You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: realtime_updates.rb
    path of file: ./repos/koala/lib/koala
    
    the code of the file until where you have to start completion: module Koala
  module Facebook
    class RealtimeUpdates
      # Manage realtime callbacks for changes to users' information.
      # See http://developers.facebook.com/docs/reference/api/realtime.
      #
      # @note: to subscribe to real-time updates, you must have an application access token
      #        or provide the app secret when initializing your RealtimeUpdates object.

      attr_reader :app_id, :app_access_token, :secret

      # Create a new RealtimeUpdates instance.
      # If you don't have your app's access token, provide the app's secret and
      # Koala will make a request to Facebook for the appropriate token.
      #
      # @param options initialization options.
      # @option options :app_id the application's ID.
      # @option options :app_access_token an application access token, if known.
      # @option options :secret the application's secret.
      #
      # @raise ArgumentError if the application ID and one of the app access token or the secret are not provided.
      def initialize(options = {})
        @app_id = options[:app_id] || Koala.config.app_id
        @app_access_token = options[:app_access_token] || Koala.config.app_access_token
        @secret = options[:secret] || Koala.config.app_secret
        unless @app_id && (@app_access_token || @secret) # make sure we have what we need
          raise ArgumentError, "Initialize must receive a hash with :app_id and either :app_access_token or :secret! (received #{options.inspect})"
        end
      end

      # The app access token, either provided on initialization or fetched from Facebook using the
      # app_id and secret.
      def app_access_token
        # If a token isn't provided but we need it, fetch it
        @app_access_token ||= Koala::Facebook::OAuth.new(@app_id, @secret).get_app_access_token
      end

      # The application API interface used to communicate with Facebook.
      # @return [Koala::Facebook::API]
      def api
        # Only instantiate the API if needed. validate_update doesn't require it, so we shouldn't
        # make an unnecessary request to get the app_access_token.
        @api ||= API.new(app_access_token)
      end

      # Subscribe to realtime updates for certain fields on a given object (user,
    
      def api
        # Only instantiate the API if needed. validate_update doesn't require it, so we shouldn't
        # make an unnecessary request to get the app_access_token.
        @api ||= API.new(app_access_token)
      end

      # Subscribe to realtime updates for certain fields on a given object (user, page, etc.).
      # See {http://developers.facebook.com/docs/reference/api/realtime the realtime updates documentation}
      # for more information on what objects and fields you can register for.
      #
      # @note Your callback_url must be set up to handle the verification request or the subscription will not be set up.
      #
      # @param object a Facebook ID (name or number)
      # @param fields the fields you want your app to be updated about
      # @param callback_url the URL Facebook should ping when an update is available
      # @param verify_token a token included in the verification request, allowing you to ensure the call is genuine
      #                     (see the docs for more information)
      # @param options (see Koala::HTTPService.make_request)
      #
      # @raise A subclass of Koala::Facebook::APIError if the subscription request failed.
      def subscribe(object, fields, callback_url, verify_token, options = {})
        args = {
          :object => object,
          :fields => fields,
          :callback_url => callback_url,
        }.merge(verify_token ? {:verify_token => verify_token} : {})
        # a subscription is a success if Facebook returns a 200 (after hitting your server for verification)
        api.graph_call(subscription_path, args, 'post', options)
      end

      # Unsubscribe from updates for a particular object or from updates.
      #
      # @param object the object whose subscriptions to delete.
      #               If no object is provided, all subscriptions will be removed.
      # @param options (see Koala::HTTPService.make_request)
      #
      # @raise A subclass of Koala::Facebook::APIError if the subscription request failed.
      def unsubscribe(object = nil, options = {})
        api.graph_call(subscription_path, object ? {:object => object} : {}, "delete", options)
      end

      # List all active subscriptions for this application.
      #
      # @param options (see Koala::HTTPService.make_request)
      #
      # @return [Array] a list of active subscriptions
      def list_subscriptions(options = {})
        api.graph_call(subscription_path, {}, "get", options)
      end

      # As a security measure (to prevent DDoS attacks), Facebook sends a verification request to your server
      # after you request a subscription.
      # This method parses the challenge params and makes sure the call is legitimate.
      #
      # @param params the request parameters sent by Facebook.  (You can pass in a Rails params hash.)
      # @param verify_token the verify token sent in the {#subscribe subscription request}, if you provided one
      #
      # @yield verify_token if you need to compute the verification token
      #                     (for instance, if your callback URL includes a record ID, which you look up
      #                     and use to calculate a hash), you can pass meet_challenge a block, which
      #                     will receive the verify_token received back from Facebook.
      #
      # @return the challenge string to be sent back to Facebook, or false if the request is invalid.
      def self.meet_challenge(params, verify_token = nil, &verification_block)
        if params["hub.mode"] == "subscribe" &&
            # you can make sure this is legitimate through two ways
            # if your store the token across the calls, you can pass in the token value
            # and we'll make sure it matches
            ((verify_token && params["hub.verify_token"] == verify_token) ||
            # alternately, if you sent a specially-constructed value (such as a hash of various secret values)
            # you can pass in a block, which we'll call with the verify_token sent by Facebook
            # if it's legit, return anything that evaluates to true; otherwise, return nil or false
            (verification_block && yield(params["hub.verify_token"])))
          params["hub.challenge"]
        else
          false
        end
      end

      # Public: As a security measure, all updates from facebook are signed using
      # X-Hub-Signature: sha1=XXXX where XXX is the sha1 of the json payload
      # using your application secret as the key.
      #
      # Example:
      #   # in Rails controller
      #   # @oauth being a previously defined Koala::Facebook::OAuth instance
      #   def receive_update
      #     if @oauth.validate_update(request.body, headers)
      #       ...
      #     end
      #   end
      def validate_update(body, headers)
        unless @secret
          raise AppSecretNotDefinedError, "You must init RealtimeUpdates with your app secret in order to validate updates"
        end

        request_signature = headers['X-Hub-Signature'] || headers['HTTP_X_HUB_SIGNATURE']
        return unless request_signature

        signature_parts = request_signature.split("sha1=")
        request_signature = signature_parts[1]
        calculated_signature = OpenSSL::HMAC.hexdigest('sha1', @secret, body)
        calculated_signature == request_signature
      end

      # The Facebook subscription management URL for your application.
      def subscription_path
        @subscription_path ||= "#{@app_id}/subscriptions"
      end
    end