CaptureHelper exposes methods to let you extract generated markup which can be used in other parts of a template or layout file. It provides a method to capture blocks into variables through capture and a way to capture a block of markup for use in a layout through content_for.

Methods
C
Instance Public methods
capture(*args, &block)

The capture method allows you to extract part of a template into a variable. You can then use this variable anywhere in your templates or layout.

Examples

The capture method can be used in ERb templates…

<% @greeting = capture do %>
  Welcome to my shiny new web page!  The date and time is
  <%= Time.now %>
<% end %>

…and Builder (RXML) templates.

@timestamp = capture do
  "The current timestamp is #{Time.now}."
end

You can then use that variable anywhere else. For example:

<html>
<head><title><%= @greeting %></title></head>
<body>
<b><%= @greeting %></b>
</body></html>
# File actionpack/lib/action_view/helpers/capture_helper.rb, line 33
def capture(*args, &block)
  # Return captured buffer in erb.
  if block_called_from_erb?(block)
    with_output_buffer { block.call(*args) }
  else
    # Return block result otherwise, but protect buffer also.
    with_output_buffer { return block.call(*args) }
  end
end
content_for(name, content = nil, &block)

Calling #content_for stores a block of markup in an identifier for later use. You can make subsequent calls to the stored content in other templates or the layout by passing the identifier as an argument to yield.

Examples

<% content_for :not_authorized do %>
  alert('You are not authorized to do that!')
<% end %>

You can then use yield :not_authorizedanywhere in your templates.

<%= yield :not_authorized if current_user.nil? %>

You can also use this syntax alongside an existing call to yieldin a layout. For example:

<%# This is the layout %>
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
  <title>My Website</title>
  <%= yield :script %>
</head>
<body>
  <%= yield %>
</body>
</html>

And now, we'll create a view that has a #content_for call that creates the scriptidentifier.

<%# This is our view %>
Please login!
<% content_for :script do %>
  <script type="text/javascript">alert('You are not authorized to view this page!')</script>
<% end %>

Then, in another view, you could to do something like this:

<%= link_to_remote 'Logout', :action => 'logout' %>
<% content_for :script do %>
  <%= javascript_include_tag :defaults %>
<% end %>

That will place <script> tags for Prototype, Scriptaculous, and application.js (if it exists) on the page; this technique is useful if you'll only be using these scripts in a few views.

Note that #content_for concatenates the blocks it is given for a particular identifier in order. For example:

<% content_for :navigation do %>
  <li><%= link_to 'Home', :action => 'index' %></li>
<% end %>
<%#  Add some other content, or use a different template: %>
<% content_for :navigation do %>
  <li><%= link_to 'Login', :action => 'login' %></li>
<% end %>

Then, in another template or layout, this code would render both links in order:

<ul><%= yield :navigation %></ul>

Lastly, simple content can be passed as a parameter:

<% content_for :script, javascript_include_tag(:defaults) %>

WARNING: #content_for is ignored in caches. So you shouldn't use it for elements that will be fragment cached.

The deprecated way of accessing a #content_for block is to use an instance variable named @content_for_#{name_of_the_content_block}. The preferred usage is now <%= yield :footer %>.

# File actionpack/lib/action_view/helpers/capture_helper.rb, line 118
def content_for(name, content = nil, &block)
  ivar = "@content_for_#{name}"
  content = capture(&block) if block_given?
  instance_variable_set(ivar, "#{instance_variable_get(ivar)}#{content}".html_safe)
  nil
end