Keynote
Flexible presenters for Rails.
A presenter is an object that encapsulates view logic. Like Rails helpers, presenters help you keep complex logic out of your templates.
Keynote provides a consistent interface for defining and instantiating presenters.
Usage
The basic idea
A simple case is making a presenter that's named after a model class and holds helper methods related to that model.
# app/presenters/user_presenter.rb
class UserPresenter < Keynote::Presenter
presents :user
def display_name
"#{user.first_name} #{user.last_name}"
end
def profile_link
link_to user, display_name, data: { user_id: user.id }
end
end
You can then instantiate it by calling the present
method (aliased to k
) in
a view, helper, controller, or another presenter.
<%# app/layouts/_header.html.erb %>
<div id="header">
...
<div class="profile_link">
<%= k(current_user).profile_link %>
</div>
</div>
If you pass anything other than a symbol or string as the first parameter of
present
/k
, Keynote will assume you want to instantiate a presenter named
after the class of that object -- in this case, the model is a User
, so
Keynote looks for a class called UserPresenter
.
Generating HTML
To make it easier to generate slightly more complex chunks of HTML, Keynote includes a modified version of Magnus Holm's Rumble library. Rumble gives us a simple block-based syntax for generating HTML fragments. Here's a small example:
build_html do
div id: :content do
h1 'Hello World', class: :main
end
end
Becomes:
<div id="content">
<h1 class="main">Hello World</h1>
</div>
You can use tag helpers like div
, span
, and a
only within a block passed
to the build_html
method. The build_html
method returns a safe string. See
the documentation for Keynote::Rumble
for more information.
A more complex example
Let's add to our original example by introducing a named presenter. In addition
to UserPresenter
, which has general-purpose methods for displaying the User
model, we'll create HeaderPresenter
, which has methods that are specific to
the layouts/header
partial.
# app/presenters/header_presenter.rb
class HeaderPresenter < Keynote::Presenter
presents :user
def profile_or_login_link
if logged_in? # defined in a helper
profile_link
else
login_link
end
end
def profile_link
build_html do
div class: 'profile_link' do
k(user).profile_link
end
end
end
def login_link
build_html do
div class: 'login_link' do
link_to 'Log In', login_url
end
end
end
end
<%# app/layouts/_header.html.erb %>
<% header = present(:header, current_user) %>
<div id="header">
...
<%= header.profile_or_login_link %>
</div>
We've avoided putting a conditional in the template, and we've also avoided
exposing the profile_or_login_link
method to other parts of the app that
shouldn't need to care about it. It's located in a class that's specific to
this context.
Delegating to models
If you want to delegate some calls on the presenter to one of the presenter's
underlying objects, it's easy to do it explicitly with ActiveSupport's
delegate
API.
# app/presenters/user_presenter.rb
class UserPresenter < Keynote::Presenter
presents :user
delegate :first_name, :last_name, to: :user
def display_name
"#{first_name} #{last_name}"
end
end
You can also generate prefixed methods like user_first_name
by passing
prefix: true
to the delegate
method.
Testing
Testing a Keynote presenter is similar to using it in views or controllers. You can test presenters with RSpec, Test::Unit, MiniTest, or MiniTest::Unit.
RSpec
Your test files should be in spec/presenters
or labeled with
type: :presenter
metadata.
Here's an example:
# spec/presenters/user_presenter_spec.rb
RSpec.describe UserPresenter do
describe "#display_name" do
it "returns the name of the user" do
user = User.new(first_name: "Alice", last_name: "Smith")
expect(present(user).display_name).to eq("Alice Smith")
end
end
end
Test::Unit and MiniTest
Your test classes should inherit from Keynote::TestCase.
class UserPresenterTest < Keynote::TestCase
setup do
user = User.new(first_name: "Alice", last_name: "Smith")
@presenter = present(user)
end
test "display name" do
assert_equal @presenter.display_name, "Alice Smith"
end
end
Rationale
Why use presenters or decorators at all?
The main alternative is to use helpers. Helpers are fine for many use cases -- Rails' built-in tag and form helpers are great. They have some drawbacks, though:
-
Every helper method you write gets mixed into the same view object as the built-in Rails helpers, URL generators, and all the other junk that comes along with
ActionView::Base
. In a freshly-generated Rails project:>> ApplicationController.new.view_context.public_methods.count => 318 >> ApplicationController.new.view_context.private_methods.count => 119
-
Helpers can't have state that isn't "global" relative to the view, which can make it hard to write helpers that work together.
-
By default, every helper is available in every view. This makes it difficult to set boundaries between different parts of your app and organize your view code cleanly.
Why not use decorators?
The biggest distinction between Keynote and similar libraries like Draper and DisplayCase is that Keynote presenters aren't decorators β undefined method calls don't fall through to an underlying model.
Applying the Decorator pattern to generating views is a reasonable thing to do. However, this practice also has some downsides.
-
Decorators make the most sense when there's exactly one object that's relevant to the methods you want to encapsulate. They're less helpful when you want to do things like define a class whose responsibility is to help render a specific part of your user interface, which may involve bringing in data from multiple models or collections.
-
When reading code that uses decorators, it often isn't obvious if a given method is defined on the decorator or the underlying model, especially when the decorator is applied in the controller instead of the view.
-
Passing decorated models between controllers and views can make it unclear whether a view (especially a nested partial) depends on a model having some specific decorator applied to it. This makes refactoring view and decorator code harder than it needs to be.
Generators
Keynote doesn't automatically generate presenters when you generate models or
resources. To generate a presenter, you can use the presenter
generator,
like so:
$ rails g presenter FooBar foo bar
create app/presenters/foo_bar_presenter.rb
create spec/presenters/foo_bar_presenter_spec.rb
That project uses RSpec, but the generator can also create test files for Test::Unit or MiniTest::Rails if applicable.
Compatibility
Keynote is supported on Rails 3.1 through 5.1. Keynote presenters are testable with Test::Unit, RSpec, and MiniTest::Rails (>= 2.0).
If you find problems with any of the above integrations, please open an issue.
Development
This repo uses Roadshow to generate a Docker Compose file for each supported version of Rails (with a compatible version of Ruby for each one).
To run specs across all versions, you can either get the Roadshow tool and
run roadshow run
, or use Docker Compose directly:
$ for fn in scenarios/*.docker-compose-yml; do docker-compose -f $fn run --rm scenario; done
To update the set of scenarios, edit scenarios.yml
and run roadshow generate
, although the Gemfiles in the scenarios
directory need to be
maintained manually.
Feel free to submit pull requests according to the usual conventions for Ruby projects.