFacebookAuthenticationSession(
        appID: String,
        completionHandler: @escaping (Result<String, Error>) -> Void
    ) {
        
        startImplicitGrantFlowAuthenticationSession(
            endpoint: URL(string: "https://www.facebook.com/v7.0/dialog/oauth")!,
            clientID: appID,
            callbackURI: "fb\(appID)://authorize",
            completionHandler: completionHandler
        )
    }
    
}