huginn/app/models/agents/change_detector_agent.rb

module Agents
  class ChangeDetectorAgent < Agent
    cannot_be_scheduled!

    description <<~MD
      The Change Detector Agent receives a stream of events and emits a new event when a property of the received event changes.

      `property` specifies a [Liquid](https://github.com/huginn/huginn/wiki/Formatting-Events-using-Liquid) template that expands to the property to be watched, where you can use a variable `last_property` for the last property value.  If you want to detect a new lowest price, try this: `{% assign drop = last_property | minus: price %}{% if last_property == blank or drop > 0 %}{{ price | default: last_property }}{% else %}{{ last_property }}{% endif %}`

      `expected_update_period_in_days` is used to determine if the Agent is working.

      The resulting event will be a copy of the received event.
    MD

    event_description <<~MD
      This will change based on the source event. If you were event from the ShellCommandAgent, your outbound event might look like:

          {
            'command' => 'pwd',
            'path' => '/home/Huginn',
            'exit_status' => '0',
            'errors' => '',
            'output' => '/home/Huginn'
          }
    MD

    def default_options
      {
        'property' => '{{output}}',
        'expected_update_period_in_days' => 1
      }
    end

    def validate_options
      unless options['property'].present? && options['expected_update_period_in_days'].present?
        errors.add(:base, "The property and expected_update_period_in_days fields are all required.")
      end
    end

    def working?
      event_created_within?(interpolated['expected_update_period_in_days']) && !recent_error_logs?
    end

    def receive(incoming_events)
      incoming_events.each do |event|
        interpolation_context.stack do
          interpolation_context['last_property'] = last_property
          handle(interpolated(event), event)
        end
      end
    end

    private

    def handle(opts, event = nil)
      property = opts['property']
      if has_changed?(property)
        created_event = create_event payload: event.payload

        log("Propagating new event as property has changed to #{property} from #{last_property}",
            outbound_event: created_event, inbound_event: event)
        update_memory(property)
      else
        log("Not propagating as incoming event has not changed from #{last_property}.", inbound_event: event)
      end
    end

    def has_changed?(property)
      property != last_property
    end

    def last_property
      self.memory['last_property']
    end

    def update_memory(property)
      self.memory['last_property'] = property
    end
  end
end