Curly

Free your views!

Curly is a template language that completely separates structure and logic. Instead of interspersing your HTML with snippets of Ruby, all logic is moved to a presenter class, with only simple placeholders in the HTML.

While the basic concepts are very similar to Mustache or Handlebars, Curly is different in some key ways:

Instead of the template controlling the variable scope and looping through data, all logic is left to the presenter object. This means that untrusted templates can safely be executed, making Curly a possible alternative to languages like Liquid.
Instead of implementing its own template resolution mechanism, Curly hooks directly into Rails, leveraging the existing resolvers.
Because of its close integration with Rails, with Curly you can easily set up caching for your views and partials.

Installing
How to use Curly
Presenters
1. Layouts and Content Blocks
2. Examples
Caching

Installing

Installing Curly is as simple as running gem install curly-templates. If you're using Bundler to manage your dependencies, add this to your Gemfile

gem 'curly-templates'

How to use Curly

In order to use Curly for a view or partial, use the suffix .curly instead of .erb, e.g. app/views/posts/_comment.html.curly. Curly will look for a corresponding presenter class named Posts::CommentPresenter. By convention, these are placed in app/presenters/, so in this case the presenter would reside in app/presenters/posts/comment_presenter.rb. Note that presenters for partials are not prepended with an underscore.

Add some HTML to the partial template along with some Curly components:

<!-- app/views/posts/_comment.html.curly -->
<div class="comment">
  <p>
    {{author_link}} posted {{time_ago}} ago.
  </p>

  {{body}}

  {{#author?}}
    <p>{{deletion_link}}</p>
  {{/author?}}
</div>

The presenter will be responsible for providing the data for the components. Add the necessary Ruby code to the presenter:

# app/presenters/posts/comment_presenter.rb
class Posts::CommentPresenter < Curly::Presenter
  presents :comment

  def body
    BlueCloth.new(@comment.body).to_html
  end

  def author_link
    link_to @comment.author.name, @comment.author, rel: "author"
  end
  
  def deletion_link
    link_to "Delete", @comment, method: :delete
  end

  def time_ago
    time_ago_in_words(@comment.created_at)
  end
  
  def author?
    @comment.author == current_user
  end
end

The partial can now be rendered like any other, e.g. by calling

render 'comment', comment: comment
render comment
render collection: post.comments

Identifiers

Curly components can specify an identifier using the so-called dot notation: {{x.y.z}}. This can be very useful if the data you're accessing is hierarchical in nature. One common example is I18n:

<h1>{{i18n.homepage.header}}</h1>

# In the presenter, the identifier is passed as an argument to the method. The
# argument will always be a String.
def i18n(key)
  translate(key)
end

The identifier is separated from the component name with a dot. If the presenter method has a default value for the argument, the identifier is optional – otherwise it's mandatory.

Attributes

In addition to an identifier, Curly components can be annotated with attributes. These are key-value pairs that affect how a component is rendered.

The syntax is reminiscent of HTML:

<div>{{sidebar rows=3 width=200px title="I'm the sidebar!"}}</div>

The presenter method that implements the component must have a matching keyword argument:

def sidebar(rows: "1", width: "100px", title:); end

All argument values will be strings. A compilation error will be raised if

an attribute is used in a component without a matching keyword argument being present in the method definition; or
a required keyword argument in the method definition is not set as an attribute in the component.

You can define default values using Ruby's own syntax.

Conditional blocks

If there is some content you only want rendered under specific circumstances, you can use conditional blocks. The {{#admin?}}...{{/admin?}} syntax will only render the content of the block if the admin? method on the presenter returns true, while the {{^admin?}}...{{/admin?}} syntax will only render the content if it returns false.

Both forms can have an identifier: {{#locale.en?}}...{{/locale.en?}} will only render the block if the locale? method on the presenter returns true given the argument "en". Here's how to implement that method in the presenter:

class SomePresenter < Curly::Presenter
  # Allows rendering content only if the locale matches a specified identifier.
  def locale?(identifier)
    current_locale == identifier
  end
end

Furthermore, attributes can be set on the block. These only need to be specified when opening the block, not when closing it:

{{#square? width=3 height=3}}
  <p>It's square!</p>
{{/square?}}

Attributes work the same way as they do for normal components.

Collection blocks

Sometimes you want to render one or more items within the current template, and splitting out a separate template and rendering that in the presenter is too much overhead. You can instead define the template that should be used to render the items inline in the current template using the collection block syntax.

Collection blocks are opened using an asterisk:

{{*comments}}
  <li>{{body}} ({{author_name}})</li>
{{/comments}}

The presenter will need to expose the method #comments, which should return a collection of objects:

class Posts::ShowPresenter < Curly::Presenter
  presents :post

  def comments
    @post.comments
  end
end

The template within the collection block will be used to render each item, and it will be backed by a presenter named after the component – in this case, comments. The name will be singularized and Curly will try to find the presenter class in the following order:

Posts::ShowPresenter::CommentPresenter
Posts::CommentPresenter
CommentPresenter

This allows you some flexibility with regards to how you want to organize these nested templates and presenters.

Note that the nested template will only have access to the methods on the nested presenter, but all variables passed to the "parent" presenter will be forwarded to the nested presenter. In addition, the current item in the collection will be passed, as well as that item's index in the collection:

class Posts::CommentPresenter < Curly::Presenter
  presents :post, :comment, :comment_counter

  def number
    # `comment_counter` is automatically set to the item's index in the collection,
    # starting with 1.
    @comment_counter
  end

  def body
    @comment.body
  end

  def author_name
    @comment.author.name
  end
end

Collection blocks are an alternative to splitting out a separate template and rendering that from the presenter – which solution is best depends on your use case.

Setting up state

Although most code in Curly presenters should be free of side effects, sometimes side effects are required. One common example is defining content for a content_for block.

If a Curly presenter class defines a setup! method, it will be called before the view is rendered:

class PostPresenter < Curly::Presenter
  presents :post

  def setup!
    content_for :title, post.title

    content_for :sidebar do
      render 'post_sidebar', post: post
    end
  end
end

Escaping Curly syntax

In order to have {{ appear verbatim in the rendered HTML, use the triple Curly escape syntax:

This is {{{escaped}}.

You don't need to escape the closing }}.

Comments

If you want to add comments to your Curly templates that are not visible in the rendered HTML, use the following syntax:

{{! This is some interesting stuff }}

Presenters

Presenters are classes that inherit from Curly::Presenter – they're usually placed in app/presenters/, but you can put them anywhere you'd like. The name of the presenter classes match the virtual path of the view they're part of, so if your controller is rendering posts/show, the Posts::ShowPresenter class will be used. Note that Curly is only used to render a view if a template can be found – in this case, at app/views/posts/show.html.curly.

Presenters can declare a list of accepted variables using the presents method:

class Posts::ShowPresenter < Curly::Presenter
  presents :post
end

A variable can have a default value:

class Posts::ShowPresenter < Curly::Presenter
  presents :post
  presents :comment, default: nil
end

Any public method defined on the presenter is made available to the template as a component:

class Posts::ShowPresenter < Curly::Presenter
  presents :post
  
  def title
    @post.title
  end
  
  def author_link
    # You can call any Rails helper from within a presenter instance:
    link_to author.name, profile_path(author), rel: "author"
  end
  
  private
  
  # Private methods are not available to the template, so they're safe to
  # use.
  def author
    @post.author
  end
end

Presenter methods can even take an argument. Say your Curly template has the content {{t.welcome_message}}, where welcome_message is an I18n key. The following presenter method would make the lookup work:

def t(key)
  translate(key)
end

That way, simple ``functions'' can be added to the Curly language. Make sure these do not have any side effects, though, as an important part of Curly is the idempotence of the templates.

Layouts and Content Blocks

Both layouts and content blocks (see content_for) use yield to signal that content can be inserted. Curly works just like ERB, so calling yield with no arguments will make the view usable as a layout, while passing a Symbol will make it try to read a content block with the given name:

# Given you have the following Curly template in
# app/views/layouts/application.html.curly
#
#   <html>
#     <head>
#       <title>{{title}}</title>
#     </head>
#     <body>
#       <div id="sidebar">{{sidebar}}</div>
#       {{body}}
#     </body>
#   </html>
#
class ApplicationLayout < Curly::Presenter
  def title
    "You can use methods just like in any other presenter!"
  end
  
  def sidebar
    # A view can call `content_for(:sidebar) { "some HTML here" }`
    yield :sidebar
  end
  
  def body
    # The view will be rendered and inserted here:
    yield
  end
end

Examples

Here is a simple Curly template – it will be looked up by Rails automatically.

<!-- app/views/posts/show.html.curly -->
<h1>{{title}}<h1>
<p class="author">{{author}}</p>
<p>{{description}}</p>

{{comment_form}}

<div class="comments">
  {{comments}}
</div>

When rendering the template, a presenter is automatically instantiated with the variables assigned in the controller or the render call. The presenter declares the variables it expects with presents, which takes a list of variables names.

# app/presenters/posts/show_presenter.rb
class Posts::ShowPresenter < Curly::Presenter
  presents :post

  def title
    @post.title
  end

  def author
    link_to(@post.author.name, @post.author, rel: "author")
  end

  def description
    Markdown.new(@post.description).to_html.html_safe
  end

  def comments
    render 'comment', collection: @post.comments
  end

  def comment_form
    if @post.comments_allowed?
      render 'comment_form', post: @post
    else
      content_tag(:p, "Comments are disabled for this post")
    end
  end
end

Caching

Caching is handled at two levels in Curly – statically and dynamically. Static caching concerns changes to your code and templates introduced by deploys. If you do not wish to clear your entire cache every time you deploy, you need a way to indicate that some view, helper, or other piece of logic has changed.

Dynamic caching concerns changes that happen on the fly, usually made by your users in the running system. You wish to cache a view or a partial and have it expire whenever some data is updated – usually whenever a specific record is changed.

Dynamic Caching

Because of the way logic is contained in presenters, caching entire views or partials by the data they present becomes exceedingly straightforward. Simply define a #cache_key method that returns a non-nil object, and the return value will be used to cache the template.

Whereas in ERB you would include the cache call in the template itself:

<% cache([@post, signed_in?]) do %>
  ...
<% end %>

In Curly you would instead declare it in the presenter:

class Posts::ShowPresenter < Curly::Presenter
  presents :post

  def cache_key
    [@post, signed_in?]
  end
end

Likewise, you can add a #cache_duration method if you wish to automatically expire the fragment cache:

class Posts::ShowPresenter < Curly::Presenter
  ...

  def cache_duration
    30.minutes
  end
end

Static Caching

Static caching will only be enabled for presenters that define a non-nil #cache_key method (see Dynamic Caching.)

In order to make a deploy expire the cache for a specific view, set the version of the view to something new, usually by incrementing by one:

class Posts::ShowPresenter < Curly::Presenter
  version 3

  def cache_key
    # Some objects
  end
end

This will change the cache keys for all instances of that view, effectively expiring the old cache entries.

This works well for views, or for partials that are rendered in views that themselves are not cached. If the partial is nested within a view that is cached, however, the outer cache will not be expired. The solution is to register that the inner partial is a dependency of the outer one such that Curly can automatically deduce that the outer partial cache should be expired:

class Posts::ShowPresenter < Curly::Presenter
  version 3
  depends_on 'posts/comment'

  def cache_key
    # Some objects
  end
end

class Posts::CommentPresenter < Curly::Presenter
  version 4
  depends_on 'posts/comment'

  def cache_key
    # Some objects
  end
end

Now, if the version of Posts::CommentPresenter is bumped, the cache keys for both presenters would change. You can register any number of view paths with depends_on.

If you use Cache Digests, Curly will automatically provide a list of dependencies. This will allow you to deploy changes to your templates and have the relevant caches automatically expire.

Thanks

Thanks to Zendesk for sponsoring the work on Curly.

Contributors

Daniel Schierbeck (@dasch)
Benjamin Quorning (@bquorning)
Jeremy Rodi (@redjazz96)
Alisson Cavalcante Agiani (@thelinuxlich)

Build Status

Copyright and License

Licensed under the Apache License Version 2.0.

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