module ActionCable
module Channel
# 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, <tt>comments_for_45</tt> 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 <tt>comments:Z2lkOi8vVGVzdEFwcC9Qb3N0LzE</tt>.
#
# 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.
module Streams
extend ActiveSupport::Concern
included do
on_unsubscribe :stop_all_streams
end
# Start streaming from the named <tt>broadcasting</tt> pubsub queue. Optionally, you can pass a <tt>callback</tt> 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.
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
transmit_subscription_confirmation
logger.info "#{self.class.name} is streaming from #{broadcasting}"
end)
end
end
# Start streaming the pubsub queue for the <tt>model</tt> in this channel. Optionally, you can pass a
# <tt>callback</tt> 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.
def stream_for(model, callback = nil, coder: nil, &block)
stream_from(broadcasting_for([ channel_name, model ]), callback || block, coder: coder)
end
# Unsubscribes all streams associated with this channel from the pubsub queue.
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
private
delegate :pubsub, to: :connection
def streams
@_streams ||= []
end
# Always wrap the outermost handler to invoke the user handler on the
# worker pool rather than blocking the event loop.
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
# May be overridden to add instrumentation, logging, specialized error
# handling, or other forms of handler decoration.
#
# TODO: Tests demonstrating this.
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
# May be overridden to change the default stream handling behavior
# which decodes JSON and transmits to the 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.
def default_stream_handler(broadcasting, coder:)
coder ||= ActiveSupport::JSON
stream_transmitter stream_decoder(coder: coder), broadcasting: broadcasting
end
def stream_decoder(handler = identity_handler, coder:)
if coder
-> message { handler.(coder.decode(message)) }
else
handler
end
end
def stream_transmitter(handler = identity_handler, broadcasting:)
via = "streamed from #{broadcasting}"
-> (message) do
transmit handler.(message), via: via
end
end
def identity_handler
-> message { message }
end
end
end
end
