How does the yield magic work in ActionView?

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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

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有刺的猬

2021-01-14 15:15
This little tiny method called execute in ActionView::Base explains it all. http://google.com/codesearch/p?hl=en#m8Vht-lU3vE/vendor/rails/actionpack/lib/action_view/base.rb&q=capture_helper.rb&d=5&l=337
```
  def execute(template)
    send(template.method, template.locals) do |*names|
      instance_variable_get "@content_for_#{names.first || 'layout'}"
    end
  end
```
The do |*names|... end block is the one receiving the yield. You'll notice that the @content_for_#{names.first} matches up with the variable being set in the content_for process.

It's called from AV::TemplateHandlers::Compilable in #render, and I would assume other places as well.
```
  def render(template)
    @view.send :execute, template
  end
```
http://google.com/codesearch/p?hl=en#m8Vht-lU3vE/vendor/rails/actionpack/lib/action_view/template_handlers/compilable.rb&q=execute&exact_package=git://github.com/payalgupta/todo-list.git&sa=N&cd=17&ct=rc&l=28
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攒了一身酷

2021-01-14 15:26
Simply:

calling yield in a method executes code that was passed to the method via a block.

For instance
```
def my_method
  yield
end

my_method { puts "Hello" }
my_method { puts "World" }
```
these 5 lines will produce the following output to the screen
```
Hello
World
```
See the following page for a nice discussion of yield in Ruby: Ruby Yield
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囚心锁ツ

2021-01-14 15:40
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:
```
<html>
  <head>
    <title> <%= @title || 'Plain Title' %> </title>
    <%= yield :head %>
  </head>
<body>
  <div id='menu'>
    <%= yield :menu %>
  </div>
  <div id='content'>
    <%= yield %>
  </div>
  <div id='footer'>
    <%= yield :footer %>
  </div>
</body>
```
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 %>
  <style type='text/css'> .edit_form div {display:inline-block;} </style>
<% 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.
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