# frozen_string_literal: true

# :markup: markdown

require "set"
require "active_support/rescuable"
require "active_support/parameter_filter"

module ActionCable
  module Channel
    # # 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.
    class Base
      include Callbacks
      include PeriodicTimers
      include Streams
      include Naming
      include Broadcasting
      include ActiveSupport::Rescuable

      attr_reader :params, :connection, :identifier
      delegate :logger, to: :connection

      class << self
        # 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.
        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

        private
          # 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.
          def clear_action_methods! # :doc:
            @action_methods = nil
          end

          # Refresh the cached action_methods when a new action_method is added.
          def method_added(name) # :doc:
            super
            clear_action_methods!
          end
      end

      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

      # 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).
      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.
      def subscribe_to_channel
        run_callbacks :subscribe do
          subscribed
        end

        reject_subscription if subscription_rejected?
        ensure_confirmation_sent
      end

      # Called by the cable connection when it's cut, so the channel has a chance to
      # cleanup with callbacks. This method is not intended to be called directly by
      # the user. Instead, override the #unsubscribed callback.
      def unsubscribe_from_channel # :nodoc:
        run_callbacks :unsubscribe do
          unsubscribed
        end
      end

      private
        # 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.
        def subscribed # :doc:
          # Override in subclasses
        end

        # Called once a consumer has cut its cable connection. Can be used for cleaning
        # up connections or marking users as offline or the like.
        def unsubscribed # :doc:
          # Override in subclasses
        end

        # 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.
        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

        def ensure_confirmation_sent # :doc:
          return if subscription_rejected?
          @defer_subscription_confirmation_counter.decrement
          transmit_subscription_confirmation unless defer_subscription_confirmation?
        end

        def defer_subscription_confirmation! # :doc:
          @defer_subscription_confirmation_counter.increment
        end

        def defer_subscription_confirmation? # :doc:
          @defer_subscription_confirmation_counter.value > 0
        end

        def subscription_confirmation_sent? # :doc:
          @subscription_confirmation_sent
        end

        def reject # :doc:
          @reject_subscription = true
        end

        def subscription_rejected? # :doc:
          @reject_subscription
        end

        def delegate_connection_identifiers
          connection.identifiers.each do |identifier|
            define_singleton_method(identifier) do
              connection.send(identifier)
            end
          end
        end

        def extract_action(data)
          (data["action"].presence || :receive).to_sym
        end

        def processable_action?(action)
          self.class.action_methods.include?(action.to_s) unless subscription_rejected?
        end

        def dispatch_action(action, data)
          logger.debug action_signature(action, data)

          if method(action).arity == 1
            public_send action, data
          else
            public_send action
          end
        rescue Exception => exception
          rescue_with_handler(exception) || raise
        end

        def action_signature(action, data)
          (+"#{self.class.name}##{action}").tap do |signature|
            arguments = data.except("action")

            if arguments.any?
              arguments = parameter_filter.filter(arguments)
              signature << "(#{arguments.inspect})"
            end
          end
        end

        def parameter_filter
          @parameter_filter ||= ActiveSupport::ParameterFilter.new(connection.config.filter_parameters)
        end

        def transmit_subscription_confirmation
          unless subscription_confirmation_sent?
            logger.debug "#{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

        def reject_subscription
          connection.subscriptions.remove_subscription self
          transmit_subscription_rejection
        end

        def transmit_subscription_rejection
          logger.debug "#{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
    end
  end
end

ActiveSupport.run_load_hooks(:action_cable_channel, ActionCable::Channel::Base)
