h3. Introduction

[[Helpers]] ("view helpers") are modules where you can write methods which are automatically usable in the associated view.  What's the associated view?  If your view is, say, @app/views/user/prefs.rhtml@, then the helper you're after is @UserHelper@, found in @app/helpers/user_helper.rb@.

The purpose of writing a method in a helper is to simplify the view.  It's best if the view file (RHTML/RXML) is short and sweet, so you can see the structure of the output.  Nitty-gritty details are best left to helper methods and partials, where they can be parametized and used repeatedly.

h3. Example

For example, in the *view* (@views/user/books.rhtml@):

<pre><code>
  You have <%= @books.size %> books in your collection:
  <ul>
    <%= book_list @books, :max => 5 %>
  </ul>
</code></pre>

In the *helper* (@helpers/user_helper.rb@):

<pre><code>
  def book_list(books, options={:max => 10})
    max = options[:max]
    list = books[0...max].map { |book|
      link = link_to "#{book.title}", :action => 'book', :id => book.id
      "<li>#{link}</li>"
    }
    # Add a "more..." link if the list is at maximum size.
    if list.size == max
      more_link = link_to "more...", :action => 'books_all'
      list[-1] = "<li>#{more_link}</li>"
    end
  end
</code></pre>

And the *result*:

<pre>
  You have 2 books in your collection:
   * Stone Alone
   * Catch-22
</pre>

_or..._

<pre>
  You have 37 books in your collection:
  * Catcher in the Rye
  * Fool's Gold
  * VB.NET Unleashed!
  * Harry the Houndog
  * more...
</pre>

And there you have it.  Ensuring the list only displays so many items, and making the last item a special link, is not trivial work, and that logic would clutter the view considerably.  Helpers are just the tonic for such a situation.

h3. Partials

If the view logic you want to capture in a helper is more HTML-heavy, you can use a [[HowtoUsePartials|partial]] instead.

h3. Extra Helpers

Final note: each view, as stated above, has one helper module automatically wired up to it.  If you are creating so many helper methods that you wish you could separate them somehow, you can:

<pre><code>
  class UserController < ApplicationController
    helper :user   # This is implied; you don't have to write this.
    helper :extra_user
  end
</code></pre>

Your user view can now draw on the methods of _two_ helper modules, and you can add more if you like.  They share the same namespace, though, so don't define the same method in both.

The additional helper we specified in the above controller is @ExtraUserHelper@, defined in @app/helpers/extra_user_helper.rb@.


Comment: Interesting.  I would never have guessed that the name "helper" was meant to refer to views, rather tha code in general. What was the rationale for putting the helper directory at the same elvel as controllers and models, rather than inside the view directory where its purpose would be more readily clear?