Action Cable Channel Streams

Streams allow channels to route broadcastings to the subscriber. A broadcasting is, as discussed elsewhere, a pubsub queue where any data placed into it is automatically sent to the clients that are connected at that time. It’s purely an online queue, though. If you’re not streaming a broadcasting at the very moment it sends out an update, you will not get that update, even if you connect after it has been sent.

Most commonly, the streamed broadcast is sent straight to the subscriber on the client-side. The channel just acts as a connector between the two parties (the broadcaster and the channel subscriber). Here’s an example of a channel that allows subscribers to get all new comments on a given page:

class CommentsChannel < ApplicationCable::Channel
  def follow(data)
    stream_from "comments_for_#{data['recording_id']}"
  end

  def unfollow
    stop_all_streams
  end
end

Based on the above example, the subscribers of this channel will get whatever data is put into the, let’s say, comments_for_45 broadcasting as soon as it’s put there.

An example broadcasting for this channel looks like so:

ActionCable.server.broadcast "comments_for_45", { author: 'DHH', content: 'Rails is just swell' }

If you have a stream that is related to a model, then the broadcasting used can be generated from the model and channel. The following example would subscribe to a broadcasting like comments:Z2lkOi8vVGVzdEFwcC9Qb3N0LzE.

class CommentsChannel < ApplicationCable::Channel
  def subscribed
    post = Post.find(params[:id])
    stream_for post
  end
end

You can then broadcast to this channel using:

CommentsChannel.broadcast_to(@post, @comment)

If you don’t just want to parlay the broadcast unfiltered to the subscriber, you can also supply a callback that lets you alter what is sent out. The below example shows how you can use this to provide performance introspection in the process:

class ChatChannel < ApplicationCable::Channel
  def subscribed
    @room = Chat::Room[params[:room_number]]

    stream_for @room, coder: ActiveSupport::JSON do |message|
      if message['originated_at'].present?
        elapsed_time = (Time.now.to_f - message['originated_at']).round(2)

        ActiveSupport::Notifications.instrument :performance, measurement: 'Chat.message_delay', value: elapsed_time, action: :timing
        logger.info "Message took #{elapsed_time}s to arrive"
      end

      transmit message
    end
  end
end

You can stop streaming from all broadcasts by calling stop_all_streams.

Methods

Instance Public methods

stop_all_streams()

Unsubscribes all streams associated with this channel from the pubsub queue.

📝 Source code 
# File actioncable/lib/action_cable/channel/streams.rb, line 122
      def stop_all_streams
        streams.each do |broadcasting, callback|
          pubsub.unsubscribe broadcasting, callback
          logger.info "#{self.class.name} stopped streaming from #{broadcasting}"
        end.clear
      end
🔎 See on GitHub

stop_stream_for(model)

Unsubscribes streams for the model.

📝 Source code 
# File actioncable/lib/action_cable/channel/streams.rb, line 117
      def stop_stream_for(model)
        stop_stream_from(broadcasting_for(model))
      end
🔎 See on GitHub

stop_stream_from(broadcasting)

Unsubscribes streams from the named broadcasting.

📝 Source code 
# File actioncable/lib/action_cable/channel/streams.rb, line 108
      def stop_stream_from(broadcasting)
        callback = streams.delete(broadcasting)
        if callback
          pubsub.unsubscribe(broadcasting, callback)
          logger.info "#{self.class.name} stopped streaming from #{broadcasting}"
        end
      end
🔎 See on GitHub

stream_for(model, callback = nil, coder: nil, &block)

Start streaming the pubsub queue for the model in this channel. Optionally, you can pass a callback that’ll be used instead of the default of just transmitting the updates straight to the subscriber.

Pass coder: ActiveSupport::JSON to decode messages as JSON before passing to the callback. Defaults to coder: nil which does no decoding, passes raw messages.

📝 Source code 
# File actioncable/lib/action_cable/channel/streams.rb, line 103
      def stream_for(model, callback = nil, coder: nil, &block)
        stream_from(broadcasting_for(model), callback || block, coder: coder)
      end
🔎 See on GitHub

stream_from(broadcasting, callback = nil, coder: nil, &block)

Start streaming from the named broadcasting pubsub queue. Optionally, you can pass a callback that’ll be used instead of the default of just transmitting the updates straight to the subscriber. Pass coder: ActiveSupport::JSON to decode messages as JSON before passing to the callback. Defaults to coder: nil which does no decoding, passes raw messages.

📝 Source code 
# File actioncable/lib/action_cable/channel/streams.rb, line 78
      def stream_from(broadcasting, callback = nil, coder: nil, &block)
        broadcasting = String(broadcasting)

        # Don't send the confirmation until pubsub#subscribe is successful
        defer_subscription_confirmation!

        # Build a stream handler by wrapping the user-provided callback with
        # a decoder or defaulting to a JSON-decoding retransmitter.
        handler = worker_pool_stream_handler(broadcasting, callback || block, coder: coder)
        streams[broadcasting] = handler

        connection.server.event_loop.post do
          pubsub.subscribe(broadcasting, handler, lambda do
            ensure_confirmation_sent
            logger.info "#{self.class.name} is streaming from #{broadcasting}"
          end)
        end
      end
🔎 See on GitHub

stream_or_reject_for(model)

Calls stream_for with the given model if it’s present to start streaming, otherwise rejects the subscription.

📝 Source code 
# File actioncable/lib/action_cable/channel/streams.rb, line 131
      def stream_or_reject_for(model)
        if model
          stream_for model
        else
          reject
        end
      end
🔎 See on GitHub