Golang : Login(Authenticate) with Facebook example

By today's Internet standard, if you want to create a website or app that allows user to login, it won't be complete without a Facebook login button.

In this tutorial, we will explore how to :

generate a Facebook Login URL string and bind a "Login with Facebook" button to the URL.
generate access token from the returned code after login successfully.
dump out all the data returned by Facebook (depending on the scope - email, userbirthday, userlocation, useraboutme).
get some data such as id, birthday, username and profile photo.

Please take note that, this tutorial uses couple of third party packages :

"github.com/golang/oauth2"

"github.com/antonholmquist/jason"

NOTE : Because I've executed this code in one of my Digital Ocean's droplet with an IP-address instead of domain name. I need to "fool" the Facebook Developers App Setting by setting the Mobile Site URL to http://<ip address>:8080/ at https://developers.facebook.com/apps/<app id>/settings/. You don't have to do this if you are executing the code in the domain name specified in the Facebook App Site URL setting.

NOTE : This tutorial assumes that you've setup your Facebook App and already have the AppID and Secret.

Here you go :

 package main

 import (
  "fmt"
  "github.com/antonholmquist/jason"
  "github.com/golang/oauth2"
  "net/http"
  "strconv"
  "strings"
 )

 type AccessToken struct {
  Token  string
  Expiry int64
 }

 func readHttpBody(response *http.Response) string {

  fmt.Println("Reading body")

  bodyBuffer := make([]byte, 5000)
  var str string

  count, err := response.Body.Read(bodyBuffer)

  for ; count > 0; count, err = response.Body.Read(bodyBuffer) {

 if err != nil {

 }

 str += string(bodyBuffer[:count])
  }

  return str

 }

 //Converts a code to an Auth_Token
 func GetAccessToken(client_id string, code string, secret string, callbackUri string) AccessToken {
  fmt.Println("GetAccessToken")
  //https://graph.facebook.com/oauth/access_token?client_id=YOUR_APP_ID&redirect_uri=YOUR_REDIRECT_URI&client_secret=YOUR_APP_SECRET&code=CODE_GENERATED_BY_FACEBOOK
  response, err := http.Get("https://graph.facebook.com/oauth/access_token?client_id=" +
 client_id + "&redirect_uri=" + callbackUri +
 "&client_secret=" + secret + "&code=" + code)

  if err == nil {

 auth := readHttpBody(response)

 var token AccessToken

 tokenArr := strings.Split(auth, "&")

 token.Token = strings.Split(tokenArr[0], "=")[1]
 expireInt, err := strconv.Atoi(strings.Split(tokenArr[1], "=")[1])

 if err == nil {
 token.Expiry = int64(expireInt)
 }

 return token
  }

  var token AccessToken

  return token
 }

 func Home(w http.ResponseWriter, r *http.Request) {
  w.Header().Set("Content-Type", "text/html; charset=utf-8")

  // generate loginURL
  fbConfig := &oauth2.Config{
 // ClientId: FBAppID(string), ClientSecret : FBSecret(string)
 // Example - ClientId: "1234567890", ClientSecret: "red2drdff6e2321e51aedcc94e19c76ee"

 ClientID: "", // change this to yours
 ClientSecret: "",
 RedirectURL:  "http://<domain name and don't forget port number if you use one>/FBLogin", // change this to your webserver adddress
 Scopes: []string{"email", "user_birthday", "user_location", "user_about_me"},
 Endpoint: oauth2.Endpoint{
 AuthURL:  "https://www.facebook.com/dialog/oauth",
 TokenURL: "https://graph.facebook.com/oauth/access_token",
 },
  }

  url := fbConfig.AuthCodeURL("")

  // Home page will display a button for login to Facebook

  w.Write([]byte("<html><title>Golang Login Facebook Example</title> <body> <a href='" + url + "'><button>Login with Facebook!</button> </a> </body></html>"))
 }

 func FBLogin(w http.ResponseWriter, r *http.Request) {
  // grab the code fragment

  w.Header().Set("Content-Type", "text/html; charset=utf-8")
  code := r.FormValue("code")

  ClientId := "" // change this to yours
  ClientSecret := ""
  RedirectURL := "http://<domain name and don't forget port number if you use one>/FBLogin"

  accessToken := GetAccessToken(ClientId, code, ClientSecret, RedirectURL)

  response, err := http.Get("https://graph.facebook.com/me?access_token=" + accessToken.Token)

  // handle err. You need to change this into something more robust
  // such as redirect back to home page with error message
  if err != nil {
 w.Write([]byte(err.Error()))
  }

  str := readHttpBody(response)

  // dump out all the data
  // w.Write([]byte(str))

  // see https://www.socketloop.com/tutorials/golang-process-json-data-with-jason-package
  user, _ := jason.NewObjectFromBytes([]byte(str))

  id, _ := user.GetString("id")
  email, _ := user.GetString("email")
  bday, _ := user.GetString("birthday")
  fbusername, _ := user.GetString("username")

  w.Write([]byte(fmt.Sprintf("Username %s ID is %s and birthday is %s and email is %s<br>", fbusername, id, bday, email)))

  img := "https://graph.facebook.com/" + id + "/picture?width=180&height=180"

  w.Write([]byte("Photo is located at " + img + "<br>"))
  // see https://www.socketloop.com/tutorials/golang-download-file-example on how to save FB file to disk

  w.Write([]byte("<img src='" + img + "'>"))
 }

 func main() {

  mux := http.NewServeMux()
  mux.HandleFunc("/", Home)
  mux.HandleFunc("/FBLogin", FBLogin)

  http.ListenAndServe(":8080", mux)
 }

Output :

References :

https://developers.facebook.com/

http://godoc.org/golang.org/x/oauth2

Golang : Login(Authenticate) with Facebook example

By Adam Ng

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