Web Development: Ruby on Rails - 15.3 Creating a Layout for Your Header and Footer (Page 4 of 4 )
Problem
You want to create a header and footer for every page on your web application. Certain pages should have special headers and footers, and you may want to dynamically determine which header and footer to use for a given request.
Solution
Many web applications let you define header and footer files, and automatically include those files at the top and bottom of every page. Rails inverts this pattern. A single file called contains both the header and footer, and the contents of each particular page are inserted into this file.
To apply a layout to every page in your web application, create a file called app/ views/layouts/application.rhtml. It should look something like this:
The key piece of information in any layout file is the directive <%= content_for_layout %>. This is replaced by the content of each individual page.
You can make customized layouts for each controller independently by creating files in the app/views/layouts folder. For example, app/views/layouts/status.rhtml is the layout for the status controller, StatusController. The layout file for PriceController would be price.rhtml.
Customized layouts override the site-wide layout; they don't add to it.
Discussion
Just like your main view templates, your layout templates have access to all the instance variables set by the action. Anything you can do in a view, you can do in a layout template. This means you can do things like set the page title dynamically in the action, and then use it in the layout:
class StatusController < ActionController:Base def index @title = "System Status" end end
Now the application.rhtml file can access @title like this:
application.rhtml doesn't just happen to be the default layout template for a Rails application's controllers. It happens this way because every controller inherits from ApplicationController. By default, a layout's name is derived from the name of the controller's class. So ApplicationController turns into application.rhtml. If you had a controller named MyFunkyController, the default filename for the layout would be app/views/layouts/my_funky.rhtml. If that file didn't exist, Rails would look for a layout corresponding to the superclass of MyFunkyController, and find it in app/views/ layouts/application.rhtml.
To change a controller's layout file, call its layout method:
class FooController < ActionController:Base # Force the layout for /foo to be app/views/layouts/bar.rhtml, # not app/view/layouts/foo.rhtml. layout 'bar' end
If you're using the render method in one of your actions (see Recipe 15.5), you can pass in a :layout argument to render and give that action a different layout from the rest of the controller. In this example, most actions of the FooController use bar.rhtml for their layout, but the count action uses count.rhtml:
class FooController < ActionController:Base layout 'bar'
def count @data = [1,2,3] render :layout => 'count' end end
You can even have an action without a layout. This code gives all of FooController's actions a layout of bar.html, except for the count action, which has no layout at all: it's responsible for all of its own HTML.
class FooController < ActionController:Base layout 'bar', :except => 'count' end
If you need to calculate the layout file dynamically, pass a method symbol into the layout method. This tells layout to call a method on each request; the return value of this method defines the layout file. The method can call action_name to determine the action name of the current request.
class FooController < ActionController:Base layout :figure_out_layout
private
def figure_out_layout if action_name =~ /pretty/ 'pretty' # use pretty.rhtml for the layout else 'standard' # use standard.rhtml end end end
Finally, layout accepts a lambda function as an argument. This lets you dynamically decide on a layout with less code:
class FooController < ActionController:Base layout lambda { |controller| controller.logged_in? ? 'user' : 'guest' } end
It's freeing for both the programmer and the designer to use a layout file instead of separate headers and footers: it's easier to see the whole picture. But if you need to use explicit headers and footers, you can. Create files called app/views/layouts/_ header.rhtml and app/views/layouts/_footer.rhtml. The underscores indicate that they are "partials" (see Recipe 15.14). To use them, set your actions up to use no layout at all, and write the following code in your view files:
<%= render :partial => 'layouts/header' %> ... your view's content goes here ... <%= render :partial => 'layouts/footer' %>
See Also
Recipe 15.5, "Displaying Templates with Render"
Recipe 15.14, "Refactoring the View into Partial Snippets of Views"
Please check back next week for the continuation of this article.
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