This only applies to RSpec below version 1.3.2. The issue has been fixed in RSpec 1.3.2, and most likely RSpec 2 and later versions.

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When you have a helper that calls content_for and want to check its behavior you should probably write a feature instead. If you still want to do it, mind the following.

Consider this helper:

module LayoutHelper
  def title(string)
    content_for :title, string
    string
  end
end

Somewhere in the layout we'd then say something like this: <title><%= yield :title %></title>.

Inspecting prepared content for content_for

If you want to inspect what the helper prepared for the view, you need to hack a bit:

it 'should work'
  helper.title('Hello Universe')
  helper.instance_variable_get('@content_for_title').should == 'Hello Universe'
end

Unfortunately, this may only work once. RSpec instanciates the helper object only once, thus keeping instance variables -- and since content_for always attaches, any second spec will break looking at @content_for_title.

Checking this more than once

When your application is up, you will actually get a fresh helper attached to the view each time your content is being rendered. We want something similar, so we define our own helper object:

let :helper do
  Spec::Rails::Example::HelperExampleGroup::HelperObject.new.tap do |helper|
    helper.send :extend, LayoutHelper
  end
end

it 'should work'
  helper.title('Hello Universe')
  helper.instance_variable_get('@content_for_title').should == 'Hello Universe'
end

it 'should still work'
  helper.title('Hello World')
  helper.instance_variable_get('@content_for_title').should == 'Hello World'
end

This applies to Rails 2. Maybe RSpec 2 / Rails 3 are smarter.