Presenters in Rails, using modules

_readme.md

Presenters in Rails

I have bemoaned the lack of a ViewModel in Rails many times, and I prefer using Presenters to simulate a ViewModel. But it turns out there is an object that does represent the state of the view in Rails apps. Its an instance of ActionView::Base that the controller calls view_context. We don't have much control of this object, but it seems like a logical place to put our view-specific behavior.

This code is an attempt at creating Presenters using modules. The module will be mixed into the view_context object. This is very similar to how a Decorator module will be mixed into a model object, only instead of being specific to the model is it specific to the view.

This means that testing your presenter is no different than testing any other module. So relying on dependencies such as other methods or instance variables can make testing difficult.

action_controller_rendering_monkeypatch.rb

module ActionController
  module Rendering
    def view_context
      # Call super to get view_context from AbstractController::Rendering
      _view_context = super

      # Do we have a presenter? If so, mix it in.
      presenter = _presenter
      _view_context.extend presenter if presenter

      _view_context
    end

    private

    # Presenter constant, if it exists
    def _presenter #:nodoc:
      _presenter_name.safe_constantize
    end

    # Default naming convention for presenters. Override to change behavior.
    def _presenter_name #:nodoc:
      "#{self.class.to_s.sub(/Controller$/, '')}#{self.action_name.camelize}Presenter"
    end
  end
end

home_index_presenter.rb

module HomeIndexPresenter
  def awesomeness
    true
  end
end

index.html.erb

<h1>Home#index</h1>
<p>Find me in app/views/home/index.html.erb</p>
<p>Awesomeness? <%= awesomeness %></p>

output.md

Home#index

Find me in app/views/home/index.html.erb

Awesomeness? true

@tute: if I am reading this correctly, it is different in that it is view-specific, versus helpers, which pollute the namespace for all views.