testing_yielded_block.rb

require "test/unit"
require "rubygems"
require "mocha"

class Controller
  def showplain(params)
    output = nil
    Cleaner.parse(params[:content]) do |cleaner|
      output = cleaner.plainversion
    end
    return output
  end
end

class Cleaner

   def self.parse(content)
     cleaner = Cleaner.new(content)
     yield cleaner
     cleaner.close
   end

   def plainversion
      ### operate on @content and return ###
   end

end

class MyTest < Test::Unit::TestCase
  
  def setup
    @controller = Controller.new
  end

  def test_showplain
    cleaner = mock('cleaner')
    cleaner.expects(:plainversion).returns('returnvalue')
    Cleaner.expects(:parse).with('somecontent').yields(cleaner)

    @controller.showplain(:content => 'somecontent')
  end
end