Presenter Pattern for Rails Applications

gem install classic_presenter

• In your Gemfile you should set:

  gem 'classic_presenter'
  

• Create a simple ActiveRecord class that you want to present:

  class Product < ActiveRecord::Base
  end
  

• Create a presenter class inheriting from ClassicPresenter::Base:

  class ProductPresenter < ClassicPresenter::Base
    def display_name
        if context.user_logged_in?
          name
        else
          context.link_to name, context.product_path
        end
    end
  
    def display_description
      context.render "products/description", :product => self
    end
  end
  

• In your view, show.html.erb for example, you can present your Product object using this syntax:

  <% present ProductPresenter, @product do |presenter| %>
  <%= presenter.display_name %>
  <%= presenter.display_description %>
  <% end %>

• You can decorates a collection of objects with your presenter:

  #products_controller.rb
  @products = ProductPresenter.map(Post.all, view_context)
  

• Inside of the presenter we have an access to the context. In the example above the context is the View(ActionView), then we can access any method like link_to, number_to_currency etc directly.

• You can also instantiate your presenter in the controller using the view_context and then to access any helper method like number_to_currency:

  #products_controller.rb
  @product = ProductPresenter.new(Product.find(params[:id]), view_context)
  
  class ProductPresenter < ClassicPresenter::Base
    def price
      context.number_to_currency(price, :precision => 3)
    end
  end
  

• Since ClassicPresenter::Base is a Decorator, it implements the “transparent interface” requirements of the Gang of Four’s definition. This means that when we call some method in ProductPresenter, it will be delegated to the decorated object (in this case Product), like this:

  #products_controller.rb
  @product = ProductPresenter.new(Product.find(params[:id]))
  
  #show.html.erb
  <%= @product.name %>
  <%= @product.display_name %>

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