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.
- A
- C
- D
- E
- M
- N
- P
- R
- S
- T
- U
- ActionCable::Channel::Callbacks
- ActionCable::Channel::PeriodicTimers
- ActionCable::Channel::Streams
- ActionCable::Channel::Naming
- ActionCable::Channel::Broadcasting
[R] | connection | |
[R] | identifier | |
[R] | params |
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 actioncable/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
# File actioncable/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
::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.
Refresh the cached ::action_methods when a new action_method is added.
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 actioncable/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
This method is called after subscription has been added to the connection and confirms or rejects the subscription.
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.
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 actioncable/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