| name | viewcomponent-patterns |
| description | Creates ViewComponents for reusable UI elements with TDD. Use when building reusable UI components, extracting complex partials, creating cards/tables/badges/modals, or when user mentions ViewComponent, components, or reusable UI. |
| allowed-tools | Read, Write, Edit, Bash, Glob, Grep |
ViewComponent Patterns for Rails 8
Overview
ViewComponents are Ruby objects for building reusable, testable view components:
- Faster than partials (no partial lookup)
- Unit testable without full request cycle
- Encapsulate view logic with Ruby
- Type-safe with explicit interfaces
TDD Workflow
ViewComponent Progress:
- [ ] Step 1: Write component test (RED)
- [ ] Step 2: Run test (fails - no component)
- [ ] Step 3: Generate component skeleton
- [ ] Step 4: Implement component
- [ ] Step 5: Run test (GREEN)
- [ ] Step 6: Add variants/slots if needed
Step 1: Component Test (RED)
require "test_helper"
class CardComponentTest < ViewComponent::TestCase
test "renders the title" do
render_inline(CardComponent.new(title: "Test Title"))
assert_selector "h3", text: "Test Title"
end
test "renders content block" do
render_inline(CardComponent.new(title: "Title")) { "Card content" }
assert_text "Card content"
end
test "renders subtitle when provided" do
render_inline(CardComponent.new(title: "Title", subtitle: "Subtitle"))
assert_selector "p", text: "Subtitle"
end
test "does not render subtitle element when not provided" do
render_inline(CardComponent.new(title: "Title"))
assert_no_selector ".subtitle"
end
end
Step 2-4: Implement Component
Base Component
class ApplicationComponent < ViewComponent::Base
include ActionView::Helpers::TagHelper
include ActionView::Helpers::NumberHelper
def not_specified_span
tag.span(I18n.t("components.common.not_specified"), class: "text-slate-400 italic")
end
end
Basic Component
class CardComponent < ApplicationComponent
def initialize(title:, subtitle: nil)
@title = title
@subtitle = subtitle
end
attr_reader :title, :subtitle
def subtitle?
subtitle.present?
end
end
<%# app/components/card_component.html.erb %>
<div class="bg-white rounded-lg shadow p-6">
<h3 class="text-lg font-semibold text-slate-900"><%= title %></h3>
<% if subtitle? %>
<p class="subtitle text-sm text-slate-500"><%= subtitle %></p>
<% end %>
<div class="mt-4">
<%= content %>
</div>
</div>
Common Patterns
Pattern 1: Status Badge
class BadgeComponent < ApplicationComponent
VARIANTS = {
success: "bg-green-100 text-green-800",
warning: "bg-yellow-100 text-yellow-800",
error: "bg-red-100 text-red-800",
info: "bg-blue-100 text-blue-800",
neutral: "bg-slate-100 text-slate-800"
}.freeze
def initialize(text:, variant: :neutral)
@text = text
@variant = variant.to_sym
end
def call
tag.span(
@text,
class: "inline-flex items-center px-2.5 py-0.5 rounded-full text-xs font-medium #{variant_classes}"
)
end
private
def variant_classes
VARIANTS.fetch(@variant, VARIANTS[:neutral])
end
end
Testing:
require "test_helper"
class BadgeComponentTest < ViewComponent::TestCase
test "renders success variant" do
render_inline(BadgeComponent.new(text: "Active", variant: :success))
assert_selector ".bg-green-100"
assert_text "Active"
end
test "renders error variant" do
render_inline(BadgeComponent.new(text: "Failed", variant: :error))
assert_selector ".bg-red-100"
end
test "defaults to neutral variant" do
render_inline(BadgeComponent.new(text: "Unknown"))
assert_selector ".bg-slate-100"
end
end
Pattern 2: Component with Slots
class CardComponent < ApplicationComponent
renders_one :header
renders_one :footer
renders_many :actions
def initialize(title: nil)
@title = title
end
end
Testing slots:
class CardComponentTest < ViewComponent::TestCase
test "renders header slot" do
render_inline(CardComponent.new) do |card|
card.with_header { "Custom Header" }
end
assert_text "Custom Header"
end
test "renders multiple action slots" do
render_inline(CardComponent.new) do |card|
card.with_action { "Action 1" }
card.with_action { "Action 2" }
end
assert_text "Action 1"
assert_text "Action 2"
end
end
Pattern 3: Collection Component
class EventCardComponent < ApplicationComponent
with_collection_parameter :event
def initialize(event:)
@event = event
end
delegate :name, :event_date, :status, to: :@event
def formatted_date
return not_specified_span if event_date.nil?
I18n.l(event_date, format: :long)
end
def status_badge
render BadgeComponent.new(text: status.humanize, variant: status_variant)
end
private
def status_variant
case status.to_sym
when :confirmed then :success
when :cancelled then :error
when :pending then :warning
else :neutral
end
end
Testing collections:
class EventCardComponentTest < ViewComponent::TestCase
test "renders single event" do
event = events(:one)
render_inline(EventCardComponent.new(event: event))
assert_text event.name
end
test "renders collection" do
events_list = [events(:one), events(:two)]
render_inline(EventCardComponent.with_collection(events_list))
assert_selector ".event-card", count: 2
end
end
Pattern 4: Modal Component
class ModalComponent < ApplicationComponent
renders_one :trigger
renders_one :title
renders_one :footer
def initialize(id:, size: :medium)
@id = id
@size = size
end
def size_classes
case @size
when :small then "max-w-md"
when :medium then "max-w-lg"
when :large then "max-w-2xl"
when :full then "max-w-full mx-4"
end
end
end
Usage in Views
<%# Simple component %>
<%= render BadgeComponent.new(text: "Active", variant: :success) %>
<%# Component with block %>
<%= render CardComponent.new(title: "Stats") do %>
<p>Content here</p>
<% end %>
<%# Component with slots %>
<%= render CardComponent.new do |card| %>
<% card.with_header do %>
<h2>Header</h2>
<% end %>
Content
<% end %>
<%# Collection %>
<%= render EventCardComponent.with_collection(@events) %>
Previews (Development)
class BadgeComponentPreview < ViewComponent::Preview
def success
render BadgeComponent.new(text: "Active", variant: :success)
end
def error
render BadgeComponent.new(text: "Failed", variant: :error)
end
end
Access at: http://localhost:3000/rails/view_components
Checklist
