A small Sinatra app that can be used to catch a service provider callback (e. g. from Facebook, Twitter or Google) using OmniAuth.

ProviderCallbackApp.rb

require 'sinatra'
require 'omniauth'

# A small Sinatra app that can be used to catch a service provider
# callback (e. g. from Facebook, Twitter or Google) using OmniAuth. 
# It can be mounted in the routes. This Sinatra app can be useful 
# if you have a Rails API app without any cookies / sessions.
#
# Examples
#
#   # Mount the app in Rails' routes file
#   mount ProviderCallbackApp, :at => "/"
#
#   # This will catch anything that goes to:
#   # * /auth/:provider/callback # :provider could be 'facebook'
#   # * /auth/failure
#   # (see Sinatra routes below)
#
# It's strongly coupled with OmniAuth. If you route to the
# '/auth/:provider/callback' URL the OmniAuth middleware will kick in and
# provide you with the provider details.
class ProviderCallbackApp < Sinatra::Base
  use Rack::Session::Cookie
  use OmniAuth::Builder do
    provider :facebook, 'APP_ID', 'APP_SECRET', scope: 'APP_SCOPE'
  end

  # Points to '/auth/failure' (overwrites thrown exception in
  # development mode).
  OmniAuth.config.on_failure = Proc.new do |env|
    if env['omniauth.error']
      error_type   = CGI::escape(env['omniauth.error'].error)
      error_reason = CGI::escape(env['omniauth.error'].error_reason)
      endpoint =
      "/auth/failure?error_type=#{error_type}&error_reason=#{error_reason}"
    else
      endpoint = '/auth/failure'
    end
    [302, {'Location' => endpoint, 'Content-Type'=> 'text/html'}, []]
  end

  # Catches the callback from a service provider. Details can
  # then be used to create a user for example or save the service
  # credentials in the database.
  #
  # Returns the user's token in a JSON object.
  get "/auth/:provider/callback" do
    if omniauth_details = request.env["omniauth.auth"]
      # TODO: Your app magic goes here!
      content_type :json
      { data: 'GOES HERE' }.to_json
    else
      content_type :json
      status 400
      { errors: { message: 'No valid provider authentication given' } }.to_json
    end
  end

  # This endpoint will be called if there is an error on the
  # provider side (app access is denied or token is expired).
  #
  # Returns JSON object with error message.
  get "/auth/failure" do
    content_type :json
    status 400
    {
      errors: {
        type: params[:error_type],
        message: (params[:message] || params[:error_reason])
      }
    }.to_json
  end
end