# Keynote
*Flexible presenters for Rails.*
[](https://travis-ci.org/rf-/keynote/builds)
[](https://codeclimate.com/github/rf-/keynote)
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.
``` ruby
# 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.
``` erb
<%# app/layouts/_header.html.erb %>
```
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](https://github.com/judofyr/rumble)
library. Rumble gives us a simple block-based syntax for generating HTML
fragments. Here's a small example:
``` ruby
build_html do
div :id => :content do
h1 'Hello World', :class => :main
end
end
```
Becomes:
``` html
Hello World
```
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`](http://rubydoc.info/gems/keynote/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.
``` ruby
# 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
```
``` erb
<%# app/layouts/_header.html.erb %>
<% header = present(:header, current_user) %>
```
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.
``` ruby
# 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:
```ruby
>> 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:
``` bash
$ 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:
``` bash
$ bundle install
$ bundle exec appraisal install
$ bundle exec rake appraisal
```
Feel free to submit pull requests according to the usual conventions for Ruby
projects.
[DisplayCase]: https://github.com/objects-on-rails/display-case
[Draper]: https://github.com/drapergem/draper