R Shiny DownloadHandler + base64decode - r

Question

I like to download a file in shiny which is created by base64enc::base64decode.
What I have so far is:
library(base64enc)
library(shiny)
downloadHandler(
filename = function()
"test.txt",
content = function(file) {
base64decode(what = "VGhpcyBpcyBhIHRlc3Qu", output = file)
}
)
and I get Warning: Error in file: argument "file" is missing, with no default
When I use base64decode without shiny, I use:
base_string <- "VGhpcyBpcyBhIHRlc3Qu"
o_file <- file("C:/User/Desktop/test.txt"), "wb")
base64decode(what = base_string, output = o_file)
close(o_file)
and everything works fine.
Is it possible to use the downloadHandler without executing the second statement first? I want to create the file just for the download.

Answer 1

If we look into the documentation of ?downloadHandler we see that the content parameter requires a file path (string) of a nonexistent temp file and writes the content to that file path.
If we look into the code of base64decode:
if (is.character(output)) {
output <- file(output, "wb")
on.exit(close(output))
}
we see that file() is called, so that you would create/open a connection to a file already and the condition of "non-existance" of that file wouldn´t be fulfilled (my understanding).
Maybe, you could use smthg like:
write.csv(base64decode(what = base_string), file)
Full app:
library(base64enc)
library(shiny)
ui <- fluidPage(
downloadLink("downloadData", "Download")
)
server <- function(input, output) {
# Our dataset
data <- mtcars
output$downloadData <- downloadHandler(
filename = function() {
paste("data-", Sys.Date(), ".stackoverflow", sep="")
},
content = function(file) {
base_string <- "VGhpcyBpcyBhIHRlc3Qu"
write.table(base64decode(what = base_string), file)
}
)
}
shinyApp(ui, server)
Edit: Given your question in the comment. You can use write.table() for an arbitrary file type. See the edited example above to write to a file of type .stackoverflow ;).