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In Inside Rails, Yehuda Katz, Rails expert and core team member, and Carl Lerche, Rails expert and full-time contributor, present expert advice and insight on the Rails platform and Rails development.

Rails 3 makes rendering options a first class citizen, and uses the same new system internally that plugins authors are expected to use. Before we get into how you can use this feature yourself, let's take a look at how Rails uses it:

ActionController.add_renderer :json do |json, options|
 json = ActiveSupport::JSON.encode(json) unless json.respond_to?(:to_str)
 json = "#{options[:callback]}(#{json})" unless options[:callback].blank?
 self.content_type ||= Mime::JSON
 self.response_body = json
end

Here, we are creating a new render :json option, which behaves exactly like render :json in Rails 2.3. In the render_json method, we use the same lower-level (but still public) APIs that are used by Rails itself to set the MIME type and response body. Using a render option entirely skips the rest of the render pipeline, so you don't have to worry about blocking normal template selection and rendering, or any other internal changes that might break your added option.

Next, let's take a look at adding a new render :pdf option. We'll use JRuby and the Flying Saucer library, which takes HTML and CSS and converts it to a PDF file. The syntax for the new option will be render :pdf => "template_name", :css => %w(main.css print.pdf), :layout => "print".

In order to understand how to do this, let's first take a look at the code necessary to use Flying Saucer in JRuby. First, you'll need to grab the Flying Saucer jars (you can get them from my Muse git repo). Next, let's write the code necessary to convert HTML and CSS to PDF with Flying Saucer:

# Both of these are .jar files
require "/path/to/itext"
require "/path/to/core-renderer"
 
module PdfUtils
 def self.string_to_pdf(input_string)
  io = StringIO.new
  renderer = org.xhtmlrenderer.pdf.ITextRenderer.new
  renderer.set_document_from_string_input input_string
  renderer.layout
  renderer.create_pdf(io.to_outputstream)
  io.string # the PDF file in a String
 end
end

It's a bit verbose, but it does work. Now that we have a way to make the PDF, let's wire it up into a render option.

ActionController.add_renderer :pdf do |template, options|
 css_files = Array.wrap(options.delete(:css))
 css = css_files.map {|file| File.read(file) }.join("\n")
 
 # Reuse the render semantics to get a string from the
 # template and options
 string = render_to_string template, options
 
 # Drop in the CSS before the  in a style tag.
 # In practice, you would probably cache the file reads
 # and the merging of the CSS and HTML.
 string.gsub!(%r{}, "<style>#{css}</style>")
 send_data PdfUtils.string_to_pdf(string), :type => Mime::PDF
end

You'd also need to register the PDF mime type:

Mime::Type.register "application/pdf", :pdf

You could now do the following in a controller:

class PostsController < ActionController::Base
 def show
  @post = Post.find(params[:id])
 
  respond_to do |format|
   format.html
   format.pdf
   # or
   format.pdf { render :pdf => "show", :css => %w(application print) }
  end
 end
end

The tricky bits of this process are now reserved to figuring out how to build and return the output, not how to inject your option into render. Pretty cool, no?

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