如何从Ruby块中的多个语句产生结果？

Question

I have a pair of Rails helpers, one of which is meant to accept a block and the other of which just renders a button. Here are simplified versions the helper definitions:

def nav_wrapper(nav_at, id, css_class, &block)
  "<ul class="complicated">\n #{yield} \n</ul>".html_safe
end

def nav_btn(nav_at, caption, id = caption.downcase.dasherize)
  "Nav button codes goes here".html_safe
end

I'm trying to set things up such that I can do something like this:

<%= nav_wrapper(@nav_at, "Top Nav", "class") do %>
  <%= nav_btn(@nav_at, "Foo", "id") %>
  <%= nav_btn(@nav_at, "Bar", "id") %>
  <%= nav_wrapper(@nav_at, "Sub Nav", "class") do %>
    <%= nav_btn(@nav_at, "SubFoo", "id") %>
    <%= nav_btn(@nav_at, "SubBar", "id") %>
  <% end %>
<% end %>

But, the yield in the nav_wrapper method only picks up the last statement of each block. So in this example, I get the Top Nav wrapper, Foo and Bar are skipped, I get the Sub Nav wrapper (being the last statement in the outer nav_wrapper block), SubFoo is skipped, and I get SubBar (being the last statement in the inner nav_wrapper block).

I know that the reason for this behavior is that the block of code is implicitly returning the last evaluated value, but I know there are lots of template helpers that render all the interstitial lines ( form_for , for example). Can someone help me figure out what the magic trick is here?

Answer 1

When a ERB template is compiled, it is converted to code which adds strings to a buffer. (Look at the source code for ERB to see what I mean.) Methods from Action View like form_for execute the block, and then retrieve the text in the ERB buffer.

Open up the lib/ruby/1.9.1/gems/1.9.1 folder, and look for actionpack. Open up whatever version of Action Pack you have, and go to lib/action_view/helpers/capture_helper.rb . There is a method in there called capture , which is used by form_for to execute a block and retrieve the text generated by ERB.

If you are writing a Rails helper, then presumably capture will be available to your code. If not, try include ActionView::Helpers::CaptureHelper .