nav_callbacks.rb

module NavWorld
  class << self
    def nav_callbacks
      @nav_callbacks ||= {}
    end

    def under(path, &block)
      nav_callbacks[path] = block
    end

    def extended(base)
      base.instance_exec do
        alias :visit_without_callbacks :visit

        def visit(path)
          (nav_callbacks[path] || lambda{}).call
          visit_without_callbacks(path)
        end
      end
    end
  end

  def nav_callbacks
    NavWorld.nav_callbacks
  end
end

World(NavWorld)

NavWorld.under("/") do
  puts "I am visiting the root, yo!"
end

NavWorld.under("/devices") do
  puts "I need me some devices!"
end