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.
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.
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>
Calling #content_for
stores a block of markup in an identifier for later use. In order to access
this stored content in other templates, helper modules or the layout, you
would pass the identifier as an argument to content_for
.
Note: yield
can still be used to retrieve the stored content,
but calling yield
doesn't work in helper modules, while
content_for
does.
<% content_for :not_authorized do %>
alert('You are not authorized to do that!')
<% end %>
You can then use content_for :not_authorized
anywhere in your
templates.
<%= content_for :not_authorized if current_user.nil? %>
This is equivalent to:
<%= yield :not_authorized if current_user.nil? %>
content_for
, however, can also be used in helper modules.
module StorageHelper
def stored_content
content_for(:storage) || "Your storage is empty"
end
end
This helper works just like normal helpers.
<%= stored_content %>
You can also use the yield
syntax alongside an existing call
to yield
in 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 script
identifier.
<%# This is our view %>
Please login!
<% content_for :script do %>
<script>alert('You are not authorized to view this page!')</script>
<% end %>
Then, in another view, you could to do something like this:
<%= link_to 'Logout', action: 'logout', remote: true %>
<% content_for :script do %>
<%= javascript_include_tag :defaults %>
<% end %>
That will place script
tags for your default set of JavaScript
files on the page; this technique is useful if you'll only be using
these scripts in a few views.
Note that #content_for concatenates (default) 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 %>
And in other place:
<% 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><%= content_for :navigation %></ul>
If the flush parameter is true #content_for replaces the blocks it is given. 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, flush: true do %>
<li><%= link_to 'Login', action: 'login' %></li>
<% end %>
Then, in another template or layout, this code would render only the last link:
<ul><%= content_for :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.
# File actionpack/lib/action_view/helpers/capture_helper.rb, line 148 def content_for(name, content = nil, options = {}, &block) if content || block_given? if block_given? options = content if content content = capture(&block) end if content options[:flush] ? @view_flow.set(name, content) : @view_flow.append(name, content) end nil else @view_flow.get(name).presence end end
#content_for? checks whether any content has been captured yet using `content_for`. Useful to render parts of your layout differently based on what is in your views.
<%# 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 class="<%= content_for?(:right_col) ? 'two-column' : 'one-column' %>">
<%= yield %>
<%= yield :right_col %>
</body>
</html>
The same as content_for
but when used with streaming flushes
straight back to the layout. In other words, if you want to concatenate
several times to the same buffer when rendering a given template, you
should use content_for
, if not, use provide
to
tell the layout to stop looking for more contents.