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- What's this?
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.