app.rb

require "sinatra"
require "omniauth-createsend"
require "createsend"

use Rack::Session::Cookie

use OmniAuth::Builder do
  provider :createsend, ENV["CREATESEND_CLIENT_ID"], ENV["CREATESEND_CLIENT_SECRET"],
    :scope => "ViewReports,CreateCampaigns,SendCampaigns"
end

get "/" do
  redirect "/auth/createsend"
end

get "/auth/:provider/callback" do
  access_token = request.env["omniauth.auth"]["credentials"]["token"]
  refresh_token = request.env["omniauth.auth"]["credentials"]["refresh_token"]
  expires_at = request.env["omniauth.auth"]["credentials"]["expires_at"]

  response = "<pre>"
  response << "Your user is successfully authenticated. Here are the details you need:<br/><br/>"
  response << "access token: #{access_token}<br/>"
  response << "refresh token: #{refresh_token}<br/>"
  response << "expires at: #{expires_at}<br/>"
  response << "<br/><br/>"

  auth = {
    :access_token => access_token,
    :refresh_token => refresh_token
  }
  cs = CreateSend::CreateSend.new auth
  clients = cs.clients

  response << "We've made an API call too. Here are your clients:<br/><br/>"
  response << MultiJson.encode(clients)
  response << "</pre>"
  response
end

get "/auth/failure" do
  content_type "application/json"
  MultiJson.encode(request.env)
end