test.R

runApp(list(
  ui = pageWithSidebar(
    headerPanel("Test"),
    sidebarPanel(
      #tags$head(tags$style(type="text/css", "
      #       loadmessage {
      #         position: fixed;
      #         top: 0px;
      #         left: 0px;
      #         width: 100%;
      #         padding: 5px 0px 5px 0px;
      #         text-align: center;
      #         font-weight: bold;
      #         font-size: 100%;
      #         color: #000000;
      #         background-color: #CCFF66;
      #         z-index: 105;
      #       }
      #    ")),
      numericInput('n', 'Number of obs', 100),
      conditionalPanel(condition="$('html').hasClass('shiny-busy')",
                       tags$div("Loading...",id="loadmessage"))
    ),
    mainPanel(plotOutput('plot'))
  ),
  server = function(input, output) {
    output$plot <- renderPlot({ Sys.sleep(2); hist(runif(input$n)) })
  }
))