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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
S
Instance Public methods
stop_all_streams()

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

# File actioncable/lib/action_cable/channel/streams.rb, line 105
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
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.

# File actioncable/lib/action_cable/channel/streams.rb, line 100
def stream_for(model, callback = nil, coder: nil, &block)
  stream_from(broadcasting_for([ channel_name, model ]), callback || block, coder: coder)
end
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.

# File actioncable/lib/action_cable/channel/streams.rb, line 75
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