gistfile1.rb

# CUSTOM EXCEPTIONS

class CallAlreadyInProgress < StandardError
end

class Dialer
  def call(number)
    raise CallAlreadyInProgress.new if call_already_in_progress?
  end
end

dialer = Dialer.new
begin
  dialer.call(number)
rescue CallAlreadyInProgress
  ui_notifier.send_message("Call already in progress")
end

# with domain events

class CallAlreadyInProgress < Event 
end

class ConnectionEstablished  < Event
end

class Dialer
  def call(number)
    if call_already_in_progress?
      CallAlreadyInProgress.new.publish()
      return 
    end
    ConnectionEstablished.new.publish()
  end
end

# those two can be in different places of our project
event_bus.register(CallAlreadyInProgress, ui_notifier.send_message("Call already in progress"))

dialer.call(number)