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---
layout: default
title: Previews
parent: How-to guide
---
# Previews
`ViewComponent::Preview`, like `ActionMailer::Preview`, provides a quick way to preview components in isolation:
```ruby
# 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](https://github.com/allmarkedup/lookbook) or [ViewComponent::Storybook](https://github.com/jonspalmer/view_component_storybook)._
## Passing parameters
Set dynamic values from URL parameters by setting them as arguments:
```ruby
# 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
{: .label }
Use `render_preview(name)` to render previews in ViewComponent unit tests:
```ruby
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:
```ruby
class ExampleComponentTest < ViewComponent::TestCase
def test_render_preview
render_preview(:with_default_title, params: { message: "Hello, world!" })
assert_text("Hello, world!")
end
end
```
## Helpers
The `ViewComponent::Preview` base class includes
[`ActionView::Helpers::TagHelper`](https://api.rubyonrails.org/classes/ActionView/Helpers/TagHelper.html), which provides the [`tag`](https://api.rubyonrails.org/classes/ActionView/Helpers/TagHelper.html#method-i-tag)
and [`content_tag`](https://api.rubyonrails.org/classes/ActionView/Helpers/TagHelper.html#method-i-content_tag) view helper methods.
## Layouts
Previews render with the application layout by default, but can use a specific layout with the `layout` option:
```ruby
# 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`:
```ruby
# 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:
```ruby
# 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:
```ruby
# 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/`:
```ruby
# test/components/previews/cell_component_preview.rb
class CellComponentPreview < ViewComponent::Preview
def default
end
end
```
```erb
<%# 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`):
```ruby
# 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`:
```ruby
# 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:
```ruby
# 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:
```ruby
# 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:
```ruby
# 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`:
```ruby
# config/application.rb
config.view_component.preview_paths << "#{Rails.root}/spec/components/previews"
```
|
