Slack app built with Sinatra

app.rb

require 'sinatra'
require 'slack-ruby-client'

Slack.configure do |config|
  config.token = ENV['SLACK_BOT_TOKEN']
end

client = Slack::Web::Client.new

modal = {
  'type': 'modal',
  'title': {
    'type': 'plain_text',
    'text': 'My App'
  },
  'submit': {
    'type': 'plain_text',
    'text': 'Submit'
  },
  'blocks': [
    {
      'type': 'input',
      'block_id': 'b',
      'element': {
        'type': 'plain_text_input'
      },
      'label': {
        'type': 'plain_text',
        'text': 'Label'
      }
    }
  ]
}

post '/slack/events' do
  # TODO: verify signature here

  if request.params['command']
    client.views_open(
      trigger_id: request.params['trigger_id'],
      view: modal
    )
    [200, {}, ''] 
  else
    body = JSON.parse(request.params['payload'])
    if body['type'] == 'view_submission'
      response_body = {
        'response_action': 'errors',
        'errors': {
          'b': 'The value is not valid'
        }
      }
      [200, {'Content-Type' => 'application/json'}, JSON.dump(response_body)] 
    else
      [200, {}, ''] 
    end
  end
end

# ruby app.rb -p 3000

Gemfile

# frozen_string_literal: true
source "https://rubygems.org"
gem "sinatra"
gem "slack-ruby-client"