Parts

All values exposed by your view are decorated and passed to your templates as parts, which allow encapsulation of view-specific behavior alongside your application's domain objects.

Unlike many third-party approaches to view object decoration, dry-view's parts are fully integrated and have access to the full rendering environment, which means that anything you can do from a template, you can also do from a part. This includes accessing the context object as well as rendering partials and building scopes.

This means that much more view logic can move out of template and into parts, which makes the templates simpler and more declarative, and puts the view logic into a place where it can be reused and refactored using typical object oriented approaches, as well as tested in isolation.

Defining a part class

To provide custom part behavior, define your own part classes in a common namespace (e.g. Parts) and configure that as your view's part_namespace Each part class must inherit from Dry::View::Part.

module Parts
  class User < Dry::View::Part
  end
end

Part class resolution

Part classes are looked up based on each exposure's name.

So for an exposure named :article, the Parts::Article class will be looked up and used to decorate the article value.

For an exposure returning an array, the exposure's name will be singularized and each element in the array will be decorated with a matching part. Then the array itself will be decorated by a matching part.

So for an exposure named :articles, the Parts::Article class will be looked up for decorating each element, and the Parts::Articles class will be looked up for decorating the entire array.

If a matching part class cannot be found, the standard Dry::View::Part class will be used.

Accessing the decorated value

When using a part within a template, or when defining your own part methods, you can call the decorated value's methods and the part object will pass them through (via #method_missing).

For example, from a template:

<!-- All the methods on the user value are still available -->
<p><%= user.name %></p>

Or when defining a custom part class:

class User < Dry::View::Part
  def display_name
    # `name` and `email` are methods on the decorated user value
    "#{name} <#{email}>"
  end
end

In case of naming collisions or when overriding a method, you can access the value directly via #_value (or #value as a convenience, as long the decorated value itself doesn't respond to #value):

class User < Dry::View::Part
  def name
    value.name.upcase
  end
end

String conversion

When used to output to the template, a part will use it's value #to_s behavior (which you can override in your part classes):

<p><%= user %></p>

Rendering partials

From a part, you can render a partial, with the part object included in the partial's own locals:

class User < Dry::View::Part
  def info_box
    render(:info_box)
  end
end

This will render an _info_box partial template (via the standard partial lookup rules) with the part still available as user.

You can also render such partials directly within templates:

<%= user.render(:info_box) %>

To make the part available by another name within the partial's cope, use the as: option:

<%= user.render(:info_box, as: :account) %>

You can also provide additional locals for the partial:

<%= user.render(:info_box, as: :account, title_label: "Your account") %>

Building scopes

You may build custom scopes from within a part using #_scope (or #scope as a convenience, as long as the decorated value doesn't respond to #scope):

class User < Dry::View::Part
  def info_box
    scope(:info_box, size: :large).render
  end
end

Accessing the context

In your part classes, you can access the context object as #_context (or #context as a convenience, as long the decorated value itself doesn't respond to #context). Parts also delegate missing methods to the context object (provided the decorated value itself doesn't respond to the method).

For example:

class User < Dry::View::Part
  def avatar_url
    # asset_path is a method defined on the context object (in this case,
    # providing static asset URLs)
    value.avatar_url || asset_path("default-user-avatar.png")
  end
end

Decorating part attributes

Your values may have their own attributes that you also want decorated as view parts. Declare these using decorate in your own view part classes:

class UserPart < Dry::View::Part
  decorate :address
end

You can pass the same options to decorate as you do to exposures, for example:

class UserPart < Dry::View::Part
  decorate :address, as: :location
end

Memoizing methods

A part object lives for the entirety of a view rendering, you can memoize expensive operations to ensure they only run once.

class User < Dry::View::Part
  def bio_html
    @bio_html ||= rich_text_renderer.render(bio)
  end

  private

  def rich_text_renderer
    @rich_text_renderer ||= MyRenderer.new
  end
end

Custom part class resolution

When defining your exposures, use the as: option to specify an alternative name or class for part decoration.

For singular values:

  • expose :article, as: :story will look up a Parts::Story class
  • expose :article, as: Parts::MyArticle will use the provided class

For arrays:

  • expose :articles, as: :stories will look up Parts::Stories for decorating the array, and Parts::Story for decorating the elements
  • expose :articles, as: [:story_collection] will look up Parts::StoryCollection for decorating the array, and Parts::Article for decorating the elements
  • expose :articles, as: [:story_collection, :story] will look up Parts::StoryCollection for decorating the array, and Parts::Story for decorating the elements
  • For the two as: structures above (with the names in the array), explicit classes can be provided instead of symbols, and they'll be used for decorating their respective items

All of these examples presume a configured part_namespace of Parts.

Providing a custom part builder

To fully customize part decoration, you can provide a replacement part builder:

class MyView < Dry::View
  config.part_builder = MyPartBuilder
end

Your part builder must conform to the following interface:

  • #initialize(namespace: nil, render_env: nil)
  • #for_render_env(render_env)
  • #call(name, value, **options)

You can also inherit from Dry::View::PartBuilder and override any of its methods, if you want to customize just a particular aspect of the standard behavior.