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