ajax下载后台任务执行生成的csv文件
点击一个按钮，生成一份csv文件，这个csvs生成过程很耗时，要放到后台执行，生成完了之后，还要把文件发送到客户端，这个采用ajax定时请求，去判断文件是否生成成功
1、页面上按钮
<%= button_to_remote "CSV", {:url => {:action => 'get_csv'},
                              :confirm => "Are u sure?"},
                              :class => "btn_red" %>
2、接受请求的action
def get_csv
    spawn do  # backgroud job
      begin
        # generating csv
        # more code here
      rescue Exception => e
        pp e
        Rails.cache.write("csv_#{Process.pid}", 'error')
        Rails.cache.write("csv_#{Process.pid}_error", e)
        return
      end
     
      # mark the csv generated completed
      Rails.cache.write("csv_#{Process.pid}", true)
    end
   
    flash[:notice] = "Generating csv"
    render :update do |page|
      page.replace_html "promote", :partial => "/common/messages"
      page.insert_html :top,
                       "progress_ajax",
                        periodically_call_remote(:url => {:action => "csv_task_progress"},
                                                 :frequency => 5)
    end
  end

3、定时请求的js和action
def periodically_call_remote(options = {})
     variable = options[:variable] ||= 'poller'
     frequency = options[:frequency] ||= 10
     code = "#{variable} = new PeriodicalExecuter(function()
{#{remote_function(options)}}, #{frequency})"
     javascript_tag(code)
end

def csv_task_progress
    if Rails.cache.read("csv_#{Process.pid}").eql?('error')
      # csv generating occurred error
      Rails.cache.write("sbps_#{Process.pid}", nil)
      flash[:error] = Rails.cache.read("csv_#{Process.pid}_error")
      render :update do |page|
        page << "poller.stop();"
        page.replace_html "promote", :partial => "/common/messages"
      end
      return
    elsif Rails.cache.read("csv_#{Process.pid}").blank?
      # return if the csv file have not generated completely
      return (render :text => nil)
    end
    # prevent the later request download csv
    Rails.ca