R shiny - having trouble with file download - r

Question

So I want to have a Shiny page which:
A) Allows the user to upload a .xls file;
B) Offers that file back to the user for download as a .csv file;
C) Prints the head of the file in the Shiny app to ensure that it was properly read.
Here is the code I am using:
# Want to read xls files with readxl package
library(readxl)
library(shiny)
## Only run examples in interactive R sessions
if (interactive()) {
ui <- fluidPage(
fileInput("file1", "Choose File", accept = ".xls"),
tags$hr(),
uiOutput("downloader"),
htmlOutput("confirmText", container = tags$h3),
tableOutput("listContents")
)
server <- function(input, output) {
theOutput <- reactiveValues(temp = NULL, df = NULL, msg = NULL, fn = NULL)
observeEvent(input$file1, {
theOutput$fn <- paste('data-', Sys.Date(), '.csv', sep='')
theOutput$temp <- read_xls(input$file1$datapath)
theOutput$msg <- paste("File Contents:")
theOutput$df <- write.csv(theOutput$temp,
file = theOutput$fn,
row.names = FALSE)
})
output$confirmText <- renderText({
theOutput$msg
})
output$listContents <- renderTable({
head(theOutput$temp)
})
output$downloader <- renderUI({
if(!is.null(input$file1)) {
downloadButton("theDownload", label = "Download")
}
})
output$theDownload <- downloadHandler(
filename = theOutput$fn,
content = theOutput$df
)
}
shinyApp(ui, server)
}
The Shiny page renders correctly, it accepts the upload with no problems, it prints out the head of the .csv with no problems, and it creates a properly formatted "data-{today's date}.csv" file in the same directory as the app.R file.
Problem is, when I hit the download button I get the error message:
Warning: Error in download$func: attempt to apply non-function
[No stack trace available]
Can someone tell me what I am doing wrong?

Answer 1

Thanks to the comments above, this is the solution I found (with my comments added, to show where the code changed):
library(readxl)
library(shiny)
if (interactive()) {
ui <- fluidPage(
fileInput("file1", "Choose File", accept = ".xls"),
tags$hr(),
uiOutput("downloader"),
htmlOutput("confirmText", container = tags$h3),
tableOutput("listContents")
)
server <- function(input, output) {
theOutput <- reactiveValues(temp = NULL, msg = NULL)
observeEvent(input$file1, {
# Do not try to automate filename and the write.csv output here!
theOutput$temp <- read_xls(input$file1$datapath)
theOutput$msg <- paste("File Contents:")
})
output$confirmText <- renderText({
theOutput$msg
})
output$listContents <- renderTable({
head(theOutput$temp)
})
output$downloader <- renderUI({
if(!is.null(input$file1)) {
downloadButton("theDownload", label = "Download")
}
})
output$theDownload <- downloadHandler(
# Filename and content need to be defined as functions
# (even if, as with filename here, there are no inputs to those functions)
filename = function() {paste('data-', Sys.Date(), '.csv', sep='')},
content = function(theFile) {write.csv(theOutput$temp, theFile, row.names = FALSE)}
) }
shinyApp(ui, server) }
The fact that content takes an argument (named here "theFile"), which is not called anywhere else, is what was throwing me off.