module Nanoc

  # Nanoc::NotificationCenter provides a way to send notifications between
  # objects. It allows blocks associated with a certain notification name to
  # be registered; these blocks will be called when the notification with the
  # given name is posted.
  #
  # It is a slightly different implementation of the Observer pattern; the
  # table of subscribers is not stored in the observable object itself, but in
  # the notification center.
  class NotificationCenter

    class << self

      # Adds the given block to the list of blocks that should be called when
      # the notification with the given name is received.
      #
      # +name+:: The name of the notification that will be posted.
      #
      # +id+:: An identifier for the block. This is only used to be able to
      #        remove the block (using the remove method) later. Defaults to
      #        nil.
      def on(name, id=nil, &block)
        initialize_if_necessary(name)

        # Add observer
        @notifications[name] << { :id => id, :block => block }
      end

      # Posts a notification with the given name. All arguments wil be passed
      # to the blocks handling the notification.
      def post(name, *args)
        initialize_if_necessary(name)

        # Notify all observers
        @notifications[name].each do |observer|
          observer[:block].call(*args)
        end
      end

      # Removes the block with the given identifier from the list of blocks
      # that should be called when the notification with the given name is
      # posted.
      #
      # +name+:: The name of the notification that will be posted.
      #
      # +id+:: The identifier of the block that should be removed.
      def remove(name, id)
        initialize_if_necessary(name)

        # Remove relevant observers
        @notifications[name].reject! { |i| i[:id] == id }
      end

    private

      def initialize_if_necessary(name)
        @notifications ||= {}       # name => observers dictionary
        @notifications[name] ||= [] # list of observers
      end

    end

  end

end