Ruby on Rails: provide vs content_for

Question

I came across the view helper function \"provide\" today. By looking into its manual I am still confused on how it is different from \"content_for\".

prov

Answer 1

First of all, what is streaming? Why would you use it?

Streaming is alternate method of rendering pages top-down (outside-in). The default rendering behavior is inside-out. Streaming must be enabled in your controller:

class MyController
  def action
    render stream: true # Streaming enabled
  end
end

According to the documentation:

Streaming may be considered to be overkill for lightweight actions like new or edit. The real benefit of streaming is on expensive actions that, for example, do a lot of queries on the database.

So, if you're not using streaming, is there still a difference?

Yes.

The difference is a template can define multiple content blocks by calling content_for multiple times. Doing so will concatenate the blocks and pass that to the layout:

# layout.html.erb
<%= yield :surprise %>

   
<%= yield %>
   
But it's not very interesting...


# template.html.erb
<%= content_for :surprise, "Hello" %>
I've got your content!
<%= content_for :surprise, ", World!" %>

# Generated HTML
Hello, World!

   
I've got your content!
   
But it's not very interesting...

Since provide doesn't continue searching the provided template, only the block passed to the first provide call will be sent to the template:

# layout.html.erb
<%= yield :title %>

# template.html.erb
<%= provide :title, "Foo" %>
<%= provide :title, "bar" %>

# Generated HTML
Foo