Easily preview email templates in your browser by adding a route for MailView email previews in your Rails app using this helper class. You can specify which mailers and which email templates to use, as well as what sample data to use.

email_previews.rb

### CUSTOM CLASS WITH MAILERS TO PREVIEW ###

class EmailPreviews < MailerPreview::Engine
  mailer NotificationMailer do
    # specify a block that returns the arguments used for this mailer
    preview(:new_follower) { User.last }
    
    # make sure I didn't miss any email templates for this mailer
    assert_full_coverage!
  end
  
  mailer Devise::Mailer do
    preview(:confirmation_instructions) { User.last }
    preview(:reset_password_instructions) { User.last }
    preview(:unlock_instructions) { User.last }
    
    #assert_full_coverage!
  end
end

mailer_preview.rb

### PLUMBING ###

module MailerPreview
  class PreviewEvaluator
    
    def self.run(mailer_class, method_name, &block)
      config = self.new
      result = config.instance_eval(&block)
      # if the block called the "arguments" method we have explicit args, 
      # otherwise we use the return value of the block if there is one
      method_args = config.arguments || (result && Array.wrap(result))
      mailer_class.send(method_name, *method_args)
    end
    
    def arguments(*args)
      @arguments = args if args.any?
      @arguments
    end
  end
  
  class MailerSetup
    
    def self.run(*args, &block)
      self.new(*args).instance_eval(&block)
    end
    
    attr_reader :preview_class, :mailer_class
    
    def initialize(preview_class, mailer_class)
      @preview_class  = preview_class
      @mailer_class   = mailer_class
      @method_names   = Set.new
    end
    
    # specify a template to preview, the block returns the arguments to use for the mailer method
    def preview(method_name, &block)
      @method_names << method_name = method_name.to_s.to_sym
      config        = self
      preview_class.send :define_method, :"#{mailer_class.name.gsub(/:+/, '_').underscore}_#{method_name.to_s.underscore}" do
        PreviewEvaluator.run(config.mailer_class, method_name, &block)
      end
    end
    
    # ensure that all templates for a mailer have been setup to be previewed
    def assert_full_coverage!
      remaining = mailer_class.public_instance_methods - mailer_class.superclass.public_instance_methods - @method_names.to_a
      
      if remaining.any?
        raise "The following #{mailer_class.name.gsub(/:+/, ' ').titleize}s are missing: #{remaining.join(', ')}"
      end
    end
  end
  
  class Engine < MailView
    def self.mailer(klass, &block)
      MailerSetup.run(self, klass, &block)
    end
  end
end

routes.rb

MyRailsApp::Application.routes.draw do
  
  mount EmailPreviews => '/email_previews', :as => :email_previews

  root :to => 'welcome#index'

end