Flowdock
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Layouts reverse the common pattern of including shared headers and footers in many templates to isolate changes in repeated setups. The inclusion pattern has pages that look like this:

  <%= render "shared/header" %>
  Hello World
  <%= render "shared/footer" %>

This approach is a decent way of keeping common structures isolated from the changing content, but it’s verbose and if you ever want to change the structure of these two includes, you’ll have to change all the templates.

With layouts, you can flip it around and have the common structure know where to insert changing content. This means that the header and footer are only mentioned in one place, like this:

  // The header part of this layout
  <%= yield %>
  // The footer part of this layout

And then you have content pages that look like this:

   hello world

At rendering time, the content page is computed and then inserted in the layout, like this:

  // The header part of this layout
  hello world
  // The footer part of this layout

NOTE: The old notation for rendering the view from a layout was to expose the magic @content_for_layout instance variable. The preferred notation now is to use yield, as documented above.

Accessing shared variables

Layouts have access to variables specified in the content pages and vice versa. This allows you to have layouts with references that won’t materialize before rendering time:

  <h1><%= @page_title %></h1>
  <%= yield %>

…and content pages that fulfill these references at rendering time:

   <% @page_title = "Welcome" %>
   Off-world colonies offers you a chance to start a new life

The result after rendering is:

  <h1>Welcome</h1>
  Off-world colonies offers you a chance to start a new life

Automatic layout assignment

If there is a template in app/views/layouts/ with the same name as the current controller then it will be automatically set as that controller’s layout unless explicitly told otherwise. Say you have a WeblogController, for example. If a template named app/views/layouts/weblog.erb or app/views/layouts/weblog.builder exists then it will be automatically set as the layout for your WeblogController. You can create a layout with the name application.erb or application.builder and this will be set as the default controller if there is no layout with the same name as the current controller and there is no layout explicitly assigned with the layout method. Nested controllers use the same folder structure for automatic layout. assignment. So an Admin::WeblogController will look for a template named app/views/layouts/admin/weblog.erb. Setting a layout explicitly will always override the automatic behaviour for the controller where the layout is set. Explicitly setting the layout in a parent class, though, will not override the child class’s layout assignment if the child class has a layout with the same name.

Inheritance for layouts

Layouts are shared downwards in the inheritance hierarchy, but not upwards. Examples:

  class BankController < ActionController::Base
    layout "bank_standard"

  class InformationController < BankController

  class VaultController < BankController
    layout :access_level_layout

  class EmployeeController < BankController
    layout nil

The InformationController uses "bank_standard" inherited from the BankController, the VaultController overwrites and picks the layout dynamically, and the EmployeeController doesn’t want to use a layout at all.

Types of layouts

Layouts are basically just regular templates, but the name of this template needs not be specified statically. Sometimes you want to alternate layouts depending on runtime information, such as whether someone is logged in or not. This can be done either by specifying a method reference as a symbol or using an inline method (as a proc).

The method reference is the preferred approach to variable layouts and is used like this:

  class WeblogController < ActionController::Base
    layout :writers_and_readers

    def index
      # fetching posts
    end

    private
      def writers_and_readers
        logged_in? ? "writer_layout" : "reader_layout"
      end

Now when a new request for the index action is processed, the layout will vary depending on whether the person accessing is logged in or not.

If you want to use an inline method, such as a proc, do something like this:

  class WeblogController < ActionController::Base
    layout proc{ |controller| controller.logged_in? ? "writer_layout" : "reader_layout" }

Of course, the most common way of specifying a layout is still just as a plain template name:

  class WeblogController < ActionController::Base
    layout "weblog_standard"

If no directory is specified for the template name, the template will by default be looked for in app/views/layouts/. Otherwise, it will be looked up relative to the template root.

Conditional layouts

If you have a layout that by default is applied to all the actions of a controller, you still have the option of rendering a given action or set of actions without a layout, or restricting a layout to only a single action or a set of actions. The :only and :except options can be passed to the layout call. For example:

  class WeblogController < ActionController::Base
    layout "weblog_standard", :except => :rss

    # ...

  end

This will assign "weblog_standard" as the WeblogController’s layout except for the rss action, which will not wrap a layout around the rendered view.

Both the :only and :except condition can accept an arbitrary number of method references, so #:except =&gt; [ :rss, :text_only ] is valid, as is :except =&gt; :rss.

Using a different layout in the action render call

If most of your actions use the same layout, it makes perfect sense to define a controller-wide layout as described above. Sometimes you’ll have exceptions where one action wants to use a different layout than the rest of the controller. You can do this by passing a :layout option to the render call. For example:

  class WeblogController < ActionController::Base
    layout "weblog_standard"

    def help
      render :action => "help", :layout => "help"
    end
  end

This will render the help action with the "help" layout instead of the controller-wide "weblog_standard" layout.

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