I want to perform some validation inside downloadHandler, so that when the condition is true a custom message is given else download of data. I have the following code example where I want to perform validation, but it seems not working.

shinyApp(
  ui = basicPage(
    textInput("jobid", label = "Enter job ID", value = ""),
    downloadLink("downloadData", "Download")

  ),
  server <- function(input, output) {
    # Our dataset
    data <- mtcars

    output$downloadData <- downloadHandler(
      filename = function() {
        paste("data-", Sys.Date(), ".csv", sep="")
      },
      content = function(file) {
        if(input$jobid ==""){
          session$sendCustomMessage(type = 'testmessage',
                                    message = 'No job id to submit')
        }else
          write.csv(data, file)
      }
    )
  }
)

How to validate??