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.
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'
Curly can also install an application layout file, replacing the .erb file commonly
created by Rails. If you wish to use this, run the curly:install
generator.
$ rails generate curly:install
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
SafeMarkdown.render(@comment.body)
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
Curly components are surrounded by curly brackets, e.g. {{hello}}
. They always map to a
public method on the presenter class, in this case #hello
. Methods ending in a question mark
can be used for conditional blocks, e.g. {{#admin?}} ... {{/admin?}}
.
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.
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. Additionally, if the presenter
method accepts arbitrary keyword arguments using the **doublesplat
syntax then all
attributes will be valid for the component, e.g.
def greetings(**names)
names.map {|name, greeting| "#{name}: #{greeting}!" }.join("\n")
end
{{greetings alice=hello bob=hi}}
<!-- The above would be rendered as: -->
alice: hello!
bob: hi!
Note that since keyword arguments in Ruby are represented as Symbol objects, which are not garbage collected in Ruby versions less than 2.2, accepting arbitrary attributes represents a security vulnerability if your application allows untrusted Curly templates to be rendered. Only use this feature with trusted templates if you're not on Ruby 2.2 yet.
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.
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.
While collection blocks allow you to define the template that should be used to render items in a collection right within the parent template, context blocks allow you to define the template for an arbitrary context. This is very powerful, and can be used to define widget-style components and helpers, and provide an easy way to work with structured data. Let's say you have a comment form on your page, and you'd rather keep the template inline. A simple template could look like:
<!-- post.html.curly -->
<h1>{{title}}</h1>
{{body}}
{{@comment_form}}
<b>Name: </b> {{name_field}}<br>
<b>E-mail: </b> {{email_field}}<br>
{{comment_field}}
{{submit_button}}
{{/comment_form}}
Note that an @
character is used to denote a context block. Like with
collection blocks, a separate presenter class is used within the
block, and a simple convention is used to find it. The name of the context component
(in this case, comment_form
) will be camel cased, and the current presenter's namespace
will be searched:
class PostPresenter < Curly::Presenter
presents :post
def title; @post.title; end
def body; markdown(@post.body); end
# A context block method *must* take a block argument. The return value
# of the method will be used when rendering. Calling the block argument will
# render the nested template. If you pass a value when calling the block
# argument it will be passed to the presenter.
def comment_form(&block)
form_for(Comment.new, &block)
end
# The presenter name is automatically deduced.
class CommentFormPresenter < Curly::Presenter
# The value passed to the block argument will be passed in a parameter named
# after the component.
presents :comment_form
# Any parameters passed to the parent presenter will be forwarded to this
# presenter as well.
presents :post
def name_field
@comment_form.text_field :name
end
# ...
end
end
Context blocks were designed to work well with Rails' helper methods such as form_for
and content_tag
, but you can also work directly with the block. For instance, if you
want to directly control the value that is passed to the nested presenter, you can call
the call
method on the block yourself:
def author(&block)
content_tag :div, class: "author" do
# The return value of `call` will be the result of rendering the nested template
# with the argument. You can post-process the string if you want.
block.call(@post.author)
end
end
If you find yourself opening a context block just in order to use a single component,
e.g. {{@author}}{{name}}{{/author}}
, you can use the shorthand syntax instead:
{{author:name}}
. This works for all component types, e.g.
{{#author:admin?}}
<p>The author is an admin!</p>
{{/author:admin?}}
The syntax works for nested contexts as well, e.g. {{comment:author:name}}
. Any
identifier and attributes are passed to the target component, which in this example
would be {{name}}
.
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
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 }}
.
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 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.
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
In order to make a Rails helper method available as a component in your template,
use the exposes_helper
method:
class Layouts::ApplicationPresenter < Curly::Presenter
# The components {{sign_in_path}} and {{root_path}} are made available.
exposes_helper :sign_in_path, :root_path
end
Presenters can be tested directly, but sometimes it makes sense to integrate with Rails on some levels. Currently, only RSpec is directly supported, but you can easily instantiate a presenter:
SomePresenter.new(context, assigns)
context
is a view context, i.e. an object that responds to render
, has all
the helper methods you expect, etc. You can pass in a test double and see what
you need to stub out. assigns
is the hash containing the controller and local
assigns. You need to pass in a key for each argument the presenter expects.
In order to test presenters with RSpec, make sure you have rspec-rails
in your
Gemfile. Given the following presenter:
# app/presenters/posts/show_presenter.rb
class Posts::ShowPresenter < Curly::Presenter
presents :post
def body
Markdown.render(@post.body)
end
end
You can test the presenter methods like this:
# You can put this in your `spec_helper.rb`.
require 'curly/rspec'
# spec/presenters/posts/show_presenter_spec.rb
describe Posts::ShowPresenter, type: :presenter do
describe "#body" do
it "renders the post's body as Markdown" do
assign(:post, double(:post, body: "**hello!**"))
expect(presenter.body).to eq "<strong>hello!</strong>"
end
end
end
Note that your spec must be tagged with type: :presenter
.
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 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.
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
In order to set any cache option, define a #cache_options
method that
returns a Hash of options:
class Posts::ShowPresenter < Curly::Presenter
...
def cache_options
{ compress: true, namespace: "my-app" }
end
end
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
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
.
Curly integrates well with the
caching mechanism in Rails 4 (or
Cache Digests in Rails 3), so the dependencies
defined with depends_on
will be tracked by Rails. This will allow you to deploy changes
to your templates and have the relevant caches automatically expire.
Thanks to Zendesk for sponsoring the work on Curly.
- Daniel Schierbeck (@dasch)
- Benjamin Quorning (@bquorning)
- Jeremy Rodi (@medcat)
- Alisson Cavalcante Agiani (@thelinuxlich)
- Łukasz Niemier (@hauleth)
- Cristian Planas (@Gawyn)
- Steven Davidovitz (@steved)
Copyright (c) 2013 Daniel Schierbeck (@dasch), Zendesk Inc.
Licensed under the Apache License Version 2.0.