ga.rb

require 'sinatra'
require 'googleauth'
require 'googleauth/web_user_authorizer'
require 'googleauth/stores/redis_token_store'
require 'google/apis/analytics_v3'
require 'google/apis/webmasters_v3'
require 'redis'

client_id = ::Google::Auth::ClientId.new(
  'xxxxxxxx',
  'xxxxxxxx'
)
scope = [::Google::Apis::AnalyticsV3::AUTH_ANALYTICS_READONLY,
         ::Google::Apis::WebmastersV3::AUTH_WEBMASTERS_READONLY]
token_store = Google::Auth::Stores::RedisTokenStore.new(redis: Redis.new)
authorizer = Google::Auth::WebUserAuthorizer.new(client_id, scope, token_store, '/oauth2callback')

get('/authorize') do
  # NOTE: Assumes the user is already authenticated to the app
  user_id = '1'
  credentials = authorizer.get_credentials(user_id, request)
  if credentials.nil?
    redirect(
      authorizer.get_authorization_url(
        login_hint: user_id,
        request: request
      )
    )
  end
  # Credentials are valid, can call APIs
  puts 'Success!!'
end

get('/oauth2callback') do
  target_url = Google::Auth::WebUserAuthorizer.handle_auth_callback_deferred(request)
  redirect target_url
end