Action Pack is a framework for handling and responding to web requests. It provides mechanisms for routing (mapping request URLs to actions), defining controllers that implement actions, and generating responses by rendering views, which are templates of various formats. In short, Action Pack provides the view and controller layers in the MVC paradigm.

It consists of several modules:

• Action Dispatch, which parses information about the web request, handles routing as defined by the user, and does advanced processing related to HTTP such as MIME-type negotiation, decoding parameters in POST/PUT bodies, handling HTTP caching logic, cookies and sessions.

• Action Controller, which provides a base controller class that can be subclassed to implement filters and actions to handle requests. The result of an action is typically content generated from views.

• Action View, which handles view template lookup and rendering, and provides view helpers that assist when building HTML forms, Atom feeds and more. Template formats that Action View handles are ERb (embedded Ruby, typically used to inline short Ruby snippets inside HTML), XML Builder and RJS (dynamically generated JavaScript from Ruby code).

With the Ruby on Rails framework, users only directly interface with the Action Controller module. Necessary Action Dispatch functionality is activated by default and Action View rendering is implicitly triggered by Action Controller. However, these modules are designed to function on their own and can be used outside of Rails.

A short rundown of some of the major features:

• Actions grouped in controller as methods instead of separate command objects and can therefore share helper methods

  CustomersController < ActionController::Base
    def show
      @customer = find_customer
    end
  
    def update
      @customer = find_customer
      if @customer.update_attributes(params[:customer])
        redirect_to :action => "show"
      else
        render :action => "edit"
      end
    end
  
    private
      def find_customer
        Customer.find params[:id]
      end
  end

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• ERb templates (static content mixed with dynamic output from ruby)

  <% for post in @posts %>
    Title: <%= post.title %>
  <% end %>
  
  All post titles: <%= @posts.collect{ |p| p.title }.join(", ") %>
  
  <% unless @person.is_client? %>
    Not for clients to see...
  <% end %>

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• “Builder” templates (great for XML content, like RSS)

  xml.rss("version" => "2.0") do
    xml.channel do
      xml.title(@feed_title)
      xml.link(@url)
      xml.description "Basecamp: Recent items"
      xml.language "en-us"
      xml.ttl "40"
  
      for item in @recent_items
        xml.item do
          xml.title(item_title(item))
          xml.description(item_description(item))
          xml.pubDate(item_pubDate(item))
          xml.guid(@recent_items.url(item))
          xml.link(@recent_items.url(item))
        end
      end
    end
  end
  

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• Filters for pre- and post-processing of the response

  class WeblogController < ActionController::Base
    # filters as methods
    before_filter :authenticate, :cache, :audit
  
    # filter as a proc
    after_filter { |c| c.response.body = Gzip::compress(c.response.body) }
  
    # class filter
    after_filter LocalizeFilter
  
    def index
      # Before this action is run, the user will be authenticated, the cache
      # will be examined to see if a valid copy of the results already
      # exists, and the action will be logged for auditing.
  
      # After this action has run, the output will first be localized then
      # compressed to minimize bandwidth usage
    end
  
    private
      def authenticate
        # Implement the filter with full access to both request and response
      end
  end
  

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• Helpers for forms, dates, action links, and text

  <%= text_field_tag "post", "title", "size" => 30 %>
  <%= link_to "New post", :controller => "post", :action => "new" %>
  <%= truncate(post.title, :length => 25) %>

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• Layout sharing for template reuse

  class WeblogController < ActionController::Base
    layout "weblog_layout"
  
    def hello_world
    end
  end
  
  Layout file (called weblog_layout):
    <html><body><%= yield %></body></html>
  
  Template for hello_world action:
    <h1>Hello world</h1>
  
  Result of running hello_world action:
    <html><body><h1>Hello world</h1></body></html>

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• Routing makes pretty URLs incredibly easy

  match 'clients/:client_name/:project_name/:controller/:action'
  
  Accessing "/clients/37signals/basecamp/project/index" calls ProjectController#index with
  { "client_name" => "37signals", "project_name" => "basecamp" } in `params`
  
  From that action, you can write the redirect in a number of ways:
  
  redirect_to(:action => "edit") =>
    /clients/37signals/basecamp/project/edit
  
  redirect_to(:client_name => "nextangle", :project_name => "rails") =>
    /clients/nextangle/rails/project/index

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• Easy testing of both controller and rendered template through ActionController::TestCase

  class LoginControllerTest < ActionController::TestCase
    def test_failing_authenticate
      process :authenticate, :user_name => "nop", :password => ""
      assert flash.has_key?(:alert)
      assert_redirected_to :action => "index"
    end
  end
  

  Learn more

• Automated benchmarking and integrated logging

  Started GET "/weblog" for 127.0.0.1 at Fri May 28 00:41:55
    Processing by WeblogController#index as HTML
  Rendered weblog/index.html.erb within layouts/application (25.7ms)
  Completed 200 OK in 29.3ms
  
  If Active Record is used as the model, you'll have the database debugging
  as well:
  
  Started POST "/posts" for 127.0.0.1 at Sat Jun 19 14:04:23
    Processing by PostsController#create as HTML
    Parameters: {"post"=>{"title"=>"this is good"}}
  SQL (0.6ms) INSERT INTO posts (title) VALUES('this is good')
  Redirected to http://example.com/posts/5
  Completed 302 Found in 221ms (Views: 215ms | ActiveRecord: 0.6ms)
  
  You specify a logger through a class method, such as:
  
  ActionController::Base.logger = Logger.new("Application Log")
  ActionController::Base.logger = Log4r::Logger.new("Application Log")

• Caching at three levels of granularity (page, action, fragment)

  class WeblogController < ActionController::Base
    caches_page :show
    caches_action :account
  
    def show
      # the output of the method will be cached as
      # ActionController::Base.page_cache_directory + "/weblog/show/n.html"
      # and the web server will pick it up without even hitting Rails
    end
  
    def account
      # the output of the method will be cached in the fragment store
      # but Rails is hit to retrieve it, so filters are run
    end
  
    def update
      List.update(params[:list][:id], params[:list])
      expire_page   :action => "show", :id => params[:list][:id]
      expire_action :action => "account"
      redirect_to   :action => "show", :id => params[:list][:id]
    end
  end
  

  Learn more

• Powerful debugging mechanism for local requests

  All exceptions raised on actions performed on the request of a local user
  will be presented with a tailored debugging screen that includes exception
  message, stack trace, request parameters, session contents, and the
  half-finished response.

  Learn more

This example will implement a simple weblog system using inline templates and an Active Record model. So let’s build that WeblogController with just a few methods:

require 'action_controller'
require 'post'

class WeblogController < ActionController::Base
  layout "weblog/layout"

  def index
    @posts = Post.find(:all)
  end

  def show
    @post = Post.find(params[:id])
  end

  def new
    @post = Post.new
  end

  def create
    @post = Post.create(params[:post])
    redirect_to :action => "show", :id => @post.id
  end
end

WeblogController::Base.view_paths = [ File.dirname(__FILE__) ]
WeblogController.process_cgi if $0 == __FILE__

The last two lines are responsible for telling ActionController where the template files are located and actually running the controller on a new request from the web-server (like to be Apache).

And the templates look like this:

weblog/layout.html.erb:
  <html><body>
  <%= yield %>
  </body></html>

weblog/index.html.erb:
  <% for post in @posts %>
    <p><%= link_to(post.title, :action => "show", :id => post.id) %></p>
  <% end %>

weblog/show.html.erb:
  <p>
    <b><%= @post.title %></b><br/>
    <b><%= @post.content %></b>
  </p>

weblog/new.html.erb:
  <%= form "post" %>

This simple setup will list all the posts in the system on the index page, which is called by accessing /weblog/. It uses the form builder for the Active Record model to make the new screen, which in turn hands everything over to the create action (that’s the default target for the form builder when given a new model). After creating the post, it’ll redirect to the show page using an URL such as /weblog/5 (where 5 is the id of the post).

The latest version of Action Pack can be installed with Rubygems:

% [sudo] gem install actionpack

Source code can be downloaded as part of the Rails project on GitHub

• github.com/rails/rails/tree/master/actionpack/

Action Pack is released under the MIT license.

API documentation is at

• api.rubyonrails.com

Bug reports and feature requests can be filed with the rest for the Ruby on Rails project here:

• rails.lighthouseapp.com/projects/8994-ruby-on-rails/tickets