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Action Cable Channel Base ↑

The channel provides the basic structure of grouping behavior into logical units when communicating over the WebSocket connection. You can think of a channel like a form of controller, but one that’s capable of pushing content to the subscriber in addition to simply responding to the subscriber’s direct requests.

Channel instances are long-lived. A channel object will be instantiated when the cable consumer becomes a subscriber, and then lives until the consumer disconnects. This may be seconds, minutes, hours, or even days. That means you have to take special care not to do anything silly in a channel that would balloon its memory footprint or whatever. The references are forever, so they won’t be released as is normally the case with a controller instance that gets thrown away after every request.

Long-lived channels (and connections) also mean you’re responsible for ensuring that the data is fresh. If you hold a reference to a user record, but the name is changed while that reference is held, you may be sending stale data if you don’t take precautions to avoid it.

The upside of long-lived channel instances is that you can use instance variables to keep reference to objects that future subscriber requests can interact with. Here’s a quick example:

class ChatChannel < ApplicationCable::Channel
  def subscribed
    @room = Chat::Room[params[:room_number]]
  end

  def speak(data)
    @room.speak data, user: current_user
  end
end

The speak action simply uses the Chat::Room object that was created when the channel was first subscribed to by the consumer when that subscriber wants to say something in the room.

Action processing ↑

Unlike subclasses of ActionController::Base, channels do not follow a RESTful constraint form for their actions. Instead, Action Cable operates through a remote-procedure call model. You can declare any public method on the channel (optionally taking a data argument), and this method is automatically exposed as callable to the client.

Example:

class AppearanceChannel < ApplicationCable::Channel
  def subscribed
    @connection_token = generate_connection_token
  end

  def unsubscribed
    current_user.disappear @connection_token
  end

  def appear(data)
    current_user.appear @connection_token, on: data['appearing_on']
  end

  def away
    current_user.away @connection_token
  end

  private
    def generate_connection_token
      SecureRandom.hex(36)
    end
end

In this example, the subscribed and unsubscribed methods are not callable methods, as they were already declared in ActionCable::Channel::Base, but appear and away are. generate_connection_token is also not callable, since it’s a private method. You’ll see that appear accepts a data parameter, which it then uses as part of its model call. away does not, since it’s simply a trigger action.

Also note that in this example, current_user is available because it was marked as an identifying attribute on the connection. All such identifiers will automatically create a delegation method of the same name on the channel instance.

Rejecting subscription requests ↑

A channel can reject a subscription request in the subscribed callback by invoking the reject method:

class ChatChannel < ApplicationCable::Channel
  def subscribed
    @room = Chat::Room[params[:room_number]]
    reject unless current_user.can_access?(@room)
  end
end

In this example, the subscription will be rejected if the current_user does not have access to the chat room. On the client-side, the Channel#rejected callback will get invoked when the server rejects the subscription request.

Methods
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Included Modules

Attributes

[R] connection
[R] identifier
[R] params

Class Public methods

action_methods()

A list of method names that should be considered actions. This includes all public instance methods on a channel, less any internal methods (defined on Base), adding back in any methods that are internal, but still exist on the class itself.

Returns ↑

  • Set - A set of all methods that should be considered actions.

Source: show | on GitHub 

# File actioncable/lib/action_cable/channel/base.rb, line 135
def action_methods
  @action_methods ||= begin
    # All public instance methods of this class, including ancestors
    methods = (public_instance_methods(true) -
      # Except for public instance methods of Base and its ancestors
      ActionCable::Channel::Base.public_instance_methods(true) +
      # Be sure to include shadowed public instance methods of this class
      public_instance_methods(false) -
      # Except the internal methods
      internal_methods).uniq

    methods.map!(&:name)
    methods.to_set
  end
end

new(connection, identifier, params = {})

Source: show | on GitHub 

# File actioncable/lib/action_cable/channel/base.rb, line 170
def initialize(connection, identifier, params = {})
  @connection = connection
  @identifier = identifier
  @params     = params

  # When a channel is streaming via pubsub, we want to delay the confirmation
  # transmission until pubsub subscription is confirmed.
  #
  # The counter starts at 1 because it's awaiting a call to #subscribe_to_channel
  @defer_subscription_confirmation_counter = Concurrent::AtomicFixnum.new(1)

  @reject_subscription = nil
  @subscription_confirmation_sent = nil
  @unsubscribed = false

  delegate_connection_identifiers
end

Class Private methods

clear_action_methods!()

action_methods are cached and there is sometimes need to refresh them. ::clear_action_methods! allows you to do that, so next time you run action_methods, they will be recalculated.

Source: show | on GitHub 

# File actioncable/lib/action_cable/channel/base.rb, line 155
def clear_action_methods! # :doc:
  @action_methods = nil
end

method_added(name)

Refresh the cached action_methods when a new action_method is added.

Source: show | on GitHub 

# File actioncable/lib/action_cable/channel/base.rb, line 160
def method_added(name) # :doc:
  super
  clear_action_methods!
end

Instance Public methods

perform_action(data)

Extract the action name from the passed data and process it via the channel. The process will ensure that the action requested is a public method on the channel declared by the user (so not one of the callbacks like subscribed).

Source: show | on GitHub 

# File actioncable/lib/action_cable/channel/base.rb, line 191
def perform_action(data)
  action = extract_action(data)

  if processable_action?(action)
    payload = { channel_class: self.class.name, action: action, data: data }
    ActiveSupport::Notifications.instrument("perform_action.action_cable", payload) do
      dispatch_action(action, data)
    end
  else
    logger.error "Unable to process #{action_signature(action, data)}"
  end
end

subscribe_to_channel()

This method is called after subscription has been added to the connection and confirms or rejects the subscription.

Source: show | on GitHub 

# File actioncable/lib/action_cable/channel/base.rb, line 206
def subscribe_to_channel
  run_callbacks :subscribe do
    subscribed unless subscription_rejected?
  end

  reject_subscription if subscription_rejected?

  ensure_confirmation_sent
end

Instance Private methods

defer_subscription_confirmation!()

Source: show | on GitHub 

# File actioncable/lib/action_cable/channel/base.rb, line 266
def defer_subscription_confirmation! # :doc:
  @defer_subscription_confirmation_counter.increment
end

defer_subscription_confirmation?()

Source: show | on GitHub 

# File actioncable/lib/action_cable/channel/base.rb, line 270
def defer_subscription_confirmation? # :doc:
  @defer_subscription_confirmation_counter.value > 0
end

ensure_confirmation_sent()

Source: show | on GitHub 

# File actioncable/lib/action_cable/channel/base.rb, line 260
def ensure_confirmation_sent # :doc:
  return if subscription_rejected?
  @defer_subscription_confirmation_counter.decrement
  transmit_subscription_confirmation unless defer_subscription_confirmation?
end

reject()

Source: show | on GitHub 

# File actioncable/lib/action_cable/channel/base.rb, line 278
def reject # :doc:
  @reject_subscription = true
end

subscribed()

Called once a consumer has become a subscriber of the channel. Usually the place to set up any streams you want this channel to be sending to the subscriber.

Source: show | on GitHub 

# File actioncable/lib/action_cable/channel/base.rb, line 234
def subscribed # :doc:
  # Override in subclasses
end

subscription_confirmation_sent?()

Source: show | on GitHub 

# File actioncable/lib/action_cable/channel/base.rb, line 274
def subscription_confirmation_sent? # :doc:
  @subscription_confirmation_sent
end

subscription_rejected?()

Source: show | on GitHub 

# File actioncable/lib/action_cable/channel/base.rb, line 282
def subscription_rejected? # :doc:
  @reject_subscription
end

transmit(data, via: nil)

Transmit a hash of data to the subscriber. The hash will automatically be wrapped in a JSON envelope with the proper channel identifier marked as the recipient.

Source: show | on GitHub 

# File actioncable/lib/action_cable/channel/base.rb, line 247
def transmit(data, via: nil) # :doc:
  logger.debug do
    status = "#{self.class.name} transmitting #{data.inspect.truncate(300)}"
    status += " (via #{via})" if via
    status
  end

  payload = { channel_class: self.class.name, data: data, via: via }
  ActiveSupport::Notifications.instrument("transmit.action_cable", payload) do
    connection.transmit identifier: @identifier, message: data
  end
end

unsubscribed()

Called once a consumer has cut its cable connection. Can be used for cleaning up connections or marking users as offline or the like.

Source: show | on GitHub 

# File actioncable/lib/action_cable/channel/base.rb, line 240
def unsubscribed # :doc:
  # Override in subclasses
end