Question

I created a sinatra based facebook app on Heroku by initiating it with the feature provided by Facebook (Get Cloud App or something like that). Everything worked fine, but when I tried to use the Scores from Facebook, I realized that the Facebook API used by Heroku (which is Mogli) does not support Scores.
So I tried to rewrite the code to use fb_graph. But since that created more confusion than everything I started with an empty file. But I just can’t make it work.

So basically I would like a simple example for a Sinatra based app on heroku which uses fb_graph. The samples should only contain authentication, since everything else seems pretty straight forward and has a lot of documentation. Just the initial authentication just doesn’t work.

I would like to use the default oauth2 approach with the callback, since it just feels more natural to me, but I’m open for everything. I know there is a Rails example, but I just can’t get my head around it and since my app will be very simple Rails seems a bit overpowered.

It would be great if anyone could give me just the few lines necessary for what I would like to do!

Answer 1

I’ve found this to be a bit of a mess. Old tutorials and code just don’t make sense anymore.

You can use the Heroku + Facebook OmniAuth example to do an external website (not a canvas app) that does server-side Facebook (and many others) authentication.

When you do that, OmniAuth will provide you with the token you need you need to pass to fb_graph. In the provided example you could add another url:

get '/me' do
  me = FbGraph::User.me(session['fb_token']).fetch
  "Hello " + me.name
end

For canvas apps, I added the following code to ‘rack-facebook-request.rb’ to my repo based on this gist

require 'base64'
require 'openssl'
require 'json'

# This is inspired by [rack-facebook-signed-request](https://github.com/gamesthatgive/rack-facebook-signed-request)
#
# Usage
#
#     use Rack::FBSignedRequest, :secret => 'SECRET'
#
class Rack::FBSignedRequest
  def initialize(app, options)
    @app = app
    @options = options
  end

  def call(env)
    @request = Rack::Request.new(env)
    if @request.POST['signed_request']
      if facebook_params = parse_signed_request(@request.params['signed_request'])
        @request.params['facebook_params'] = facebook_params
        env['rack.request.query_hash'] = @request.params
        env['REQUEST_METHOD'] = 'GET'
        puts 'Valid signed request. Changed REQUEST_METHOD to GET.'

        if facebook_params['user_id']
          env['fb_user_id'] = facebook_params['user_id']
          env['fb_access_token'] = facebook_params['oauth_token']
          puts 'Request has been authorized.'
        else
          puts 'Request is not authorized.'
        end

      else
        puts 'Not a valid signed request'
      end
    else
       puts 'Not a signed_request'
    end

    @app.call(env)
  end

  private

  # The following code from omniauth

  def parse_signed_request(value)
    signature, encoded_payload = value.split('.')

    decoded_hex_signature = base64_decode_url(signature)
    decoded_payload = JSON(base64_decode_url(encoded_payload))

    unless decoded_payload['algorithm'] == 'HMAC-SHA256'
      raise NotImplementedError, "unkown algorithm: #{decoded_payload['algorithm']}"
    end

    if valid_signature?(@options[:secret], decoded_hex_signature, encoded_payload)
      decoded_payload
    end
  end

  def valid_signature?(secret, signature, payload, algorithm = OpenSSL::Digest::SHA256.new)
    OpenSSL::HMAC.digest(algorithm, secret, payload) == signature
  end

  def base64_decode_url(value)
    value += '=' * (4 - value.size.modulo(4))
    Base64.decode64(value.tr('-_', '+/'))
  end
end

This will give you the current fb_user_id and fb_access_token if your app is authorised. Remember that you have to use javascript or a link in the iFrame to request app authorisation in this case.