How to get the cursor position in a Shiny textareaInput

does anyone know how I can, inside a shiny application, get the cursor position inside a textAreaInput ?

library(shiny)

ui <- fluidPage(
  textAreaInput("hop"
                ,label="textarea",value = "Supercalifragilisticexpialidocious"),
  verbatimTextOutput("out")
)

server <- function(input, output, session) {

  output$out <- renderText({
    "here I would like to get the cursor position (an interger?) \n inside que textArea"

  })

}

shinyApp(ui, server)

I think I have to use javascript, but I don't know where to start.

Regards

this is a solution I found :

library(shiny)

ui <- fluidPage(tags$head(tags$script(
  'Shiny.addCustomMessageHandler("prout",
  function(NULL) {

   var ctl = document.getElementById("hop");
    var startPos = ctl.selectionStart;
  var endPos = ctl.selectionEnd;
  alert(startPos + ", " + endPos);

  });'
    )),
  textAreaInput("hop"
                ,label="textarea",value = "Supercalifragilisticexpialidocious"),
  verbatimTextOutput("out"),
  actionButton("hop","hop")
)

server <- function(input, output, session) {

  output$out <- renderText({
    "here I would like to get the cursor position (an interger?) \n inside que textArea"

  })

  observeEvent(input$hop,{
    message("hop")
    session$sendCustomMessage(type="prout",message=list(NULL))
  })
}

shinyApp(ui, server)