Is there a way to tell whether sliderInput was moved relative to the initial value?

Question

I am building a simple survey-like shiny app. I record user responses to several questions using sliders. In the case the participant response is equal to the initial value of the slider, I would like to know whether the participant intentionally left it in that position or not.

I'm consider either of the following options, but I don't know how to do neither of both: a- enforce participants to move the slider before submitting the response. b- set starting slider value at 'NULL' or something similar (meaning the slider cursor is hidden until first click on the slider).

Answer 1

I would go for option 3, make a flag which is set whenever the slider is changed and depending on that you do something different. For instance:

library(shiny)

ui <- fluidPage(sliderInput("sld", "Value", 1, 10, 1), 
                actionButton("go", "Validate"), 
                verbatimTextOutput("out"))

server <- function(input, output, session) {
   has_changed <- reactiveVal(FALSE)

   observeEvent(input$sld, {
         ## whenever the slider changes flag it
         has_changed(TRUE)
      }, 
     ## use ignoreInit to avoid firing on startup
     ignoreInit = TRUE)


   output$out <- renderPrint({
      input$go
      if (isolate(has_changed())) {
         "Changed!"
      } else {
         "Never Changed"
      }})
}

shinyApp(ui, server)