Note: This gem is in the process of a name / API change, see https://github.com/github/actionview-component/issues/206

ActionView::Component

ActionView::Component is a framework for building view components in Rails.

Current Status: Used in production at GitHub. Because of this, all changes will be thoroughly vetted, which could slow down the process of contributing. We will do our best to actively communicate status of pull requests with any contributors. If you have any substantial changes that you would like to make, it would be great to first open an issue to discuss them with us.

Roadmap

This gem is meant to serve as a precursor to upstreaming the ActionView::Component class into Rails. It also serves to enable the usage of ActionView::Component in older versions of Rails.

Preliminary support for rendering components was merged into Rails 6.1.0.alpha in https://github.com/rails/rails/pull/36388. Assuming ActionView::Component makes it into Rails, this gem will then exist to serve as a backport.

Design philosophy

As the goal of this gem is to be upstreamed into Rails, it is designed to integrate as seamlessly as possible, with the least surprise.

Compatibility

actionview-component is tested for compatibility with combinations of Ruby 2.5/2.6/2.7 and Rails 5.0.0/5.2.3/6.0.0/6.1.0.alpha.

Installation

Add this line to your application's Gemfile:

gem "actionview-component"

And then execute:

$ bundle

In config/application.rb, add:

require "action_view/component/railtie"

Guide

What are components?

ActionView::Components are Ruby classes that are used to render views. They take data as input and return output-safe HTML. Think of them as an evolution of the presenter/decorator/view model pattern, inspired by React Components.

Why components?

In working on views in the Rails monolith at GitHub (which has over 3700 templates), we have run into several key pain points:

Testing

Currently, Rails encourages testing views via integration or system tests. This discourages us from testing our views thoroughly, due to the costly overhead of exercising the routing/controller layer, instead of just the view. It also often leads to partials being tested for each view they are included in, cheapening the benefit of DRYing up our views.

Code Coverage

Many common Ruby code coverage tools cannot properly handle coverage of views, making it difficult to audit how thorough our tests are and leading to gaps in our test suite.

Data Flow

Unlike a method declaration on an object, views do not declare the values they are expected to receive, making it hard to figure out what context is necessary to render them. This often leads to subtle bugs when we reuse a view across different contexts.

Standards

Our views often fail even the most basic standards of code quality we expect out of our Ruby classes: long methods, deep conditional nesting, and mystery guests abound.

What are the benefits?

Testing

ActionView::Component allows views to be unit-tested. In the main GitHub codebase, our unit tests run in around 25ms/test, vs. ~6s/test for integration tests.

Code Coverage

ActionView::Component is at least partially compatible with code coverage tools. We’ve seen some success with SimpleCov.

Data flow

By clearly defining the context necessary to render a component, we’ve found them to be easier to reuse than partials.

Performance

In early benchmarks, we’ve seen performance improvements over the existing rendering pipeline. For a test page with nested renders 10 levels deep, we’re seeing around a 5x increase in speed over partials:

Comparison:
           component:     6515.4 i/s
             partial:     1251.2 i/s - 5.21x  slower

Rails 6.1.0.alpha, joelhawksley/actionview-component-demo, /benchmark route, via RAILS_ENV=production rails s, measured with evanphx/benchmark-ips

When should I use components?

Components are most effective in cases where view code is reused or needs to be tested directly.

Building components

Components are subclasses of ActionView::Component::Base and live in app/components. You may wish to create an ApplicationComponent that is a subclass of ActionView::Component::Base and inherit from that instead.

Component class names end in -Component.

Component module names are plural, as they are for controllers. (Users::AvatarComponent)

Components support ActiveModel validations. Components are validated after initialization, but before rendering.

Content passed to an ActionView::Component as a block is captured and assigned to the content accessor.

Quick start

Use the component generator to create a new ActionView::Component.

The generator accepts the component name and the list of accepted properties as arguments:

bin/rails generate component Example title content
      invoke  test_unit
      create  test/components/example_component_test.rb
      create  app/components/example_component.rb
      create  app/components/example_component.html.erb

ActionView::Component includes template generators for the erb, haml, and slim template engines and will use the template engine specified in your Rails config (config.generators.template_engine) by default.

If you want to override this behavior, you can pass the template engine as an option to the generator:

bin/rails generate component Example title content --template-engine slim
      invoke test_unit
      create  test/components/example_component_test.rb
      create  app/components/example_component.rb
      create  app/components/example_component.html.slim

Implementation

An ActionView::Component is a Ruby file and corresponding template file (in any format supported by Rails) with the same base name:

app/components/test_component.rb:

class TestComponent < ActionView::Component::Base
  validates :content, :title, presence: true

  def initialize(title:)
    @title = title
  end

  private

  attr_reader :title
end

app/components/test_component.html.erb:

<span title="<%= title %>"><%= content %></span>

We can render it in a view as:

<%= render(TestComponent.new(title: "my title")) do %>
  Hello, World!
<% end %>

Which returns:

<span title="my title">Hello, World!</span>

Error case

If the component is rendered with a blank title:

<%= render(TestComponent.new(title: "")) do %>
  Hello, World!
<% end %>

An error will be raised:

ActiveModel::ValidationError: Validation failed: Title can't be blank

Content Areas

A component can declare additional content areas to be rendered in the component. For example:

app/components/modal_component.rb:

class ModalComponent < ActionView::Component::Base
  validates :user, :header, :body, presence: true

  with_content_areas :header, :body

  def initialize(user:)
    @user = user
  end
end

app/components/modal_component.html.erb:

<div class="modal">
  <div class="header"><%= header %></div>
  <div class="body"><%= body %>"></div>
</div>

We can render it in a view as:

<%= render(ModalComponent.new(user: {name: 'Jane'})) do |component| %>
  <% component.with(:header) do %>
      Hello <%= user[:name] %>
    <% end %>
  <% component.with(:body) do %>
    <p>Have a great day.</p>
  <% end %>
<% end %>

Which returns:

<div class="modal">
  <div class="header">Hello Jane</div>
  <div class="body"><p>Have a great day.</p></div>
</div>

Content for content areas can be passed as arguments to the render method or as named blocks passed to the with method. This allows a few different combinations of ways to render the component:

Required render argument optionally overridden or wrapped by a named block

app/components/modal_component.rb:

class ModalComponent < ActionView::Component::Base
  validates :header, :body, presence: true

  with_content_areas :header, :body

  def initialize(header:)
    @header = header
  end
end

<%= render(ModalComponent.new(header: "Hi!")) do |component| %>
  <% help_enabled? && component.with(:header) do %>
    <span class="help_icon"><%= component.header %></span>
  <% end %>
  <% component.with(:body) do %>
    <p>Have a great day.</p>
  <% end %>
<% end %>

Required argument passed by render argument or by named block

app/components/modal_component.rb:

class ModalComponent < ActionView::Component::Base
  validates :header, :body, presence: true

  with_content_areas :header, :body

  def initialize(header: nil)
    @header = header
  end
end

app/views/render_arg.html.erb:

<%= render(ModalComponent.new(header: "Hi!")) do |component| %>
  <% component.with(:body) do %>
    <p>Have a great day.</p>
  <% end %>
<% end %>

app/views/with_block.html.erb:

<%= render(ModalComponent) do |component| %>
  <% component.with(:header) do %>
    <span class="help_icon">Hello</span>
  <% end %>
  <% component.with(:body) do %>
    <p>Have a great day.</p>
  <% end %>
<% end %>

Optional argument passed by render argument, by named block, or neither

app/components/modal_component.rb:

class ModalComponent < ActionView::Component::Base
  validates :body, presence: true

  with_content_areas :header, :body

  def initialize(header: nil)
    @header = header
  end
end

app/components/modal_component.html.erb:

<div class="modal">
  <% if header %>
    <div class="header"><%= header %></div>
  <% end %>
  <div class="body"><%= body %>"></div>
</div>

app/views/render_arg.html.erb:

<%= render(ModalComponent.new(header: "Hi!")) do |component| %>
  <% component.with(:body) do %>
    <p>Have a great day.</p>
  <% end %>
<% end %>

app/views/with_block.html.erb:

<%= render(ModalComponent.new) do |component| %>
  <% component.with(:header) do %>
    <span class="help_icon">Hello</span>
  <% end %>
  <% component.with(:body) do %>
    <p>Have a great day.</p>
  <% end %>
<% end %>

app/views/no_header.html.erb:

<%= render(ModalComponent.new) do |component| %>
  <% component.with(:body) do %>
    <p>Have a great day.</p>
  <% end %>
<% end %>

Conditional Rendering

Components can implement a #render? method which indicates if they should be rendered, or not at all.

For example, you might have a component that displays a "Please confirm your email address" banner to users who haven't confirmed their email address. The logic for rendering the banner would need to go in either the component template:

<!-- app/components/confirm_email_component.html.erb -->
<% if user.requires_confirmation? %>
  <div class="alert">
    Please confirm your email address.
  </div>
<% end %>

or the view that renders the component:

<!-- app/views/_banners.html.erb -->
<% if current_user.requires_confirmation? %>
  <%= render(ConfirmEmailComponent.new(user: current_user)) %>
<% end %>

The #render? hook allows you to move this logic into the Ruby class, leaving your views more readable and declarative in style:

# app/components/confirm_email_component.rb
class ConfirmEmailComponent < ApplicationComponent
  def initialize(user:)
    @user = user
  end

  def render?
    @user.requires_confirmation?
  end
end

<!-- app/components/confirm_email_component.html.erb -->
<div class="banner">
  Please confirm your email address.
</div>

<!-- app/views/_banners.html.erb -->
<%= render(ConfirmEmailComponent.new(user: current_user)) %>

Testing

Components are unit tested directly. The render_inline test helper is compatible with Capybara matchers, allowing us to test the component above as:

require "action_view/component/test_case"

class MyComponentTest < ActionView::Component::TestCase
  test "render component" do
    render_inline(TestComponent.new(title: "my title")) { "Hello, World!" }

    assert_selector("span[title='my title']", "Hello, World!")
  end
end

In general, we’ve found it makes the most sense to test components based on their rendered HTML.

Action Pack Variants

To test a specific variant you can wrap your test with the with_variant helper method as:

test "render component for tablet" do
  with_variant :tablet do
    render_inline(TestComponent.new(title: "my title")) { "Hello, tablets!" }

    assert_selector("span[title='my title']", "Hello, tablets!")
  end
end

Previewing Components

ActionView::Component::Preview provides a way to see how components look by visiting a special URL that renders them. In the previous example, the preview class for TestComponent would be called TestComponentPreview and located in test/components/previews/test_component_preview.rb. To see the preview of the component with a given title, implement a method that renders the component. You can define as many examples as you want:

# test/components/previews/test_component_preview.rb

class TestComponentPreview < ActionView::Component::Preview
  def with_default_title
    render(TestComponent.new(title: "Test component default"))
  end

  def with_long_title
    render(TestComponent.new(title: "This is a really long title to see how the component renders this"))
  end
end

The previews will be available in http://localhost:3000/rails/components/test_component/with_default_title and http://localhost:3000/rails/components/test_component/with_long_title.

Previews use the application layout by default, but you can also use other layouts from your app:

# test/components/previews/test_component_preview.rb

class TestComponentPreview < ActionView::Component::Preview
  layout "admin"

  ...
end

By default, the preview classes live in test/components/previews. This can be configured using the preview_path option. For example, if you want to use lib/component_previews, set the following in config/application.rb:

config.action_view_component.preview_path = "#{Rails.root}/lib/component_previews"

Configuring TestController

By default components tests and previews expect your Rails project to contain an ApplicationController class from which Controller classes inherit. This can be configured using the test_controller option. For example, if your controllers inherit from BaseController, set the following in config/application.rb:

config.action_view_component.test_controller = "BaseController"

Setting up RSpec

If you're using RSpec, you can configure component specs to have access to test helpers. Add the following to spec/rails_helper.rb:

require "action_view/component/test_helpers"

RSpec.configure do |config|
    # ...

    # Ensure that the test helpers are available in component specs
    config.include ActionView::Component::TestHelpers, type: :component
end

Specs created by the generator should now have access to test helpers like render_inline.

To use component previews, set the following in config/application.rb:

config.action_view_component.preview_path = "#{Rails.root}/spec/components/previews"

Initializer requirement

In Ruby 2.6.x and below, ActionView::Component requires the presence of an initialize method in each component. However, initialize is no longer required for projects using 2.7.x and above.

Frequently Asked Questions

Can I use other templating languages besides ERB?

Yes. This gem is tested against ERB, Haml, and Slim, but it should support most Rails template handlers.

What happened to inline templates?

Inline templates have been removed (for now) due to concerns raised by @soutaro regarding compatibility with the type systems being developed for Ruby 3.

Isn't this just like X library?

ActionView::Component is far from a novel idea! Popular implementations of view components in Ruby include, but are not limited to:

Resources

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/github/actionview-component. This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the Contributor Covenant code of conduct. We recommend reading the contributing guide as well.