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Previews

ViewComponent::Preview, like ActionMailer::Preview, provides a quick way to preview components in isolation:

# test/components/previews/example_component_preview.rb
class ExampleComponentPreview < ViewComponent::Preview
  def with_default_title
    render(ExampleComponent.new(title: "Example component default"))
  end

  def with_content_block
    render(ExampleComponent.new(title: "This component accepts a block of content")) do
      tag.div do
        content_tag(:span, "Hello")
      end
    end
  end
end

Then access the resulting previews at:

  • /rails/view_components/example_component/with_default_title
  • /rails/view_components/example_component/with_content_block

For a more interactive experience, consider using Lookbook or ViewComponent::Storybook.

Passing parameters

Set dynamic values from URL parameters by setting them as arguments:

# test/components/previews/example_component_preview.rb
class ExampleComponentPreview < ViewComponent::Preview
  def with_dynamic_title(title: "Example component default")
    render(ExampleComponent.new(title: title))
  end
end

Then pass in a value: /rails/view_components/example_component/with_dynamic_title?title=Custom+title.

Previews as test cases

Since 2.56.0

Use render_preview(name) to render previews in ViewComponent unit tests:

class ExampleComponentTest < ViewComponent::TestCase
  def test_render_preview
    render_preview(:with_default_title)

    assert_text("Example component default")
  end
end

Parameters can also be passed:

class ExampleComponentTest < ViewComponent::TestCase
  def test_render_preview
    render_preview(:with_default_title, params: { message: "Hello, world!" })

    assert_text("Hello, world!")
  end
end

By default, the preview class is inferred from the name of the current test file. Use from to set it explicitly:

class ExampleTest < ViewComponent::TestCase
  def test_render_preview
    render_preview(:with_default_title, from: ExampleComponentPreview)

    assert_text("Hello, world!")
  end
end

Helpers

The ViewComponent::Preview base class includes ActionView::Helpers::TagHelper, which provides the tag and content_tag view helper methods.

Layouts

Previews render with the application layout by default, but can use a specific layout with the layout option:

# test/components/previews/example_component_preview.rb
class ExampleComponentPreview < ViewComponent::Preview
  layout "admin"

  ...
end

To set a custom layout for individual previews and the previews index page, set: default_preview_layout:

# config/application.rb
# Set the default layout to app/views/layouts/component_preview.html.erb
config.view_component.default_preview_layout = "component_preview"

Preview paths

Preview classes live in test/components/previews, which can be configured using the preview_paths option:

# config/application.rb
config.view_component.preview_paths << "#{Rails.root}/lib/component_previews"

Previews route

Previews are served from /rails/view_components by default. To use a different endpoint, set the preview_route option:

# config/application.rb
config.view_component.preview_route = "/previews"

Preview templates

Given a preview test/components/previews/cell_component_preview.rb, template files can be defined at test/components/previews/cell_component_preview/:

# test/components/previews/cell_component_preview.rb
class CellComponentPreview < ViewComponent::Preview
  def default
  end
end
<%# test/components/previews/cell_component_preview/default.html.erb %>
<table class="table">
  <tbody>
    <tr>
      <%= render CellComponent.new %>
    </tr>
  </tbody>
</div>

To use a different location for preview templates, pass the template argument: (the path should be relative to config.view_component.preview_path):

# test/components/previews/cell_component_preview.rb
class CellComponentPreview < ViewComponent::Preview
  def default
    render_with_template(template: 'custom_cell_component_preview/my_preview_template')
  end
end

Values from params can be accessed through locals:

# test/components/previews/cell_component_preview.rb
class CellComponentPreview < ViewComponent::Preview
  def default(title: "Default title", subtitle: "A subtitle")
    render_with_template(locals: {
      title: title,
      subtitle: subtitle
    })
  end
end

Which enables passing in a value: /rails/view_components/cell_component/default?title=Custom+title&subtitle=Another+subtitle.

Configuring preview controller

Extend previews to add authentication, authorization, before actions, etc. using the preview_controller option:

# config/application.rb
config.view_component.preview_controller = "MyPreviewController"

Enabling previews

Previews are enabled by default in test and development environments. To enable or disable previews, use the show_previews option:

# config/environments/test.rb
config.view_component.show_previews = false

Source previews

A source preview is a syntax highlighted source code example of the usage of a view component, shown below the preview. Source previews are disabled by default. To enable or disable source previews, use the show_previews_source option:

# config/environments/test.rb
config.view_component.show_previews_source = true

To render the source preview in a different location, use the view helper preview_source from within the preview template or preview layout.

Use with RSpec

When using previews with RSpec, replace test/components with spec/components and update preview_paths:

# config/application.rb
config.view_component.preview_paths << "#{Rails.root}/spec/components/previews"