Implementation of the wrapper pattern for Rails / ActionView

wrapper_helper.rb

# Implementation of the wrapper pattern, borrowed from Template Toolkit:
# http://template-toolkit.org/docs/manual/Directives.html#section_WRAPPER
#
# The method captures a block and passes its content to a partial template.
# The partial template calls <%= yield %> to render the block content, e.g.:
#
# <% wrapper "shared/widget", { :title => "Hello" } do %>
#   ...some ERb...
# <% end %>
#
# In shared/_widget.html.erb:
#
# <div class="widget">
#   <h3><%= title %></h3>
#   <div class="content">
#     <%= yield %>
#   </div>
# </div>

module WrapperHelper
  def wrapper(template_name, local_assigns = {}, content = nil, &block)
    content = capture(&block) if block_given?
    instance_variable_set(:@content_for_layout, content)
    concat render_partial(:partial => template_name, :locals => local_assigns)
    nil
  end
end

wrapper_helper_spec.rb

require 'spec_helper'

describe WrapperHelper do
  before do
    helper.stub!(:render_partial)
    helper.stub!(:capture)
    helper.stub!(:concat)
  end

  describe "#wrapper" do
    context "without a block" do
      it "assigns @content_for_layout to the supplied value" do
        helper.should_receive(:instance_variable_set).with(:@content_for_layout, "content")
        helper.wrapper("template", nil, "content")
      end

      it "renders the requested partial template and locals" do
        local_assigns = {:foo => "bar"}
        helper.should_receive(:render_partial).with(:partial => "template", :locals => local_assigns)
        helper.wrapper("template", local_assigns, "content")
      end

      it "concatenates the rendered content" do
        helper.stub!(:render_partial).and_return("wrapped content")
        helper.should_receive(:concat).with("wrapped content")
        helper.wrapper("template", nil, "content")
      end

      it "returns nil" do
        helper.wrapper("template", nil, "content").should be_nil
      end
    end

    context "with a block" do
      it "captures the block" do
        helper.should_receive(:capture)
        helper.wrapper("template") { "content" }
      end

      it "assigns @content_for_layout the captured block" do
        helper.should_receive(:capture).and_return("content")
        helper.should_receive(:instance_variable_set).with(:@content_for_layout, "content")
        helper.wrapper("template") { "content" }
      end

      it "renders the requested partial template" do
        helper.should_receive(:render_partial).with(:partial => "template", :locals => {})
        helper.wrapper("template") { "content" }
      end

      it "concatenates the rendered content" do
        helper.stub!(:render_partial).and_return("wrapped content")
        helper.should_receive(:concat).with("wrapped content")
        helper.wrapper("template") { "content" }
      end

      it "returns nil" do
        helper.wrapper("template") { "content" }.should be_nil
      end
    end
  end
end