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Broadcasting is how other parts of your application can send messages to a channel's subscribers. As explained in Channel, most of the time, these broadcastings are streamed directly to the clients subscribed to the named broadcasting. Let's explain with a full-stack example:

class WebNotificationsChannel < ApplicationCable::Channel
  def subscribed
    stream_from "web_notifications_#{current_user.id}"
  end
end

# Somewhere in your app this is called, perhaps from a NewCommentJob:
ActionCable.server.broadcast \
  "web_notifications_1", { title: "New things!", body: "All that's fit for print" }

# Client-side CoffeeScript, which assumes you've already requested the right to send web notifications:
App.cable.subscriptions.create "WebNotificationsChannel",
  received: (data) ->
    new Notification data['title'], body: data['body']
Namespace
Methods
B
Instance Public methods
broadcast(broadcasting, message, coder: ActiveSupport::JSON)

Broadcast a hash directly to a named broadcasting. This will later be JSON encoded.

# File actioncable/lib/action_cable/server/broadcasting.rb, line 23
def broadcast(broadcasting, message, coder: ActiveSupport::JSON)
  broadcaster_for(broadcasting, coder: coder).broadcast(message)
end
broadcaster_for(broadcasting, coder: ActiveSupport::JSON)

Returns a broadcaster for a named broadcasting that can be reused. Useful when you have an object that may need multiple spots to transmit to a specific broadcasting over and over.

# File actioncable/lib/action_cable/server/broadcasting.rb, line 29
def broadcaster_for(broadcasting, coder: ActiveSupport::JSON)
  Broadcaster.new(self, String(broadcasting), coder: coder)
end