Method: GraphQL::Subscriptions::ActionCableSubscriptions#setup_stream

Defined in:
lib/graphql/subscriptions/action_cable_subscriptions.rb

#setup_stream(channel, initial_event) ⇒ Object

Every subscribing channel is listening here, but only one of them takes any action. This is so we can reuse payloads when possible, and make one payload to send to all subscribers.

But the problem is, any channel could close at any time, so each channel has to be ready to take over the primary position.

To make sure there's always one-and-only-one channel building payloads, let the listener belonging to the first event on the list be the one to build and publish payloads.



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# File 'lib/graphql/subscriptions/action_cable_subscriptions.rb', line 168

def setup_stream(channel, initial_event)
  topic = initial_event.topic
  event_stream = stream_event_name(initial_event)
  channel.stream_from(event_stream, coder: @action_cable_coder) do |message|
    events_by_fingerprint = @events[topic]
    object = nil
    events_by_fingerprint.each do |_fingerprint, events|
      if !events.empty? && events.first == initial_event
        # The fingerprint has told us that this response should be shared by all subscribers,
        # so just run it once, then deliver the result to every subscriber
        first_event = events.first
        first_subscription_id = first_event.context.fetch(:subscription_id)
        object ||= load_action_cable_message(message, first_event.context)
        result = execute_update(first_subscription_id, first_event, object)
        if !result.nil?
          # Having calculated the result _once_, send the same payload to all subscribers
          events.each do |event|
            subscription_id = event.context.fetch(:subscription_id)
            deliver(subscription_id, result)
          end
        end
      end
    end
    nil
  end
end