Dynamic update of a label withMathJax in Shiny UI - r

Question

I'm using shiny::updateSliderInput to update the label of a slider. I would like the label for the slider to render with the Greek character xi (ξ) and a subscript. The subscript is target of the updateSliderInput call.
I've been successful generating the label without the updateSliderInput call, but correctly using the withMathJax() call for the label update has not worked as well as I would prefer.
A reproducible example of a shiny app for this issue is:
library(shiny)
shinyApp(ui = shinyUI(fluidPage(sliderInput("order", withMathJax("Order, \\(k\\)"), min = 3, max = 7, value = 4, step = 1),
sliderInput("iknots", "iKnots", min = 0, max = 10, value = 5, step = 1),
sliderInput("xi1", withMathJax(), min = 0, max = 10, value = 1, step = 0.1))),
server = shinyServer(function(input, output, clientData, session) {
observe({
k <- as.integer(input$order)
l <- as.integer(input$iknots)
updateSliderInput(session, "xi1", label = paste0("\\(\\xi_{", k + l, "}\\)"))
})
}))
Upon initial loading of the app in browser we have a the desirable label, the ξ9 is correct.
After adjusting either of the first two sliders, the label for the third slider does not render as expected.
I've tried using withMathJax in label argument of updateSliderInput but have had undesirable labels. Changing the updateSliderInput line in the above to
updateSliderInput(session, "xi1", label = withMathJax(paste0("\\(\\xi_{", k + l, "}\\)")))
and setting the sliderInput to have the either label = withMathJax() or label = "" results in the initial label of
and updated label of
How can the subscript on the ξ be correctly updated in the slider label?

Answer 1

You might need to use renderUI and uiOutput to create the last slider otherwise the javascript used to display the MathJax is not called when you update the label.
You could do:
library(shiny)
shinyApp(ui = shinyUI(fluidPage(sliderInput("order", withMathJax("Order, \\(k\\)"), min = 3, max = 7, value = 4, step = 1),
sliderInput("iknots", "iKnots", min = 0, max = 10, value = 5, step = 1),
uiOutput("lastSlider"))),
server = shinyServer(function(input, output, clientData, session) {
output$lastSlider<- renderUI({
k <- as.integer(input$order)
l <- as.integer(input$iknots)
sliderInput("xi1",label = withMathJax(paste0("\\(\\xi_{", k + l, "}\\)")), min = 0, max = 10, step = 0.1,value=input$xi1)
})
}))