package main

import (
  "github.com/go-martini/martini"
  "github.com/martini-contrib/oauth2"
  "github.com/martini-contrib/sessions"
)

func main() {
  m := martini.Classic()
  m.Use(sessions.Sessions("my_session", sessions.NewCookieStore([]byte("secret123"))))
  m.Use(oauth2.Google(&oauth2.Options{
    ClientId:      "client_id",
    ClientSecret:  "client_secret",
    RedirectURL:   "redirect_url",
    Scopes:        []string{"https://www.googleapis.com/auth/drive"},
    LoginCallback: func(tokens oauth2.Tokens, t *oauth.Transport, s sessions.Session, w http.ResponseWriter, r *http.Request) {
    	fmt.Printf("access token: %s", tokens.Access())
	// Do whatever you want with session, or http stuff like sending additional header/cookies, 
	// or read additional information from request
    },
  }))

  // Tokens are injected to the handlers
  m.Get("/", func(tokens oauth2.Tokens) string {
    if tokens.IsExpired() {
      return "not logged in, or the access token is expired"
    }
    return "logged in"
  })

  // Routes that require a logged in user
  // can be protected with oauth2.LoginRequired handler.
  // If the user is not authenticated, they will be
  // redirected to the login path.
  m.Get("/restrict", oauth2.LoginRequired, func(tokens oauth2.Tokens) string {
    return tokens.Access()
  })

  m.Run()
}

m.Get("/login-required", oauth2.LoginRequired, func() ...)

oauth2.PathLogin = "/oauth2login"
oauth2.PathLogout = "/oauth2logout"
...