module ActionCable::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.
Public Instance Methods
Unsubscribes all streams associated with this channel from the pubsub queue.
# File lib/action_cable/channel/streams.rb, line 104 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
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 lib/action_cable/channel/streams.rb, line 99 def stream_for(model, callback = nil, coder: nil, &block) stream_from(broadcasting_for([ channel_name, model ]), callback || block, coder: coder) 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 lib/action_cable/channel/streams.rb, line 74 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
Private Instance Methods
May be overridden to change the default stream handling behavior which decodes JSON and transmits to client.
TODO: Tests demonstrating this.
TODO: Room for optimization. Update transmit API to be coder-aware so we can no-op when pubsub and connection are both JSON-encoded. Then we can skip decode+encode if we're just proxying messages.
# File lib/action_cable/channel/streams.rb, line 148 def default_stream_handler(broadcasting, coder)) coder ||= ActiveSupport::JSON stream_transmitter stream_decoder(coder: coder), broadcasting: broadcasting end
# File lib/action_cable/channel/streams.rb, line 169 def identity_handler -> message { message } end
# File lib/action_cable/channel/streams.rb, line 153 def stream_decoder(handler = identity_handler, coder)) if coder -> message { handler.(coder.decode(message)) } else handler end end
May be overridden to add instrumentation, logging, specialized error handling, or other forms of handler decoration.
TODO: Tests demonstrating this.
# File lib/action_cable/channel/streams.rb, line 132 def stream_handler(broadcasting, user_handler, coder: nil) if user_handler stream_decoder user_handler, coder: coder else default_stream_handler broadcasting, coder: coder end end
# File lib/action_cable/channel/streams.rb, line 161 def stream_transmitter(handler = identity_handler, broadcasting)) via = "streamed from #{broadcasting}" -> (message) do transmit handler.(message), via: via end end
# File lib/action_cable/channel/streams.rb, line 114 def streams @_streams ||= [] end
Always wrap the outermost handler to invoke the user handler on the worker pool rather than blocking the event loop.
# File lib/action_cable/channel/streams.rb, line 120 def worker_pool_stream_handler(broadcasting, user_handler, coder: nil) handler = stream_handler(broadcasting, user_handler, coder: coder) -> message do connection.worker_pool.async_invoke handler, :call, message, connection: connection end end