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Flexible presenters for Rails.
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README.md

Keynote

Flexible presenters for Rails.

Travis CI Code Climate

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.

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 – 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, 3.2, 4.0, 4.1, and 4.2. 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

You can run Keynote's tests across all supported versions of Rails as follows:

$ bundle install
$ bundle exec appraisal install
$ bundle exec rake appraisal

Feel free to submit pull requests according to the usual conventions for Ruby projects.

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