Rails How does the respond to block work

Question

I m going through the Getting Started with Rails guide and got confused with section 6 7  After generating a scaffold I find the following auto-generated block in my controller   def index    posts   Post all    respond to do  format      format html    index html erb     format json    render  json   gt   posts     end end   I d like to understand how the respond to block actually works  What type of variable is format  Are  html and  json methods of the format object  The documentation for   ActionController  MimeResponds  ClassMethods  respond to   doesn t answer the question

User · Answer

This is a block of Ruby code that takes advantage of a Rails helper method. If you aren't familiar with blocks yet, you will see them a lot in Ruby.

respond_to is a Rails helper method that is attached to the Controller class (or rather, its super class). It is referencing the response that will be sent to the View (which is going to the browser).

The block in your example is formatting data - by passing in a 'format' paramater in the block - to be sent from the controller to the view whenever a browser makes a request for html or json data.

If you are on your local machine and you have your Post scaffold set up, you can go to http://localhost:3000/posts and you will see all of your posts in html format. But, if you type in this: http://localhost:3000/posts.json, then you will see all of your posts in a json object sent from the server.

This is very handy for making javascript heavy applications that need to pass json back and forth from the server. If you wanted, you could easily create a json api on your rails back-end, and only pass one view - like the index view of your Post controller. Then you could use a javascript library like Jquery or Backbone (or both) to manipulate data and create your own interface. These are called asynchronous UIs and they are becomming really popular (Gmail is one). They are very fast and give the end-user a more desktop-like experience on the web. Of course, this is just one advantage of formatting your data.

The Rails 3 way of writing this would be this:

    class PostsController < ApplicationController
      # GET /posts
      # GET /posts.xml


      respond_to :html, :xml, :json

      def index
        @posts = Post.all

        respond_with(@posts)
      end

#
# All your other REST methods
#

end

By putting respond_to :html, :xml, :json at the top of the class, you can declare all the formats that you want your controller to send to your views.

Then, in the controller method, all you have to do is respond_with(@whatever_object_you_have)

It just simplifies your code a little more than what Rails auto-generates.

If you want to know about the inner-workings of this...

From what I understand, Rails introspects the objects to determine what the actual format is going to be. The 'format' variables value is based on this introspection. Rails can do a whole lot with a little bit of info. You'd be surprised at how far a simple @post or :post will go.

For example, if I had a _user.html.erb partial file that looked like this:

_user.html.erb

<li>    
    <%= link_to user.name, user %>
</li>

Then, this alone in my index view would let Rails know that it needed to find the 'users' partial and iterate through all of the 'users' objects:

index.html.erb

 <ul class="users">
   <%= render @users %>     
 </ul>

would let Rails know that it needed to find the 'user' partial and iterate through all of the 'users' objects:

You may find this blog post useful: http://archives.ryandaigle.com/articles/2009/8/6/what-s-new-in-edge-rails-cleaner-restful-controllers-w-respond_with

You can also peruse the source: https://github.com/rails/rails

User · Answer

I am new to Ruby and got stuck at this same code  The parts that I got hung up on were a little more fundamental than some of the answers I found here  This may or may not help someone    respond to is a method on the superclass ActionController  it takes a block  which is like a delegate  The block is from do until end  with  format  as an argument to the block  respond to executes your block  passing a Responder into the format argument    http   api rubyonrails org v4 1 classes ActionController Responder html   The Responder does NOT contain a method for  html or  json  but we call these methods anyways  This part threw me for a loop  Ruby has a feature called method missing  If you call a method that doesn t exist  like json or html   Ruby calls the method missing method instead    http   ruby-metaprogramming rubylearning com html ruby metaprogramming 2 html   The Responder class uses its method missing as a kind of registration  When we call  json   we are telling it to respond to requests with the  json extension by serializing to json  We need to call html with no arguments to tell it to handle  html requests in the default way  using conventions and views     It could be written like this  using JS-like pseudocode       get an instance to a responder from the base class var format   get responder       register html to render in the default way     by way of the views and conventions  format register  html       register json as well  the argument to  json is the second    argument to method missing   json  is the first   which contains    optional ways to configure the response  In this case  serialize as json  format register  json   renderOptions    This part confused the heck out of me  I still find it unintuitive  Ruby seems to use this technique quite a bit  The entire class  responder  becomes the method implementation  In order to leverage method missing  we need an instance of the class  so we re obliged to pass a callback into which they pass the method-like object  For someone who has coded in C-like languages for 20 some years  this is very backwards and unintuitive to me  Not that it s bad  But it s something a lot of people with that kind of background need to get their head around  and I think might be what the OP was after   p s  note that in RoR 4 2 respond to was extracted into responders gem

User · Answer

Format  is your response type  Could be json or html  for example  It s the format of the output your visitor will receive

User · Answer

What type of variable is format    From a java POV  format is an implemtation of an anonymous interface  This interface has one method named for each mime type  When you invoke one of those methods  passing it a block   then if rails feels that the user wants that content type  then it will invoke your block   The twist  of course  is that this anonymous glue object doesn t actually implement an interface - it catches the method calls dynamically and works out if its the name of a mime type that it knows about   Personally  I think it looks weird  the block that you pass in is executed  It would make more sense to me to pass in a hash of format labels and blocks  But - that s how its done in RoR  it seems

User · Answer

There is one more thing you should be aware of - MIME   If you need to use a MIME type and it isn t supported by default  you can register your own handlers in config initializers mime types rb   Mime  Type register  text markdown    markdown

User · Answer

This is a little outdated  by Ryan Bigg does a great job explaining this here   http   ryanbigg com 2009 04 how-rails-works-2-mime-types-respond to  In fact  it might be a bit more detail than you were looking for  As it turns out  there s a lot going on behind the scenes  including a need to understand how the MIME types get loaded

User · Answer

The meta-programming behind responder registration  see Parched Squid s answer  also allows you to do nifty stuff like this   def index    posts   Post all    respond to do  format      format html    index html erb     format json    render  json   gt   posts       format csv     render  csv   gt   posts       format js   end end   The csv line will cause to csv to be called on each post when you visit  posts csv   This makes it easy to export data as CSV  or any other format  from your rails site   The js line will cause a javascript file  posts js  or  posts js coffee  to be rendered executed   I ve found that to be a light-weight way to create an Ajax enabled site using jQuery UI pop-ups

User · Answer

From what I know  respond to is a method attached to the ActionController  so you can use it in every single controller  because all of them inherits from the ActionController  Here is the Rails respond to method   def respond to  amp block    responder   Responder new self    block call responder    responder respond end   You are passing it a block  like I show here   respond to  lt  lt   BEGINNING OF THE BLOCK   gt  gt  do  format    format html   format xml    render  xml   gt   whatever   end  lt  lt   END OF THE BLOCK   gt  gt    The  format  part is the argument that the block is expecting  so inside the respond to method we can use that  How    Well  if you notice we pass the block with a prefixed  amp  in the respond to method  and we do that to treat that block as a Proc   Since the argument has the   xml     html  we can use that as methods to be called   What we basically do in the respond to class is call methods   html   xml   json  to an instance of a Responder class

User · Answer

I d like to understand how the respond to block actually works  What   type of variable is format  Are  html and  json methods of the format   object    In order to understand what format is  you could first look at the source for respond to  but quickly you ll find that what really you need to look at is the code for retrieve response from mimes   From here  you ll see that the block that was passed to respond to  in your code   is actually called and passed with an instance of Collector  which within the block is referenced as format   Collector basically generates methods  I believe at Rails start-up  based on what mime types rails knows about   So  yes  the  html and  json are methods defined  at runtime  on the Collector  aka format  class

[ruby-on-rails] Rails: How does the respond_to block work?

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