module ActionController #:nodoc:
  module MimeResponds #:nodoc:
    def self.included(base)
      base.send(:include, ActionController::MimeResponds::InstanceMethods)
    end

    module InstanceMethods
      # Without web-service support, an action which collects the data for displaying a list of people
      # might look something like this:
      #
      #   def index
      #     @people = Person.find(:all)
      #   end
      # 
      # Here's the same action, with web-service support baked in:
      # 
      #   def index
      #     @people = Person.find(:all)
      # 
      #     respond_to do |format|
      #       format.html
      #       format.xml { render :xml => @people.to_xml }
      #     end
      #   end
      # 
      # What that says is, "if the client wants HTML in response to this action, just respond as we 
      # would have before, but if the client wants XML, return them the list of people in XML format." 
      # (Rails determines the desired response format from the HTTP Accept header submitted by the client.)
      # 
      # Supposing you have an action that adds a new person, optionally creating their company 
      # (by name) if it does not already exist, without web-services, it might look like this:
      # 
      #   def create
      #     @company = Company.find_or_create_by_name(params[:company][:name])
      #     @person  = @company.people.create(params[:person])
      # 
      #     redirect_to(person_list_url)
      #   end
      # 
      # Here's the same action, with web-service support baked in:
      # 
      #   def create
      #     company  = params[:person].delete(:company)
      #     @company = Company.find_or_create_by_name(company[:name])
      #     @person  = @company.people.create(params[:person])
      # 
      #     respond_to do |format|
      #       format.html { redirect_to(person_list_url) }
      #       format.js
      #       format.xml  { render :xml => @person.to_xml(:include => @company) }
      #     end
      #   end
      # 
      # If the client wants HTML, we just redirect them back to the person list. If they want Javascript 
      # (wants.js), then it is an RJS request and we render the RJS template associated with this action. 
      # Lastly, if the client wants XML, we render the created person as XML, but with a twist: we also 
      # include the person’s company in the rendered XML, so you get something like this:
      # 
      #   <person>
      #     <id>...</id>
      #     ...
      #     <company>
      #       <id>...</id>
      #       <name>...</name>
      #       ...
      #     </company>
      #   </person>
      # 
      # Note, however, the extra bit at the top of that action:
      # 
      #   company  = params[:person].delete(:company)
      #   @company = Company.find_or_create_by_name(company[:name])
      # 
      # This is because the incoming XML document (if a web-service request is in process) can only contain a 
      # single root-node. So, we have to rearrange things so that the request looks like this (url-encoded):
      # 
      #   person[name]=...&person[company][name]=...&...
      # 
      # And, like this (xml-encoded):
      # 
      #   <person>
      #     <name>...</name>
      #     <company>
      #       <name>...</name>
      #     </company>
      #   </person>
      # 
      # In other words, we make the request so that it operates on a single entity—a person. Then, in the action, 
      # we extract the company data from the request, find or create the company, and then create the new person 
      # with the remaining data.
      # 
      # Note that you can define your own XML parameter parser which would allow you to describe multiple entities 
      # in a single request (i.e., by wrapping them all in a single root note), but if you just go with the flow 
      # and accept Rails' defaults, life will be much easier.
      # 
      # If you need to use a MIME type which isn't supported by default, you can register your own handlers in
      # environment.rb as follows.
      # 
      #   Mime::Type.register "image/jpg", :jpg
      def respond_to(*types, &block)
        raise ArgumentError, "respond_to takes either types or a block, never both" unless types.any? ^ block
        block ||= lambda { |responder| types.each { |type| responder.send(type) } }
        responder = Responder.new(block.binding)
        block.call(responder)
        responder.respond
      end
    end
    
    class Responder #:nodoc:
      default_block_format = <<-END
        Proc.new { 
          @template.template_format = '%s'
          render :action => "\#{action_name}", :content_type => Mime::%s
        }
      END

      DEFAULT_BLOCKS = [:html, :js, :xml].inject({}) do |memo, ext| 
        default_block = default_block_format % [ext, ext.to_s.upcase]
        memo.update(ext => default_block)
      end
      
      def initialize(block_binding)
        @block_binding = block_binding
        @mime_type_priority = eval(
          "(params[:format] && Mime::EXTENSION_LOOKUP[params[:format]]) ? " +
          "[ Mime::EXTENSION_LOOKUP[params[:format]] ] : request.accepts", 
          block_binding
        )

        @order     = []
        @responses = {}
      end

      def custom(mime_type, &block)
        mime_type = mime_type.is_a?(Mime::Type) ? mime_type : Mime::Type.lookup(mime_type.to_s)
        
        @order << mime_type
        
        if block_given?
          @responses[mime_type] = Proc.new do
            eval <<-END, @block_binding
              @template.template_format = '#{mime_type.to_sym}'
              response.content_type     = '#{mime_type.to_s}'
            END
            block.call
          end
        else
          if source = DEFAULT_BLOCKS[mime_type.to_sym]
            @responses[mime_type] = eval(source, @block_binding)
          else
            raise ActionController::RenderError, "Expected a block but none was given for custom mime handler #{mime_type}"
          end
        end
      end

      def any(*args, &block)
        args.each { |type| send(type, &block) }
      end
      
      def method_missing(symbol, &block)
        mime_constant = symbol.to_s.upcase
        
        if Mime::SET.include?(Mime.const_get(mime_constant))
          custom(Mime.const_get(mime_constant), &block)
        else
          super
        end
      end
      
      def respond
        for priority in @mime_type_priority
          if priority == Mime::ALL
            @responses[@order.first].call
            return
          else
            if priority === @order
              @responses[priority].call
              return # mime type match found, be happy and return
            end
          end
        end
        
        if @order.include?(Mime::ALL)
          @responses[Mime::ALL].call
        else
          eval 'render(:nothing => true, :status => "406 Not Acceptable")', @block_binding
        end
      end
    end
  end
end