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
.
# File actioncable/lib/action_cable/channel/streams.rb, line 120 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
Unsubscribes all streams associated with this channel from the pubsub queue.
# File actioncable/lib/action_cable/channel/streams.rb, line 115 def stop_stream_for(model) stop_stream_from(broadcasting_for(model)) end
Unsubscribes streams for the model
.
# File actioncable/lib/action_cable/channel/streams.rb, line 106 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
Unsubscribes streams from the named broadcasting
.
# File actioncable/lib/action_cable/channel/streams.rb, line 101 def stream_for(model, callback = nil, coder: nil, &block) stream_from(broadcasting_for(model), callback || block, coder: coder) end
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 76 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
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 129 def stream_or_reject_for(model) if model stream_for model else reject end end
Calls stream_for
with the given model
if it's present to start streaming, otherwise rejects the subscription.
© 2004–2021 David Heinemeier Hansson
Licensed under the MIT License.