Action View Base

Action View templates can be written in several ways. If the template file has a .erb extension, then it uses the erubi template system which can embed Ruby into an HTML document. If the template file has a .builder extension, then Jim Weirich’s Builder::XmlMarkup library is used.

ERB

You trigger ERB by using embeddings such as <% %>, <% -%>, and <%= %>. The <%= %> tag set is used when you want output. Consider the following loop for names:

<b>Names of all the people</b>
<% @people.each do |person| %>
  Name: <%= person.name %><br/>
<% end %>

The loop is set up in regular embedding tags <% %>, and the name is written using the output embedding tag <%= %>. Note that this is not just a usage suggestion. Regular output functions like print or puts won’t work with ERB templates. So this would be wrong:

<%# WRONG %>
Hi, Mr. <% puts "Frodo" %>

If you absolutely must write from within a function use concat.

When on a line that only contains whitespaces except for the tag, <% %> suppresses leading and trailing whitespace, including the trailing newline. <% %> and <%- -%> are the same. Note however that <%= %> and <%= -%> are different: only the latter removes trailing whitespaces.

Using sub templates

Using sub templates allows you to sidestep tedious replication and extract common display structures in shared templates. The classic example is the use of a header and footer (even though the Action Pack-way would be to use Layouts):

<%= render "application/header" %>
Something really specific and terrific
<%= render "application/footer" %>

As you see, we use the output embeddings for the render methods. The render call itself will just return a string holding the result of the rendering. The output embedding writes it to the current template.

But you don’t have to restrict yourself to static includes. Templates can share variables amongst themselves by using instance variables defined using the regular embedding tags. Like this:

<% @page_title = "A Wonderful Hello" %>
<%= render "application/header" %>

Now the header can pick up on the @page_title variable and use it for outputting a title tag:

<title><%= @page_title %></title>

Passing local variables to sub templates

You can pass local variables to sub templates by using a hash with the variable names as keys and the objects as values:

<%= render "application/header", { headline: "Welcome", person: person } %>

These can now be accessed in application/header with:

Headline: <%= headline %>
First name: <%= person.first_name %>

The local variables passed to sub templates can be accessed as a hash using the local_assigns hash. This lets you access the variables as:

Headline: <%= local_assigns[:headline] %>

This is useful in cases where you aren’t sure if the local variable has been assigned. Alternatively, you could also use defined? headline to first check if the variable has been assigned before using it.

By default, templates will accept any locals as keyword arguments. To restrict what locals a template accepts, add a locals: magic comment:

<%# locals: (headline:) %>

Headline: <%= headline %>

In cases where the local variables are optional, declare the keyword argument with a default value:

<%# locals: (headline: nil) %>

<% unless headline.nil? %>
Headline: <%= headline %>
<% end %>

Read more about strict locals in Action View Overview in the guides.

Template caching

By default, Rails will compile each template to a method in order to render it. When you alter a template, Rails will check the file’s modification time and recompile it in development mode.

Builder

Builder templates are a more programmatic alternative to ERB. They are especially useful for generating XML content. An XmlMarkup object named xml is automatically made available to templates with a .builder extension.

Here are some basic examples:

xml.em("emphasized")                                 # => <em>emphasized</em>
xml.em { xml.b("emph & bold") }                      # => <em><b>emph &amp; bold</b></em>
xml.a("A Link", "href" => "http://onestepback.org")  # => <a href="http://onestepback.org">A Link</a>
xml.target("name" => "compile", "option" => "fast")  # => <target option="fast" name="compile"\>
                                                     # NOTE: order of attributes is not specified.

Any method with a block will be treated as an XML markup tag with nested markup in the block. For example, the following:

xml.div do
  xml.h1(@person.name)
  xml.p(@person.bio)
end

would produce something like:

<div>
  <h1>David Heinemeier Hansson</h1>
  <p>A product of Danish Design during the Winter of '79...</p>
</div>

Here is a full-length RSS example actually used on Basecamp:

xml.rss("version" => "2.0", "xmlns:dc" => "http://purl.org/dc/elements/1.1/") do
  xml.channel do
    xml.title(@feed_title)
    xml.link(@url)
    xml.description "Basecamp: Recent items"
    xml.language "en-us"
    xml.ttl "40"

    @recent_items.each do |item|
      xml.item do
        xml.title(item_title(item))
        xml.description(item_description(item)) if item_description(item)
        xml.pubDate(item_pubDate(item))
        xml.guid(@person.firm.account.url + @recent_items.url(item))
        xml.link(@person.firm.account.url + @recent_items.url(item))

        xml.tag!("dc:creator", item.author_name) if item_has_creator?(item)
      end
    end
  end
end

For more information on Builder please consult the source code.