How do you flash a session message in Sinatra ?

Rack:: Flash is used to flash a message in Sinatra.
Example Usage :

require 'sinatra/base'
require 'rack-flash'

class MyApp < Sinatra::Base enable :sessions use Rack::Flash post '/set-flash' do # Set a flash entry flash[: notice] = "Thanks for signing up!" # Get a flash entry flash[:notice] # => "Thanks for signing up!"
    
    # Set a flash entry for only the current request
    flash.now[: notice] = "Thanks for signing up!"
  end
end