TODO
====

respond_to is a way to quickly & compactly define presentation behaviors.
Automatically responding is a nice goodie, but it feels awkward writing
all those _type methods.

  # Presentation modules?
  class TracksController < ApplicationController
    def reorder
      Collection.transaction do
        @collection.tracks.find(:all, :lock => true).each do |track|
          track.position = params['collection_track'].index(track.id.to_s) + 1
          track.save!
        end
      end
    end

    # Creates and includes a module in the controller then calls the action.
    respond_to :html do
      def reorder
        super
      rescue => rollback
        flash[:error] = rollback.message
      ensure
        redirect_back_or_to_collection
      end
    end
  end



Here's a controller before and after automatic respond_to.

  # BEFORE
  class TracksController::Before < ApplicationController
    before_filter :find_collection

    # Reorder tracks in a collection.
    def reorder
      Collection.transaction do
        @collection.tracks.find(:all, :lock => true).each do |track|
          track.position = params['collection_track'].index(track.id.to_s) + 1
          track.save!
        end
      end

      respond_to do |type|
        type.html { redirect_back_or_to_collection }
        type.js   { render_tracks_update }
        type.xml  { render :xml => @collection.to_xml }
      end
    rescue ActiveRecord::RecordNotSaved => maybe_invalid
      respond_to do |type|
        type.html { flash[:error] = ... }
        type.js   { page.replace_html @collection.dom_id(:tracks), ... }
        type.xml  { ... }
      end
    rescue => rollback
      respond_to do |type|
        type.html do
          flash[:error] = rollback.message
          redirect_back_or_to_collection
        end
        type.js   { ... }
        type.xml  { ... }
      end
    end
  end

  # AFTER: reorder with implicit Accepts
  class TracksController < ApplicationController
    before_filter :find_collection

    # Html reordering.
    def reorder_html
      yield
    rescue => rollback
      flash[:error] = rollback.message
    ensure
      redirect_back_or_to_collection
    end

    # Ajax reordering.
    def reorder_js
      yield
      render_tracks_update
    rescue ActiveRecord::RecordNotSaved => maybe_invalid
      render_errors_update @collection.dom_id(:tracks), @collection
    rescue => rollback
      render_exception_update @collection.dom_id('tracks-info')
    end

    # Xml reordering.
    def reorder_xml
      yield
      header['Location'] = collection_url(@collection)
      render :xml => @collection.to_xml
    rescue ActiveRecord::RecordNotSaved
      render :xml => @collection.errors.to_xml, :status => '400 Bad Request'
    end

    protected
      # Reorder tracks in a collection.
      def reorder
        Collection.transaction do
          @collection.tracks.find(:all, :lock => true).each do |track|
            track.position = params['collection_track'].index(track.id.to_s) + 1
            track.save!
          end
        end
      end
  end

# Other ideas?