RailsConf: Presenters

RailsConf 2014 Talk, Tuesday, 2:30, Ballroom 4, [Concerns, Decorators, Presenters, Service Objects, Helpers, Help Me Decide!](http://www.railsconf.com/program#prop_445)

# Presenters

Code: [Github Fat Code Refactoring Techniques Pull Request Presenters](https://github.com/justin808/fat-code-refactoring-techniques/pull/11/files)

Example: Added `FollowersPresenter` and `FollowedPresenter` to encapsulate
multiple instance variables in the show\_follow view.

**WARNING**: This code change is an over-simplified example of a good case of when
to encapsulate the state passed to a view. As you look at this example, consider
a case where there is far more logic in the view and the controller setup of the
instance variables for the view. 




## A Name for a PORO

-   Some Object for Complicated controller setup for a view
-   Terminology is controversial in the context of prior literature.
-   Trying to describe a object that sits between a controller method and
    a view.
-   Could have used "Facade", but..I didn't like it, and some others gave me a
    thumbs up on the term "Presenter".

## "Presenter"

-   Object used to facilitate presentation of data in the view, typically
    constructed in the controller method or maybe in the view.
-   Smooth sounding.
-   Easy to spell.

## Why?

1.  You're trying to reduce the number of instance variables assigned by the
    controller action to one.
2.  You want to consolidate the view specific Ruby code in one place, rather than
    being spread inside the ERB or HAML code.

## Presenter Applicable Situations

1.  Controller is initializing multiple instance variables for the view.
2.  Controller has several private methods supporting the public method
    associated with the action just to set up the context for the view.
3.  View has lots of inline Ruby code for calculating instance variables
4.  Logic is specific to one controller method and one view, or maybe a couple
    different response types for the same controller method.
5.  Numerous view helper methods interacting just for one action.
6.  If you find same code in multiple presenters for same model, then see if you
    can move to a decorator or concern. A module is another option for DRY'ing up
    duplication.

## Presenter Solution

-   Create directory `app/presenters` and add this code to
      `application.rb`
    
```ruby
  config.autoload_paths += %W(
    #{config.root}/app/presenters
  )
```

-   Create subdirectory `app/presenters/users`
-   Create presenter: `User::FollowPresenter`
-   Controller instantiates the `User::FollowPresenter` that is used by the view.
-   Move ruby code from view and the controller into the presenter.
-   Possibly include this snippet of code so that view instance methods
    are available:
    
```ruby
        include Draper::ViewHelpers
```

## Presenter Advantages

1.  Clearly bundles the code around the work the controller is doing and what the
    view needs.
2.  Simplifies code at the controller and view layers.
3.  Provides the possibility of unit testing the Presenter.
4.  Easy to group related methods and memoize instance variables, which can be
    highly beneficial if fragment caching is used, as compared to instantiating
    instance variables in the controller..
5.  Good place for the calculation of a complicated cache key rather than a view
    helper.

## Presenter Disadvantages

1.  There might be simpler techniques than creating another class and object, such
    as putting some logic in the Draper decorator or a view helper. A few lines of
    inline ruby code may work better than moving this code to a separate class.
2.  Other options for controller simplification include breaking up a controller
    into several classes and using controller concerns. However, those may not
    tackle the issue of passing too many instance variables into the view.
@justin808 You're right when you say that the method would just be a pass through, but unfortunately, without it, you are introducing some unnecessary coupling to 3rd party objects. In my opinion, that is one of the purposes of having a Presenter object. To eliminate the coupling between the View layer and the Model/data layer.

There is a portion of an article on "The Pragmatic Bookshelf" that addresses this issue exactly...under the subhead "Law of Demeter" in the article "Tell, Don't Ask" it says:

>The disadvantage, of course, is that you end up writing many small wrapper methods that do very little but delegate container traversal and such. The cost tradeoff is between that inefficiency and higher class coupling.

>The higher the degree of coupling between classes, the higher the odds that any change you make will break something somewhere else. This tends to create fragile, brittle code.

>Depending on your application, the development and maintenance costs of high class coupling may easily swamp run-time inefficiencies in most cases.

Reference: https://pragprog.com/articles/tell-dont-ask

There is also a really great talk by Ben Orenstein that touches on these points and why they're beneficial. Check it out: https://www.youtube.com/watch?v=C0H-LyZy9Ko

You also mentioned in your previous comment:

>However, if there was some sort of modification to the @presenter.user.name, such as needing to to modify the value if it were too long, then that would be a fine example to justify a new method.

I think the problem here is that when you change the model, you have to remember that you need to change things not only in the presenter, but also in the view (and perhaps in other classes - real apps are always more complex). If the presenter was the only one coupled to the Model, then this would be far less of an issue.

Of course, one could go crazy with this stuff and it might not be practical to follow this religiously for every circumstance, however, it has surely helped me to keep coupling low and save me tons of headaches especially when more features and complexity are introduced into the system.