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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.

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.

Methods
#
C
I

Attributes

[R] lookup_context
[R] view_renderer

Class Public methods

cache_template_loading()

# File actionview/lib/action_view/base.rb, line 171
def cache_template_loading
  ActionView::Resolver.caching?
end

cache_template_loading=(value)

# File actionview/lib/action_view/base.rb, line 175
def cache_template_loading=(value)
  ActionView::Resolver.caching = value
end

inspect()

# File actionview/lib/action_view/base.rb, line 191
def inspect
  "#<ActionView::Base:#{'%#016x' % (object_id << 1)}>"
end

Instance Public methods

_run(method, template, locals, buffer, add_to_stack: true, has_strict_locals: false, &block)

# File actionview/lib/action_view/base.rb, line 245
def _run(method, template, locals, buffer, add_to_stack: true, has_strict_locals: false, &block)
  _old_output_buffer, _old_virtual_path, _old_template = @output_buffer, @virtual_path, @current_template
  @current_template = template if add_to_stack
  @output_buffer = buffer

  if has_strict_locals
    begin
      public_send(method, buffer, **locals, &block)
    rescue ArgumentError => argument_error
      raise(
        ArgumentError,
        argument_error.
          message.
            gsub("unknown keyword:", "unknown local:").
            gsub("missing keyword:", "missing local:").
            gsub("no keywords accepted", "no locals accepted")
      )
    end
  else
    public_send(method, locals, buffer, &block)
  end
ensure
  @output_buffer, @virtual_path, @current_template = _old_output_buffer, _old_virtual_path, _old_template
end

compiled_method_container()

# File actionview/lib/action_view/base.rb, line 270
    def compiled_method_container
      raise NotImplementedError, <<~msg.squish
        Subclasses of ActionView::Base must implement `compiled_method_container`
        or use the class method `with_empty_template_cache` for constructing
        an ActionView::Base subclass that has an empty cache.
      msg
    end

in_rendering_context(options)

# File actionview/lib/action_view/base.rb, line 278
def in_rendering_context(options)
  old_view_renderer  = @view_renderer
  old_lookup_context = @lookup_context

  if !lookup_context.html_fallback_for_js && options[:formats]
    formats = Array(options[:formats])
    if formats == [:js]
      formats << :html
    end
    @lookup_context = lookup_context.with_prepended_formats(formats)
    @view_renderer = ActionView::Renderer.new @lookup_context
  end

  yield @view_renderer
ensure
  @view_renderer = old_view_renderer
  @lookup_context = old_lookup_context
end