class ActionCable::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.

Attributes

connection[R]

identifier[R]

params[R]

Public Class Methods

action_methods() click to toggle source

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.

# File lib/action_cable/channel/base.rb, line 113
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)).uniq.map(&:to_s)
    methods.to_set
  end
end

new(connection, identifier, params = {}) click to toggle source

# File lib/action_cable/channel/base.rb, line 140
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

  delegate_connection_identifiers
end

Private Class Methods

clear_action_methods!() click to toggle source

::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.

# File lib/action_cable/channel/base.rb, line 129
def clear_action_methods! # :doc:
  @action_methods = nil
end

method_added(name) click to toggle source

Refresh the cached ::action_methods when a new action_method is added.

Calls superclass method

# File lib/action_cable/channel/base.rb, line 134
def method_added(name) # :doc:
  super
  clear_action_methods!
end

Public Instance Methods

perform_action(data) click to toggle source

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).

# File lib/action_cable/channel/base.rb, line 160
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() click to toggle source

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

# File lib/action_cable/channel/base.rb, line 175
def subscribe_to_channel
  run_callbacks :subscribe do
    subscribed
  end

  reject_subscription if subscription_rejected?
  ensure_confirmation_sent
end

Private Instance Methods

action_signature(action, data) click to toggle source

# File lib/action_cable/channel/base.rb, line 268
def action_signature(action, data)
  "#{self.class.name}##{action}".tap do |signature|
    if (arguments = data.except("action")).any?
      signature << "(#{arguments.inspect})"
    end
  end
end

defer_subscription_confirmation!() click to toggle source

# File lib/action_cable/channel/base.rb, line 222
def defer_subscription_confirmation! # :doc:
  @defer_subscription_confirmation_counter.increment
end

defer_subscription_confirmation?() click to toggle source

# File lib/action_cable/channel/base.rb, line 226
def defer_subscription_confirmation? # :doc:
  @defer_subscription_confirmation_counter.value > 0
end

delegate_connection_identifiers() click to toggle source

# File lib/action_cable/channel/base.rb, line 242
def delegate_connection_identifiers
  connection.identifiers.each do |identifier|
    define_singleton_method(identifier) do
      connection.send(identifier)
    end
  end
end

dispatch_action(action, data) click to toggle source

# File lib/action_cable/channel/base.rb, line 258
def dispatch_action(action, data)
  logger.info action_signature(action, data)

  if method(action).arity == 1
    public_send action, data
  else
    public_send action
  end
end

ensure_confirmation_sent() click to toggle source

# File lib/action_cable/channel/base.rb, line 216
def ensure_confirmation_sent # :doc:
  return if subscription_rejected?
  @defer_subscription_confirmation_counter.decrement
  transmit_subscription_confirmation unless defer_subscription_confirmation?
end

extract_action(data) click to toggle source

# File lib/action_cable/channel/base.rb, line 250
def extract_action(data)
  (data["action"].presence || :receive).to_sym
end

processable_action?(action) click to toggle source

# File lib/action_cable/channel/base.rb, line 254
def processable_action?(action)
  self.class.action_methods.include?(action.to_s) unless subscription_rejected?
end

reject() click to toggle source

# File lib/action_cable/channel/base.rb, line 234
def reject # :doc:
  @reject_subscription = true
end

reject_subscription() click to toggle source

# File lib/action_cable/channel/base.rb, line 287
def reject_subscription
  connection.subscriptions.remove_subscription self
  transmit_subscription_rejection
end

subscribed() click to toggle source

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

# File lib/action_cable/channel/base.rb, line 195
def subscribed # :doc:
  # Override in subclasses
end

subscription_confirmation_sent?() click to toggle source

# File lib/action_cable/channel/base.rb, line 230
def subscription_confirmation_sent? # :doc:
  @subscription_confirmation_sent
end

subscription_rejected?() click to toggle source

# File lib/action_cable/channel/base.rb, line 238
def subscription_rejected? # :doc:
  @reject_subscription
end

transmit(data, via: nil) click to toggle source

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.

# File lib/action_cable/channel/base.rb, line 207
def transmit(data, via: nil) # :doc:
  logger.debug "#{self.class.name} transmitting #{data.inspect.truncate(300)}".tap { |m| m << " (via #{via})" if via }

  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

transmit_subscription_confirmation() click to toggle source

# File lib/action_cable/channel/base.rb, line 276
def transmit_subscription_confirmation
  unless subscription_confirmation_sent?
    logger.info "#{self.class.name} is transmitting the subscription confirmation"

    ActiveSupport::Notifications.instrument("transmit_subscription_confirmation.action_cable", channel_class: self.class.name) do
      connection.transmit identifier: @identifier, type: ActionCable::INTERNAL[:message_types][:confirmation]
      @subscription_confirmation_sent = true
    end
  end
end

transmit_subscription_rejection() click to toggle source

# File lib/action_cable/channel/base.rb, line 292
def transmit_subscription_rejection
  logger.info "#{self.class.name} is transmitting the subscription rejection"

  ActiveSupport::Notifications.instrument("transmit_subscription_rejection.action_cable", channel_class: self.class.name) do
    connection.transmit identifier: @identifier, type: ActionCable::INTERNAL[:message_types][:rejection]
  end
end

unsubscribed() click to toggle source

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

# File lib/action_cable/channel/base.rb, line 201
def unsubscribed # :doc:
  # Override in subclasses
end