如何使用变量contentType实现R Shiny downloadHandler？

Question

As per the docs , the downloadHandler should accept contentType as a string. Since the contentType is not a constant, I am trying to pass a variable or reactive. But the app fails whenever the contentType is not strictly a string. How could I pass a variable value?

I have tried function(){} , reactive , eventReactive . What confuses me is that it works just fine for filename = function(){} .

ui <- fluidPage(
  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) {
      write.csv(data, file)
    },
    contentType = function() {
      "text/csv"
    }
  )
}

shinyApp(ui, server)

In this case, the error message is is.na() applied to non-(list or vector) of type 'closure' and the file is not downloaded correctly ( server error ).

Answer 1

ui <- fluidPage(
    radioButtons("csvs", label = "Type to download", choices = c(".txt", ".csv"), selected = ".csv"),
    downloadLink("downloadData", "Download")
)

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

    output$downloadData <- downloadHandler(
        filename = function() {
            paste("data-", Sys.Date(), input$csvs, sep="")
        },
        content = function(file) {
            write.csv(data, file)
        }
    )
}

shinyApp(ui, server)

An easier and working way is to let the user pick it with a radioButton