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README.md

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 6.1, 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 2.3/2.4/2.5/2.6 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/base"

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.

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.

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, title: "my title") do %>
  Hello, World!
<% end %>

Which returns:

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

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)

Error case

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

Testing

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!" }.css("span").to_html
    )
  end
end

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

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.

License

The gem is available as open source under the terms of the MIT License.

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