visitor_helper.rb

module VisitorHelper
  # give this method an object or collection, optional other arguments, and a block.  When you call visit within that block, the block itself will be called with the argument(s)
  #
  #  <ul>
  #    <% visit tree do |node| %>
  #      <li>
  #        <%= node.name %>
  #        <ul><% visit node.children %></ul>
  #      </li>
  #    <% end %>
  #  </ul>
  #  
  # For nested visitors, you can provide :as => <name> to distinguish which visitor to call
  #
  #  <% visit tree, 1, :as => :outer_visit do |node, depth| %>
  #    <p>
  #      <%= "#{depth}. node.name" %>
  #      <% visit node.children, :as => :inner_visit do |child| %>
  #        <em><%= child.name %></em>
  #        <% inner_visit child.children %>
  #      <% end %>
  #    </p>
  #    <% outer_visit node.children, depth + 1 %>
  #  <% end %>
  #   
  #
  def visit(collection, *args, &visitor)
    opts = args.extract_options!
    visit_name = opts[:as] || :visit
    
    metaclass.send :define_method, visit_name do |collection, *args|
      Array(collection).inject('') {|out, i| out << instance_exec(i, *args, &visitor)}
    end
    
    send visit_name, collection, *args
    
  ensure
    metaclass.send :remove_method, visit_name
  end
end