Catching errors within an R Shiny downloadHandler

When an error occurs within a downloadHandler object, the app will still pop up a download window. Generally the only indication that an error has occurred is that the default filename and type are not right in the pop-up. In this example:

ui <- fluidPage(
  downloadLink("downloadData", "Download")
)

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

shinyApp(ui, server)

The stop() here simulates an error. When running the script, the only indication that the widget did not work correctly is that the default filename is downloadData.htm instead of data-[date]-.csv.

Is there a way to catch this error and prevent the download window from launching?

Solution

This is a modification of @Eliot Dixon's answer that removes the requirement to know in advance if the downloadHandler logic will fail. The key is to have an always-hidden downloadButton and an always-visible actionButton. When the actionButton is clicked, run the complicated logic. If it succeeds, save the resulting data to a reactiveVal and then use shinyjs::click() to simulate clicking on the real, but hidden, downloadButton.

library(shiny)
library(shinyjs)

ui <- fluidPage(
    useShinyjs(),
    selectInput("dataExists", "Data exists", choice = c("TRUE", "FALSE")),
    conditionalPanel("false", downloadButton("downloadData", "Download")),
    actionButton("click", "Download")
)

server <- function(input, output) {
    computed_data <- reactiveVal()
    
    observeEvent(input$click, {
        # Put all content logic here.  
        # If it fails, showNotification.
        # If it succeeds, store result and click button.
        if(input$dataExists == "FALSE"){
            showNotification("Data error.")
        } else {
            computed_data(mtcars)
            click("downloadData")
        }
    })
    
    output$downloadData <- downloadHandler(
        filename = function() {
            paste("data-", Sys.Date(), ".csv", sep="")
        },
        content = function(file) {
            write.csv(computed_data(), file)
        }
    )
}

shinyApp(ui, server)