auth.rb

require 'sinatra/base'
require 'slack-ruby-client'

# Load Slack app info into a hash called `config` from the environment variables assigned during setup
# See the "Running the app" section of the README for instructions.
SLACK_CONFIG = {
  slack_client_id: ENV['SLACK_CLIENT_ID'],
  slack_api_secret: ENV['SLACK_API_SECRET'],
  slack_redirect_uri: ENV['SLACK_REDIRECT_URI'],
  slack_verification_token: ENV['SLACK_VERIFICATION_TOKEN']
}

# Check to see if the required variables listed above were provided, and raise an exception if any are missing.
missing_params = SLACK_CONFIG.select { |key, value| value.nil? }
if missing_params.any?
  error_msg = missing_params.keys.join(", ").upcase
  raise "Missing Slack config variables: #{error_msg}"
end

# Set the OAuth scope of your bot. We're just using `bot` for this demo, as it has access to
# all the things we'll need to access. See: https://api.slack.com/docs/oauth-scopes for more info.
BOT_SCOPE = 'bot'

# This hash will contain all the info for each authed team, as well as each team's Slack client object.
# In a production environment, you may want to move some of this into a real data store.
$teams = {}

# Since we're going to create a Slack client object for each team, this helper keeps all of that logic in one place.
def create_slack_client(slack_api_secret)
  Slack.configure do |config|
    config.token = slack_api_secret
    fail 'Missing API token' unless config.token
  end
  Slack::Web::Client.new
end

# Slack uses OAuth for user authentication. This auth process is performed by exchanging a set of
# keys and tokens between Slack's servers and yours. This process allows the authorizing user to confirm
# that they want to grant our bot access to their team.
# See https://api.slack.com/docs/oauth for more information.
class Auth < Sinatra::Base
  # This is the HTML markup for our "Add to Slack" button.
  # Note that we pass the `client_id`, `scope` and "redirect_uri" parameters specific to our application's configs.
  add_to_slack_button = %(
    <a href=\"https://slack.com/oauth/authorize?scope=#{BOT_SCOPE}&client_id=#{SLACK_CONFIG[:slack_client_id]}&redirect_uri=#{SLACK_CONFIG[:redirect_uri]}\">
      <img alt=\"Add to Slack\" height=\"40\" width=\"139\" src=\"https://platform.slack-edge.com/img/add_to_slack.png\"/>
    </a>
  )

  # If a user tries to access the index page, redirect them to the auth start page
  get '/' do
    redirect '/begin_auth'
  end

  # OAuth Step 1: Show the "Add to Slack" button, which links to Slack's auth request page.
  # This page shows the user what our app would like to access and what bot user we'd like to create for their team.
  get '/begin_auth' do
    status 200
    body add_to_slack_button
  end

  # OAuth Step 2: The user has told Slack that they want to authorize our app to use their account, so
  # Slack sends us a code which we can use to request a token for the user's account.
  get '/finish_auth' do
    client = Slack::Web::Client.new
    # OAuth Step 3: Success or failure
    begin
      response = client.oauth_access(
        {
          client_id: SLACK_CONFIG[:slack_client_id],
          client_secret: SLACK_CONFIG[:slack_api_secret],
          redirect_uri: SLACK_CONFIG[:slack_redirect_uri],
          code: params[:code] # (This is the OAuth code mentioned above)
        }
      )
      # Success:
      # Yay! Auth succeeded! Let's store the tokens and create a Slack client to use in our Events handlers.
      # The tokens we receive are used for accessing the Web API, but this process also creates the Team's bot user and
      # authorizes the app to access the Team's Events.
      team_id = response['team_id']
      $teams[team_id] = {
        user_access_token: response['access_token'],
        bot_user_id: response['bot']['bot_user_id'],
        bot_access_token: response['bot']['bot_access_token']
      }

      $teams[team_id]['client'] = create_slack_client(response['bot']['bot_access_token'])
      # Be sure to let the user know that auth succeeded.
      status 200
      body "Yay! Auth succeeded! You're awesome!"
    rescue Slack::Web::Api::Error => e
      # Failure:
      # D'oh! Let the user know that something went wrong and output the error message returned by the Slack client.
      status 403
      body "Auth failed! Reason: #{e.message}<br/>#{add_to_slack_button}"
    end
  end
end