The facebook-oauth2 app demonstrates:

• Using the goauth2 library to fetch json information on the logged-in Facebook user.

Here are the contents of the app:

facebook-oauth2/app/
	models
		user.go   # User struct and in-memory data store
	controllers
		app.go    # All code

Browse the code on Github

OAuth2 Overview

The entire OAuth process is governed by this configuration:

var FACEBOOK = &oauth.Config{
	ClientId:     "95341411595",
	ClientSecret: "8eff1b488da7fe3426f9ecaf8de1ba54",
	AuthURL:      "https://graph.facebook.com/oauth/authorize",
	TokenURL:     "https://graph.facebook.com/oauth/access_token",
	RedirectURL:  "http://loisant.org:9000/Application/Auth",
}

Here’s an overview of the process:

1. The app sends the user to AuthURL.
2. While there, the user agrees to the authorization.
3. Facebook sends the user back to RedirectURL, adding a parameter code.
4. The app retrieves an OAuth access token from TokenURL using the code.
5. The app subsequently uses the access token to authenticate web service requests.

Code walk

Let’s take a look at the first bit of code:

func (c Application) Index() revel.Result {
	u := c.connected()
	me := map[string]interface{}{}
	if u != nil && u.AccessToken != "" {
		// Use the access token to request user info
		...
	}

	authUrl := FACEBOOK.AuthCodeURL("foo")
	return c.Render(me, authUrl)
}

It doesn’t do much since we don’t have an access token yet. All it does is generate an Authorization URL. (“foo” is the “state”, which is a parameter that facebook propagates back to us as a parameter to the RedirectURL. We do not need to use it here.)

Here’s the interesting bit of the template:

{{if .me}}
<h3>You're {{.me.name}} on Facebook</h3>
{{else}}
<a href="{{.authUrl}}">login</a>
{{end}}

If we had information on the user, we would tell them their name. Since we don’t, we just ask the user to log in to Facebook.

Assuming the user does so, the next time we see them is when Facebook sends them to Auth:

func (c Application) Auth(code string) revel.Result {
	t := &oauth.Transport{Config: FACEBOOK}
	tok, err := t.Exchange(code)
	if err != nil {
		revel.ERROR.Println(err)
		return c.Redirect(Application.Index)
	}

	user := c.connected()
	user.AccessToken = tok.AccessToken
	return c.Redirect(Application.Index)
}

The t.Exchange(code) bit makes a request to the TokenURL to get the access token. If successful, we store it on the user. Either way, the user ends up back at Index:

func (c Application) Index() revel.Result {
	u := c.connected()
	me := map[string]interface{}{}
	if u != nil && u.AccessToken != "" {
		resp, _ := http.Get("https://graph.facebook.com/me?access_token=" +
			url.QueryEscape(u.AccessToken))
		defer resp.Body.Close()
		if err := json.NewDecoder(resp.Body).Decode(&me); err != nil {
			revel.ERROR.Println(err)
		}

Now we have an AccessToken, so we make a request to get the associated user’s information. The information gets returned in JSON, so we decode it into a simple map and pass it into the template.