Pistaa

Simple structure for Ruby on Rails that allows templates for engines to accept injected content from other engines.

Suppose you are creating a modular product that consists of a main engine and some engines that can be used to extend the main engine. When some of the optional engines are enabled, you probably want to alter a template in the main engine, to show functionality that is included in one of the optional engines. But if you change the main engine to cater the needs of all optional engine, the main engine becomes a spaghetti of code for the optional engines.

Pistaa provides another solution to this problem. It allows you to define slots where other engines can inject partials so your main engine only has to call Pistaa to render a slot and automatically all injected partials will be rendered.

Add Pistaa to the gemspec of your main engine.

s.add_dependency "pistaa"

Change a template in the main engine to render a slot where other engines can inject partials. So let's change news/show.html.erb.

<%= render_pistaa_slot :news_body %>

Then, in one of the optional engines, register a partial in the slot.

module MyEngine
  class Engine < ::Rails::Engine
    def self.activate
      Pistaa[:news_body][:my_engine_content] = 'my_engine/news/my_engine_content'
    end

    config.to_prepare &method(:activate).to_proc
  end
end

And make sure the template my_engine/news/_my_engine_content.html.erb exists. This will be rendered just like any other partial, so you can use all the instance variables that are available to the template in the main engine.

Because Pistaa does not hijack the rendering logic in any way (it only provides simple helpers that loop over the partials in a slot and calls render partial: 'path/to/partial' for each partial), all functionality that Rails uses for rendering views is still available. So you can render other partials for the injected partial, you can use any template language you like and you can even call Pistaa again from the injected partial.

Because it essentially is simple partial rendering, overriding templates also works as expected. So if the application that uses both the main engine and MyEngine defines my_engine/news/_my_engine_content.html.erb, it will override the template from MyEngine.

If the main application overrides the template news/show.html.erb, Pistaa exposes some helpers that allow the overriding template to control the order of the partials in the slot. Check app/helpers/pistaa_helper.rb for more information. Smart combination of render_pistaa_slot, render_pistaa_slot_item and hide_pistaa_slot_item will probably serve your needs.

I'm aware of one other project that tries to solve this problem, but in a completely different way. Deface is tool that was used in older versions of Spree and also provides a way to alter templates from engines. It parses ERB templates to an XML structure using Nokogiri, exposes a DSL to manipulate the Nokogiri document and renders the document back to ERB. Although it works, it is quite error prone. Currently, it's stable versions only support Rails 3.

The way hide_pistaa_slot_item, render_pistaa_slot_item and render_pistaa_slot work, is inspired by Drupal's render arrays.

The first thing that will be added to this gem, is a way to control the order of the partials from the engines.