title: Shiny - Detect a change in input with a warning in Ui.R
body: <p>I have an app that has many inputs and that uses an <code>actionButton()</code> to do some calculation that takes a non trivial amount of time including reading some files and plotting a map. </p>
  
  <p>During some user testing, some of the feedback that the need to re-press the <code>actionButton()</code> after adjusting an input was non intuitive. I would like to render a warning message (read "helpful reminder") that users need re-press the <code>actionButton()</code>. I'd like it if this message only rendered after inputs are changed since the last press of the <code>actionButton()</code>.</p>
  
  <p>So far I have tried using global variables inside an 'eventReactive()' and trying to use <code>identical()</code>  with no luck.</p>
  
  <p><strong>My question is:</strong> How can I render a helpful reminder to re-press an <code>actionButton()</code> when any inputs have changed since last time the button was pressed?.</p>
  
  <p>Here is a minimal example of an app with many inputs that takes some time to output something</p>
  
  <pre><code>## app.R ##
  library(shiny)
  library(shinydashboard)
  
  ui &lt;- dashboardPage(
    dashboardHeader(),
    dashboardSidebar(
      #arbritray inputs
      sliderInput("in1", label = "Input 1", value=1,min=0,max=5),
      sliderInput("in2", label = "Input 2", value=2,min=0,max=5),
      sliderInput("in3", label = "Input 3", value=3,min=0,max=5),
      actionButton("StartCalculation","Calculate")
    ),
    dashboardBody(
      textOutput("answer")
    )
  )
  
  server &lt;- function(input, output) {
  
    out &lt;- eventReactive(input$StartCalculation,{
          Sys.sleep(2) #simulate long computation
      input$in1+input$in2+input$in3
    })
  
    output$answer &lt;- renderText({
      out()
    })
  }
  
  shinyApp(ui, server)
  </code></pre>