Ruby on Rails Filters

Filters run in respect to controller actions - before, after or around them. These methods can halt the action processing by redirecting or set up common data to every action in the controller.

Rails support three types of filter methods:

Before filters

After filters

Around filters

Before Filters

Rails before filters are executed before the code in action controller is executed. The before filters are defined at the top of a controller class that calls them. To set it up, you need to call before_filter method.

Example:

 
class UserController < ApplicationController   
before_filter :verify_password   
def verify_password  
...    
end    
end 

In this example, method verify_password is applied as a before filter. Before any action method will be called, verify_password method is called.

After Filters

Rails after filters are executed after the code in action controller is executed. Just like before filters, after filters are also defined at the top of a controller class that calls them. To set it up, you need to call after_filter method.

Example:


class UserController < ApplicationController before_filter :verify_password def verify_password ... end end class PhotoController < ApplicationController after_filter :resize_photo def resize_photo ... end end

In this example, method resize_photo is applied as an after filter.

Around Filters

Rails around filters contain codes that are executed both before and after controller's code is executed. They are generally used when you need both before and after filter. Its implementation is little bit different and more complex than other two filters. It is generally defined by a common class which contains both before and after methods.

Example:

class ActionLogger    
def before(controller)   
@start_time = Time.new   
end    
def after(controller)   
@end_time = Time.now    
@elapsed_time = @end_time.to_f - @start_time.to_f   
@action = controller.action_name    
# next save this logging detail to a file or database    
table    
end    
end  

In the ActionLogger class, before method captures time an action is started and after method captures time an action completes, the elapsed time.
Let us see how ActionLogger class works as an around filter. In your controller class, simply add around_filter method and pass an instance of ActionLogger as a parameter.

class PhotoController < ApplicationController    
around_filter ActionLogger.new    
end