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View Components for Rails.


Say you're writing a Rails online shop - the shopping cart is reappearing again and again in every view. You're thinking about a clean solution for that part. A mixture of controller code, before-filters, partials and helpers?

No. That sucks. Take Cells.

Cells are View Components for Rails. They look and feel like controllers. They don't have no DoubleRenderError. They can be rendered everywhere in your controllers or views. They are cacheable, testable, fast and wonderful. They bring back OOP to your view and improve your software design.

And the best: You can have as many cells in your page as you need!


It's a gem!

Rails >= 3.0:

gem install cells

Rails 2.3:

gem install cells -v 3.3.9


Creating a cell is nothing more than

$ rails generate cell cart show -e haml
  create  app/cells/
  create  app/cells/cart
  create  app/cells/cart_cell.rb
  create  app/cells/cart/show.html.haml
  create  test/cells/cart_test.rb

That looks very familiar.

Render the cell

Now, render your cart. Why not put it in layouts/application.html.erb for now?

<div id="header">
  <%= render_cell :cart, :show, :user => @current_user %>

Feels like rendering a controller action. For good encapsulation we pass the current user from outside into the cell - a dependency injection.


Time to improve our cell code. Let's start with app/cells/cart_cell.rb:

class CartCell < Cell::Rails
  def show(args)
    user    = args[:user]
    @items  = user.items_in_cart

    render  # renders show.html.haml

Is that a controller? Hell, yeah. We even got a #render method as we know it from the good ol' ActionController.


Since a plain call to #render will start rendering app/cells/cart/show.html.haml we should put some meaningful markup there.

  You have #{@items.size} items in your shopping cart.

ERB? Haml? Builder?

Yes, Cells support all template types that are supported by Rails itself. Remember- it's a controller!


Yes, Cells have helpers just like controllers. If you need some specific helper, do

class CartCell < Cell::Rails
  helper MyExtraHelper

and it will be around in your cart views.


Yeah, we do support rendering partials in views. Nevertheless, we discourage partials at all.

The distinction between partials and views is making things more complex, so why should we have two kinds of view types? Use ordinary views instead, they're fine.

  = render :view => 'items'

Rendering Global Partials

Sometimes you need to render a global partial from app/views within a cell. For instance, the gmaps4rails helper depends on a global partial. While this breaks encapsulation it's still possible in cells - just add the global view path.

class MapCell < Cell::Rails
  append_view_path "app/views"

View Inheritance

This is where OOP comes back to your view.

  • Inherit code into your cells by deriving more abstract cells.

  • Inherit views from parent cells.


Let render_cell take care of creating the right cell. Just configure your super-cell properly.

class LoginCell < Cell::Rails
  build do
    UnauthorizedUserCell unless logged_in?

A call to

render_cell(:login, :box)

will render the configured UnauthorizedUserCell instead of the original LoginCell if the login test fails.


Cells do strict view caching. No cluttered fragment caching. Add

class CartCell < Cell::Rails
  cache :show, :expires_in => 10.minutes

and your cart will be re-rendered after 10 minutes.

You can expand the state's cache key - why not use a versioner block to do just this?

class CartCell < Cell::Rails
  cache :show do |cell, options|

The block's return value is appended to the state key: "cells/cart/show/0ecb1360644ce665a4ef".

Check the API to learn more.


Another big advantage compared to monolithic controller/helper/partial piles is the ability to test your cells isolated.


So what if you wanna test the cart cell? Use the generated test/cells/cart_cell_test.rb test.

class CartCellTest < Cell::TestCase
  test "show" do
    invoke :show, :user => @user_fixture
    assert_select "#cart", "You have 3 items in your shopping cart."

Don't forget to put require 'cell/test_case' in your project's test/test_helper.rb file.

Then, run your tests with

$ rake test:cells

That's easy, clean and strongly improves your component-driven software quality. How'd you do that with partials?


If you prefer RSpec examples, use the rspec-cells gem for specing.

it "should render the posts count" do
  render_cell(:posts, :count).should have_selector("p", :content => "4 posts!")

To run your specs we got a rake task, too!

$ rake spec:cells

Mountable Cells

Cells 3.8 got rid of the ActionController dependency. This essentially means you can mount Cells to routes or use them like a Rack middleware. All you need to do is derive from Cell::Base.

class PostCell < Cell::Base

In your routes.rb file, mount the cell like a Rack app.

match "/posts" => proc { |env|
  [ 200, {}, [ Cell::Base.render_cell_for(:post, :show) ]]

Cells in ActionMailer

ActionMailer doesn't have request object, so if you inherit from Cell::Rails you will receive an error. Cell::Base will fix that problem, but you will not be able to use any of routes inside your cells.

You can fix that with actionmailer_with_request which (suprise!) brings request object to the ActionMailer.

Using Rails Gems Like simple_form Outside Of Rails

Cells can be used outside of Rails. A new module brought in 3.8.5 provides the Rails view “API” making it possible to use gems like the popular simple_form outside Rails!

All you need to do is providing the cell with some helpers, usually it's the polymorphic routing paths required by the gems.

module RoutingHelpers
  def musician_path(model)

Then, use the Cell::Rails::HelperAPI module and it should work fine (depending on the quality of the gem you're desiring to use).

require 'cell/base'
require "cell/rails/helper_api"
require "simple_form"

class BassistCell < Cell::Base
  include Cell::Rails::HelperAPI

  self._helpers = RoutingHelpers

  def show
    @musician = Musician.find(:first)

Your views can now use the gem's helpers.

<%= simple_form_for @musician do |f| %>
  <%= f.input :name %>
  <%= f.button :submit %>
<% end %>

Note that this currently “only” works with Rails 3.2-4.0.

Cells is Rails::Engine aware!

Now Rails::Engines can contribute to Cells view paths. By default, any 'app/cells' found inside any Engine is automatically included into Cells view paths. If you need to, you can customize the view paths changing/appending to the 'app/cell_views' path configuration. See the @Cell::EngineIntegration@ for more details.


By default, generated cells inherit from Cell::Rails. If you want to change this, specify your new class name in config/application.rb:

module MyApp
  class Application < Rails::Application
    config.generators do |g|
      g.base_cell_class "ApplicationCell"

Rails 2.3 note

In order to copy the cells rake tasks to your app, run

$ script/generate cells_install

Capture Support

If you need a global @#content_for@ use the cells-capture@ gem.

More features

Cells can do more.

No Limits

Have as many cells in your page as you need - no limitation to your render_cell calls.

Cell Nesting

Have complex cell hierarchies as you can call render_cell within cells, too.

Go for it, you'll love it!


Copyright © 2007-2011, Nick Sutterer

Copyright © 2007-2008, Solide ICT by Peter Bex and Bob Leers

Released under the MIT License.