Presenter Sanity

March 24, 2013 Brian Butz

I’m not a huge fan of Rails view helpers, which is a post for a different day, but put simply I prefer to encapsulate presentation logic in a presenter object rather than mix it in globally across all templates. There’s a few ways to do this, the simplest being the Good Ol’ Plain Ol’ Ruby Object:


  class FooPresenter
    def description_for(foo)
      Date.today.tuesday? ? "50% off! #{foo.description}" : foo.description
    end
  end

Usually, I’ll instantiate the presenter in a controller and then call it within the view:


  class FoosController
    def index
      @foos = Foo.all
      @foo_presenter = FooPresenter.new
    end
  end

  
    
    
  

Everything is simple and feels where it should be, but we’ve gotten this at the cost of losing view helpers mixed in from Rails. One of the helpers I find particularly helpful is strip_tags. If we want access to strip_tags we can do as Rails does when mixing it into the view scope and include it in our presenter:


  class FooPresenter
    include ActionView::Helpers::SanitizeHelper
    def description_for(foo)
      description = strip_tags(foo.description)
      html = Date.today.tuesday? ? "50% off! #{description}" : description
      html.html_safe
    end
  end

This feels a bit wrong as well. For one, we’ve increased the number of public methods on our presenter by at least 4, and only really needed one of them. Two, we have little control over how we are stripping the tags, and so any unit tests we’ve written for our presenter must integrate and essentially test parts of Rails. What we really want is something that can do what strip_tags is doing. Luckily, and due Rails becoming more modular, we already something to do this for us in the Rails library html-scanner. Under the hood, strip_tag is, unless configured differently, passing your string down to the sanitize method on an instance of an HTML::FullSanitizer object. We can do something similar in our presenter:


  require 'html/sanitizer'
  class FooPresenter
    def initialize(sanitizer=HTML::FullSanitizer.new)
      @sanitizer = sanitizer
    end
    def description_for(foo)
      description = @sanitizer.sanitize(foo.description)
      html = Date.today.tuesday? ? "50% off! #{description}" : description
      html.html_safe
    end
  end

Now we have an object that is easy to test purely as a unit, and also has the ability to be extended with different sanitizers.

While this works well with strip_tags, unfortunately not every helper in Rails is as nicely decoupled. The work being done in the number_to_* methods, for instance, are written completely in modules. It may be nice at some point to pull these into corresponding objects, but a short term solution could be to just have an object that includes the module:


  class NumberCruncher
    include ActionView::Helpers::NumberHelper
  end

  > NumberCruncher.new.number_to_currency(1)
  => "$1.00"

About the Author

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