In Sinatra, best way to serve iPhone layout vs. normal layout?

Question

I'm writing a Sinatra app which needs to render different layouts based on whether the user is using an iPhone or a regular browser. I can detect the browser type using Rack-Mobile-Detect but I'm not sure of the best way to tell Sinatra which layout to use.

Also, I have a feeling that how I choose to do this may also break page caching. Is that true?

Example code:

require 'sinatra/base'
require 'haml'
require 'rack/mobile-detect'

class Orca < Sinatra::Base

  use Rack::MobileDetect

  helpers do
    def choose_layout
      if request.env['X_MOBILE_DEVICE'] == :iPhone
        # use iPhone layout
      else
        # use normal layout
      end
    end
  end

  before do
    # should I use a before filter?
    choose_layout()  
  end

  get '/' do
    haml :home # with proper layout
  end

end #Class Orca

Answer 1

This is what I ended up doing:

require 'sinatra/base'
require 'haml'
require 'rack/mobile-detect'

class Orca < Sinatra::Base

  use Rack::MobileDetect

  # HAML template options
  # Use HTML5 doctype
  set :haml, {:format => :html5 }

  helpers do

    def get_layout
      # For AJAX (XMLHttpRequest) requests, don't use a layout
      if request.xhr? then 
        @layout = false
        exit
      end

      # For non-AJAX (XMLHttpRequest) requests, choose correct layout
      # For each mobile device, you will need a layout_<device>.haml file
      # in the Views directory
      @layout = case request.env['X_MOBILE_DEVICE']
                when /iPhone|iPod/ then :layout_iphone

              # when "Android" then :layout_android

                else true # use default Sinatra layout
                end
    end

  end # helpers

  before do
    get_layout() 
  end # before filter

  get '/' do
    # Will use iPhone layout for iPhone|iPod, 
    # Sinatra default layout for desktop browsers
    haml :home, :layout => @layout
  end

end # Class