Shiny app illustrating a slider that appears to wait for the user to enter a number in a numericInput field

server.R

function(input,output) {
  output$sliderout <- renderText({
    if (exists_as_numeric(input$numinput)) {
      isolate(input$sliderinput)
    }
  })
  output$numout <- renderText({
    input$numinput
  })
}

ui.R

library(shiny)

basicPage(
  title = "Slider 'Waits' for Numerical Input",
  sliderInput(inputId = "sliderinput", label = "Slider",
                  min = 0, max = 100, step = 1, value = 0),
  verbatimTextOutput("sliderout"),
  numericInput(inputId = "numinput", label = "Enter a Number", value = NULL),
  verbatimTextOutput("numout")
)