From: Andrew Stewart <boss@...>
Date: 2008-01-18T02:12:44+09:00
Subject: Re: Ruby syntax in "respond_to do |format| line -- can clarify?


On 17 Jan 2008, at 16:21, Joshua Beall wrote:
>     respond_to do |format|
>       format.html # index.html.erb
>       format.xml  { render :xml => @posts }
>     end
>
> But I still don't understand where |format| is coming from, or what  
> it's
> doing.  From what I've seen in Ruby, I think of something like | 
> format|
> as being part of a foreach type block.  As in  
> "possible_formats.each do
> |format|".

Take heart, the code isn't entirely obvious if you're unfamiliar with  
Ruby!

The format between the pipes ("|format|") at the start of the block  
is a block variable.  It could be called anything though the  
convention is to use "format".  When respond_to was first introduced  
lots of people used "wants" to make the code in the block read nicely  
and it almost prevailed.

Anyway, the block variable is set within the respond_to method:

http://dev.rubyonrails.org/browser/trunk/actionpack/lib/ 
action_controller/mime_responds.rb#L102

module ActionController
   module MimeResponds
     module InstanceMethods

       def respond_to(*types, &block)
         raise ArgumentError, "respond_to takes either types or a  
block, never both" unless types.any? ^ block
         block ||= lambda { |responder| types.each { |type|  
responder.send(type) } }
         responder = Responder.new(self)
         block.call(responder)
         responder.respond
       end

       class Responder
         # ...
       end

     end
   end
end

The &block in the respond_to method's signature is set to the  
do...end block from your snippet.  The first two lines of the method  
are irrelevant to your situation.  The third line creates an object  
that can respond appropriately to different mime types.  The fourth  
line is the one you're interested in: it executes the block, passing  
in the responder object.

So if you look at your block again:

   do |format|
     format.html
     format.xml
   end

...and you imagine executing it as a method, the block variable,  
format, should look like a method argument.  Accordingly it is set to  
the responder object passed in by the line:

   block.call(responder)

Thus the format.html and format.xml lines in the block are  
sending :html and :xml messages to the object in format, i.e. to  
responder.  As it happens responder doesn't have an html method, nor  
an xml method, so these calls are handled by its method_missing method.

http://dev.rubyonrails.org/browser/trunk/actionpack/lib/ 
action_controller/mime_responds.rb#L139

I recommend reading up on blocks and procs.  Here's one article to  
get you started:

http://onestepback.org/index.cgi/Tech/Ruby/RubyBindings.rdoc

Regards,
Andy Stewart

-------
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