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.
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 rails/rails#36388. Assuming ActionView::Component makes it into Rails 6.1, this gem will then exist to serve as a backport.
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.
actionview-component is tested for compatibility with combinations of Ruby 2.4/2.5/2.6 and Rails 5.0.0/5.2.3/6.0.0/6.1.0.alpha.
Add this line to your application's Gemfile:
gem "actionview-component"And then execute:
$ bundleIn config/application.rb, add:
require "action_view/component"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.
In working on views in the Rails monolith at GitHub (which has over 3700 templates), we have run into several key pain points:
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.
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.
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.
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.
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.
ActionView::Component is at least partially compatible with code coverage tools. We’ve seen some success with SimpleCov.
By clearly defining the context necessary to render a component, we’ve found them to be easier to reuse than partials.
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
Components are most effective in cases where view code is reused or needs to be tested directly.
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.
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.erbAn 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
endapp/components/test_component.html.erb:
<span title="<%= title %>"><%= content %></span>We can render it in a view as:
<%= render(TestComponent, title: "my title") do %>
Hello, World!
<% end %>Which returns:
<span title="my title">Hello, World!</span>Components can be rendered via:
render(TestComponent, foo: :bar)
render(component: TestComponent, locals: { foo: :bar })
The following syntax has been deprecated and will be removed in v2.0.0:
render(TestComponent.new(foo: :bar))
If the component is rendered with a blank title:
<%= render(TestComponent, title: "") do %>
Hello, World!
<% end %>An error will be raised:
ActiveModel::ValidationError: Validation failed: Title can't be blank
Components are unit tested directly. The render_inline test helper wraps the result in Nokogiri.HTML, allowing us to test the component above as:
require "action_view/component/test_helpers"
class MyComponentTest < Minitest::Test
include ActionView::Component::TestHelpers
def test_render_component
assert_equal(
%(<span title="my title">Hello, World!</span>),
render_inline(TestComponent, title: "my title") { "Hello, World!" }.to_html
)
end
endIn general, we’ve found it makes the most sense to test components based on their rendered HTML.
To test a specific variant you can wrap your test with the with_variant helper method as:
def test_render_component_for_tablet
with_variant :tablet do
assert_equal(
%(<span title="my title">Hello, tablets!</span>),
render_inline(TestComponent, title: "my title") { "Hello, tablets!" }.css("span").to_html
)
end
endActionView::Component::Previews provide 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, title: "Test component default")
end
def with_long_title
render(TestComponent, title: "This is a really long title to see how the component renders this")
end
endThe 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"
...
endBy 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"Yes. This gem is tested against ERB, Haml, and Slim, but it should support most Rails template handlers.
Inline templates have been removed (for now) due to concerns raised by @soutaro regarding compatibility with the type systems being developed for Ruby 3.
ActionView::Component is far from a novel idea! Popular implementations of view components in Ruby include, but are not limited to:
- Rethinking the View Layer with Components, RailsConf 2019
- Introducing ActionView::Component with Joel Hawksley, Ruby on Rails Podcast
- Rails to Introduce View Components, Dev.to
- ActionView::Components in Rails 6.1, Drifting Ruby
- Demo repository, actionview-component-demo
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.
The gem is available as open source under the terms of the MIT License.