class ActionView::TemplateRenderer

def resolve_layout(layout, keys, formats)
  details = @details.dup
  details[:formats] = formats
  case layout
  when String
    begin
      if layout =~ /^\//
        with_fallbacks { find_template(layout, nil, false, keys, details) }
      else
        find_template(layout, nil, false, keys, details)
      end
    rescue ActionView::MissingTemplate
      all_details = @details.merge(:formats => @lookup_context.default_formats)
      raise unless template_exists?(layout, nil, false, keys, all_details)
    end
  when Proc
    resolve_layout(layout.call(formats), keys, formats)
  else
    layout
  end
end