R shiny: display "loading..." message while function is running

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:

DATA <- reactive({
  if(input$DownloadButton>0) {
    RunDownload()
  } else {
    NULL
  }
})

output$Download <- renderText({
  if(NROW(DATA())>0) {
    paste0(Sys.time(),": ",NROW(DATA()), " items downloaded")
  } else {
    ''
  }
})

actionButton() is a function that downloads data from internet. input$DownloadButton 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 paste0(Sys.time(),": ",NROW(DATA()), " items downloaded") when execution ends.