ActionController::Responder

Responder is responsible for exposing a resource to different mime requests, usually depending on the HTTP verb. The responder is triggered when <code>respond_with</code> is called. The simplest case to study is a GET request:

  class PeopleController < ApplicationController
    respond_to :html, :xml, :json

    def index
      @people = Person.find(:all)
      respond_with(@people)
    end
  end

When a request comes in, for example for an XML response, three steps happen:

  1) the responder searches for a template at people/index.xml;

  2) if the template is not available, it will invoke <code>#to_xml</code> on the given resource;

  3) if the responder does not <code>respond_to :to_xml</code>, call <code>#to_format</code> on it.

Builtin HTTP verb semantics

The default Rails responder holds semantics for each HTTP verb. Depending on the content type, verb and the resource status, it will behave differently.

Using Rails default responder, a POST request for creating an object could be written as:

  def create
    @user = User.new(params[:user])
    flash[:notice] = 'User was successfully created.' if @user.save
    respond_with(@user)
  end

Which is exactly the same as:

  def create
    @user = User.new(params[:user])

    respond_to do |format|
      if @user.save
        flash[:notice] = 'User was successfully created.'
        format.html { redirect_to(@user) }
        format.xml { render :xml => @user, :status => :created, :location => @user }
      else
        format.html { render :action => "new" }
        format.xml { render :xml => @user.errors, :status => :unprocessable_entity }
      end
    end
  end

The same happens for PUT and DELETE requests.

Nested resources

You can supply nested resources as you do in <code>form_for</code> and <code>polymorphic_url</code>. Consider the project has many tasks example. The create action for TasksController would be like:

  def create
    @project = Project.find(params[:project_id])
    @task = @project.comments.build(params[:task])
    flash[:notice] = 'Task was successfully created.' if @task.save
    respond_with(@project, @task)
  end

Giving an array of resources, you ensure that the responder will redirect to <code>project_task_url</code> instead of <code>task_url</code>.

Namespaced and singleton resources require a symbol to be given, as in polymorphic urls. If a project has one manager which has many tasks, it should be invoked as:

  respond_with(@project, :manager, @task)

Check <code>polymorphic_url</code> documentation for more examples.