Slots

Since 2.12.0

In addition to the content accessor, ViewComponents can accept content through slots. Think of slots as a way to render multiple blocks of content, including other components.

Slots are defined with renders_one and renders_many:

• renders_one defines a slot that will be rendered at most once per component: renders_one :header
• renders_many defines a slot that can be rendered multiple times per-component: renders_many :posts

If a second argument isn’t provided to these methods, a passthrough slot is registered. Any content passed through can be rendered inside these slots without restriction.

For example:

# blog_component.rb
class BlogComponent < ViewComponent::Base
  renders_one :header
  renders_many :posts
end

To render a renders_one slot, call the name of the slot.

To render a renders_many slot, iterate over the name of the slot:

<%# blog_component.html.erb %>
<h1><%= header %></h1>

<% posts.each do |post| %>
  <%= post %>
<% end %>

<%# index.html.erb %>
<%= render BlogComponent.new do |component| %>
  <% component.with_header do %>
    <%= link_to "My blog", root_path %>
  <% end %>

  <% BlogPost.all.each do |blog_post| %>
    <% component.with_post do %>
      <%= link_to blog_post.name, blog_post.url %>
    <% end %>
  <% end %>
<% end %>

Returning:

<h1><a href="/">My blog</a></h1>

<a href="/blog/first-post">First post</a>
<a href="/blog/second-post">Second post</a>

Predicate methods

Since 2.50.0

To test whether a slot has been passed to the component, use the provided #{slot_name}? method.

<%# blog_component.html.erb %>
<% if header? %>
  <h1><%= header %></h1>
<% end %>

<% if posts? %>
  <div class="posts">
    <% posts.each do |post| %>
      <%= post %>
    <% end %>
  </div>
<% else %>
  <p>No post yet.</p>
<% end %>

Component slots

Slots can also render other components. Pass the name of a component as the second argument to define a component slot.

Arguments passed when calling a component slot will be used to initialize the component and render it. A block can also be passed to set the component’s content.

# blog_component.rb
class BlogComponent < ViewComponent::Base
  # Since `HeaderComponent` is nested inside of this component, we've to
  # reference it as a string instead of a class name.
  renders_one :header, "HeaderComponent"

  # `PostComponent` is defined in another file, so we can refer to it by class name.
  renders_many :posts, PostComponent

  class HeaderComponent < ViewComponent::Base
    attr_reader :classes

    def initialize(classes:)
      @classes = classes
    end

    def call
      content_tag :h1, content, { class: classes }
    end
  end
end

<%# blog_component.html.erb %>
<%= header %>

<% posts.each do |post| %>
  <%= post %>
<% end %>

<%# index.html.erb %>
<%= render BlogComponent.new do |component| %>
  <% component.with_header(classes: "") do %>
    <%= link_to "My Site", root_path %>
  <% end %>

  <% component.with_post(title: "My blog post") do %>
    Really interesting stuff.
  <% end %>

  <% component.with_post(title: "Another post!") do %>
    Blog every day.
  <% end %>
<% end %>

Referencing slots

As the content passed to slots is registered after a component is initialized, it can’t be referenced in an initializer. One way to reference slot content is using the before_render lifecycle method:

# blog_component.rb
class BlogComponent < ViewComponent::Base
  renders_one :image
  renders_many :posts

  def before_render
    @post_container_classes = "PostContainer--hasImage" if image.present?
  end
end

<%# blog_component.html.erb %>
<% posts.each do |post| %>
  <div class="<%= @post_container_classes %>">
    <%= image if image? %>
    <%= post %>
  </div>
<% end %>

Lambda slots

It’s also possible to define a slot as a lambda that returns content to be rendered (either a string or a ViewComponent instance). Lambda slots are useful in cases where writing another component may be unnecessary, such as working with helpers like content_tag or as wrappers for another ViewComponent with specific default values:

class BlogComponent < ViewComponent::Base
  renders_one :header, -> (classes:) do
    # This isn't complex enough to be its own component yet, so we'll use a
    # lambda slot. If it gets much bigger, it should be extracted out to a
    # ViewComponent and rendered here with a component slot.
    content_tag :h1 do
      link_to title, root_path, { class: classes }
    end
  end

  # It's also possible to return another ViewComponent with preset default values:
  renders_many :posts, -> (title:, classes:) do
    PostComponent.new(title: title, classes: "my-default-class " + classes)
  end
end

Lambda slots are able to access state from the parent ViewComponent:

class TableComponent < ViewComponent::Base
  renders_one :header, -> do
    HeaderComponent.new(selectable: @selectable)
  end

  def initialize(selectable: false)
    @selectable = selectable
  end
end

To provide content for a lambda slot via a block, add a block parameter. Render the content by calling the block’s call method, or by passing the block directly to content_tag:

class BlogComponent < ViewComponent::Base
  renders_one :header, -> (classes:, &block) do
    content_tag :h1, class: classes, &block
  end
end

Note: While a lambda is called when the with_* method is called, a returned component isn’t rendered until first use.

Rendering collections

Since 2.23.0

renders_many slots can also be passed a collection, using the plural setter (links in this example):

# navigation_component.rb
class NavigationComponent < ViewComponent::Base
  renders_many :links, "LinkComponent"

  class LinkComponent < ViewComponent::Base
    def initialize(name:, href:)
      @name = name
      @href = href
    end
  end
end

<%# navigation_component.html.erb %>
<% links.each do |link| %>
  <%= link %>
<% end %>

<%# index.html.erb %>
<%= render(NavigationComponent.new) do |component| %>
  <% component.with_links([
    { name: "Home", href: "/" },
    { name: "Pricing", href: "/pricing" },
    { name: "Sign Up", href: "/sign-up" },
  ]) %>
<% end %>

#with_content

Since 2.31.0

Slot content can also be set using #with_content:

<%= render BlogComponent.new do |component| %>
  <% component.with_header(classes: "title").with_content("My blog") %>
<% end %>

Polymorphic slots

Since 2.42.0

Polymorphic slots can render one of several possible slots.

For example, consider this list item component that can be rendered with either an icon or an avatar visual. The visual slot is passed a hash mapping types to slot definitions:

class ListItemComponent < ViewComponent::Base
  renders_one :visual, types: {
    icon: IconComponent,
    avatar: lambda { |**system_arguments|
      AvatarComponent.new(size: 16, **system_arguments)
    }
  }
end

Note: the types hash’s values can be any valid slot definition, including a component class, string, or lambda.

Filling in the visual slot is done by calling the appropriate slot method:

<%= render ListItemComponent.new do |component| %>
  <% component.with_visual_avatar(src: "http://some-site.com/my_avatar.jpg", alt: "username") do %>
    Profile
  <% end >
<% end %>
<%= render ListItemComponent.new do |component| %>
  <% component.with_visual_icon(icon: :key) do %>
    Security Settings
  <% end >
<% end %>

To see whether a polymorphic slot has been passed to the component, use the #{slot_name}? method.

<% if visual? %>
  <%= visual %>
<% else %>
  <span class="visual-placeholder">N/A</span>
<% end %>