Facebook Login using FB Login Kit for Swift

FBLogin.swift

import UIKit
import MBProgressHUD

import FacebookCore
import FacebookLogin
import FBSDKCoreKit

@IBAction func facebookAction(_ sender: UIButton) {
    showHUDWithMessage("Facebook Login")
    if let accessToken = AccessToken.current {
      debugPrint("Already Logged in - accessToken", accessToken)
      self.graphRequestForME({ (result) in })
    } else {
      let loginManager = LoginManager()
      loginManager.logIn([ .publicProfile, .userFriends, .email ], viewController: self) { [weak self] loginResult in
        guard let strongSelf = self else { return }
        mainQueue() {
          switch loginResult {
          case .failed(let error):
            debugPrint(error)
            strongSelf.progressHUD.showFailureMessage("Login Failed")
            let result = Result<User>.failure(error)
            strongSelf.delegate?.facebookLogin(result)
          case .cancelled:
            debugPrint("FB Login User cancelled")
            strongSelf.progressHUD.showFailureMessage("Login cancelled")
            let error = LoginError.cancelled
            let result = Result<User>.failure(error)
            strongSelf.delegate?.facebookLogin(result)
          case .success(let grantedPermissions, let declinedPermissions, let accessToken):
            debugPrint("---- FB Permissions ----")
            debugPrint("FB grantedPermissions", grantedPermissions)
            debugPrint("FB declinedPermissions", declinedPermissions)
            
            strongSelf.user.accessToken = accessToken
            strongSelf.user.facebookID = accessToken.userId
            strongSelf.graphRequestForME({ (result) in })
          } // switch loginResult
        }
      }
    }
  } // IBAction
  
  func graphRequestForME(_ callback: @escaping (Result<User>) -> Void) {
    FBSDKGraphRequest(graphPath: "me",
      parameters: ["fields": "id, name, first_name, last_name, email, picture.width(720).height(720)"]).start(completionHandler: { [weak self] (connection, result, error) in
        guard let strongSelf = self else { return }
        mainQueue() {
          if error == nil {
            strongSelf.progressHUD.showSucessMessage("Facebook Login Sucess")
            guard let currentAccessToken = FBSDKAccessToken.current() else { return }
            strongSelf.user = strongSelf.user.userFromGraphResult(result as! NSDictionary)
            strongSelf.user.tokenString = currentAccessToken.tokenString
            let result = Result.success(strongSelf.user)
            strongSelf.delegate?.facebookLogin(result)
          } else {
            strongSelf.progressHUD.showFailureMessage("Facebook Login Failed")
            delay(1) {
              let result = Result<User>.failure(error!)
              strongSelf.delegate?.facebookLogin(result)
            }
          } // if error
        } // mainQueue
    })
  }