Demo on why observe() and isolate() can be preferable to reactive()

server.R

# server.R

shinyServer(
  function(input, output, session){
    
# This approach triggers a delay when the box is filled before the actionButton is actioned
#    busyReactive <- reactive({
#        delay <- as.integer(input$delay)
#        if(input$trigger > 0) {Sys.sleep(delay)}
#        return(input$trigger)
#    })
 
    observe({
        input$trigger
        if(is.null(input$trigger) || input$trigger == 0){return()}
        isolate(Sys.sleep(input$delay))
     })
 
    
    delayValues <- reactiveValues(total = 0)
    
    observe({
        input$trigger
        if(is.null(input$trigger) || input$trigger == 0){return()} # needed for math below
        delayValues$total <- isolate(delayValues$total) + isolate(input$delay)
    })
    
    output$back <- renderText({
        if(is.null(input$trigger) || input$trigger == 0){return()} 
        paste("Shiny is back!", "The actionButton was activated", input$trigger, "times. You have waited a total of about", delayValues$total, "seconds.", sep = " ")
    })

    output$busy <- renderText({
        paste0("Shiny is busy... ", "Expect to wait about ", input$delay, " seconds.")
    })

  }
)

ui.R

# ui.R

shinyUI(
  pageWithSidebar(
    headerPanel(h3("Use of hasClass('shiny-busy') with conditionalPanel - Demo"))
    , 
    sidebarPanel(
      helpText(a("Reference: shiny-discuss/05C_X-XMwyk", href = "https://groups.google.com/forum/#!topic/shiny-discuss/05C_X-XMwyk", target="_blank"))
    )
    , 
    mainPanel(
      h5("The actionButton triggers a time-consuming action:")
      , 
      numericInput(inputId = "delay", label = "Select the delay (in seconds)", value = 2, min = 0), tags$br()
      , 
      actionButton("trigger","actionButton")
      ,
      conditionalPanel(condition = "$('html').hasClass('shiny-busy')"
        , tags$h5(textOutput("busy"))
      )
      , 
      conditionalPanel(condition = "!$('html').hasClass('shiny-busy')"
        , tags$h5(textOutput("back"))
      )
    )
  )
)