title: R shiny: display "loading..." message while function is running
body: <p>I use Shiny GUI R package. I'm looking for a way to display a message like "loading..." after the actionButton was pressed. The function takes several minutes to execute, so I need to inform the user somehow that the button actually triggered some event. Now the server.R code looks like this:</p>
  
  <pre><code>DATA &lt;- reactive({
    if(input$DownloadButton&gt;0) {
      RunDownload()
    } else {
      NULL
    }
  })
  
  output$Download &lt;- renderText({
    if(NROW(DATA())&gt;0) {
      paste0(Sys.time(),": ",NROW(DATA()), " items downloaded")
    } else {
      ''
    }
  })
  </code></pre>
  
  <p><code>actionButton()</code> is a function that downloads data from internet. <code>input$DownloadButton</code> is actionButton. So after the button was pressed the user waits for several minutes and only then sees a message saying that download was successful. I would like to show a message "Loading..." just after the actionButton was pressed and then another message like <code>paste0(Sys.time(),": ",NROW(DATA()), " items downloaded")</code> when execution ends.</p>