Google Drive - Set Authorizer and User

GoogleDriveViewControllerAuthorizer.swift

import GTMSessionFetcher

extension ViewController: GIDSignInDelegate, GIDSignInUIDelegate {
    func sign(_ signIn: GIDSignIn!, didSignInFor user: GIDGoogleUser!, withError error: Error!) {
        // A nil error indicates a successful login
        if error == nil {
            // Include authorization headers/values with each Drive API request.
            self.googleDriveService.authorizer = user.authentication.fetcherAuthorizer()
            self.googleUser = user
        } else {
            self.googleDriveService.authorizer = nil
            self.googleUser = nil
        }
        // ...        
    }
}