Rails 4 view variant detection as a controller concern

My team is currently working on an app, where different content need to be served depending on what device you use to access the page: desktop, tablet or a phone. Luckily, Rails 4.1 came out with an ActionPack Variants which just do this task. We might want the behaviour application-wide, so we need to share it accross all controllers. The first thought is to put the code in a base controller, that all the other controllers inherit from (eg. ApplicationController), like this:

class ApplicationController < ActionController::Base
  protect_from_forgery with: :exception
  before_filter :detect_device

  private
  def detect_device
    case request.user_agent
      when /iPad/i
        request.variant = :tablet
      when /ip(hone|od)/i
        request.variant = :phone
      when /Android/i && /mobile/i
        request.variant = :phone
      when /Android/i
        request.variant = :tablet
      when /Windows Phone/i
        request.variant = :tablet
    end
  end
end

Despite the fact it is not particularly DRY (this is a separate feature and it’s probable we’re gona reuse it), the app we’re builing is divided into several zones (client, emploee, admin), where each zone has a separate master controller, that inherits directly from ActionController::Base. The variants behaviour should be present only for client and emploee. How to do it then?

Extract the code to a concern!

When you created a Rails 4 app, you’ve probably noticed new directiories under controllers and models, which is called concerns. The idea of concerns was strongly promoted by DHH and now it is bundled in Rails together with a handy ActiveSupport::Concern wrapper. Let’s do it then, the separate concern will look like:

# coding: utf-8
#app/controllers/concerns/detect_format_variant.rb
module DetectFormatVariant
  extend ActiveSupport::Concern

  included do
    before_filter :detect_device
  end

  private
  def detect_device
    case request.user_agent
      when /iPad/i
        request.variant = :tablet
      when /ip(hone|od)/i
        request.variant = :phone
      when /Android/i && /mobile/i
        request.variant = :phone
      when /Android/i
        request.variant = :tablet
      when /Windows Phone/i
        request.variant = :tablet
    end
  end
end

Then in every controller we want this behaviour, we just need to include it:

class ClientController < ActionController::Base
  protect_from_forgery with: :exception

  include DetectFormatVariant
end

Not only it is DRYer, but it’s also self-descriptive. Concerns help you keep your controller clean and is a perfect tool to share code between controllers.