How does the yield magic work in ActionView?

Question

I was looking at how content_for works and observed the block.call in the capture_erb_with_buffer method. It apparently magically writes to the buf

Answer 1

I'm not certain how yield functions on the ERB level, but I do know how it works when applied to layouts.

Heres a sample layout.html.erb file:

  
    
    <%= yield :head %>
  

  

    <%= yield :menu %>
  
  

    <%= yield %>
  
  

    <%= yield :footer %>
  

I have defined 4 yields(:head, :menu, :footer, and default) and an instance variable @title.

Now controller actions can render views that slot into these spots. Note that the view renders before the layout, so I can define a variable like @title in a view and have it defined in the layout.

Sample views: An About page

<% @title = 'About' %>
<% content_for :menu do %>
  <%= link_to 'Back to Home', :action => :home %>
<% end %>

We rock!

<% content_for :footer do %>
  An Illinois based company.
<% end %>

An Edit page

<% @title = 'Edit' %>
<% content_for :head do %>
  
<% end %>

<% form_for :thing, :html => {:class => 'edit_form'} do |f| %>
   ...
<% end %>

You can mix and match what yields you want to put data in, and what occurs in the content_for :something will be inserted in the matching yield :something in the layout file.

It even works for partials, a partial can insert its own content_for :something block which will get added with any other content_for calls.