How to exit downloadHandler when file doesn't exist - r

Question

I have a Shiny app with a downloadButton, which enables the user to download some log files.
As the logfiles are also handled by logrotate, it might be that at a certain time no logfile exists, which currently breaks the app when trying to download it.
How can I prevent that? Or how can I display a modalDialog with the information, that no log-file currently exists?
I tried to include req(F) or return(FALSE) but they don't work.
The current approach works, as I create an empty data.frame which is then exported, but it's not a very nice solution.
library(shiny)
library(data.table)
## Write random log file. Uncomment the next line to make the example work.
#fwrite(x = iris, file = "logs.log")
ui <- fluidPage(
downloadButton("showLogs", label="", title="Logs herunterladen", icon = icon("book-open"))
)
server <- function(input, output, session) {
output$showLogs <- downloadHandler(
filename = function() {
paste('logs-', Sys.Date(), '.csv', sep='')
},
content = function(file) {
logfile <- list.files(path = ".", pattern = basename("logs.log"))
if (length(logfile) != 0) {
logfile <- fread(logfile, sep = ";", header = F)
fwrite(logfile, file, sep = ";", row.names = FALSE)
} else {
## Problem is in here
# req(F)
# return(FALSE)
fwrite(data.frame("No log-Files"), file, sep = ";", row.names = FALSE)
}
}
)
}
shinyApp(ui, server)

Answer 1

Here is a solution without reactiveTimer.
library(shiny)
library(data.table)
library(shinyjs)
## Write random log file. Uncomment the next line to make the example work.
#fwrite(x = iris, file = "logs.log")
ui <- fluidPage(
useShinyjs(),
downloadButton("showLogs", label="", style = "display:none;"),
actionButton("btn", "Download")
)
server <- function(input, output, session) {
observeEvent(input$btn, {
logfile <- list.files(path = ".", pattern = basename("logs.log"))
if(length(logfile)){
runjs("$('#showLogs').click();")
}
})
output$showLogs <- downloadHandler(
filename = function() {
paste('logs-', Sys.Date(), '.csv', sep='')
},
content = function(file) {
logfile <- list.files(path = ".", pattern = basename("logs.log"))
logfile <- fread(logfile, sep = ";", header = F)
fwrite(logfile, file, sep = ";", row.names = FALSE)
}
)
}
shinyApp(ui, server)

Answer 2

This app throws an alert if you click the button while no log file exists, and there's no download.
library(shiny)
library(data.table)
library(shinyjs)
## Write random log file. Uncomment the next line to make the example work.
#fwrite(x = iris, file = "logs.log")
ui <- fluidPage(
useShinyjs(),
downloadButton("showLogs", label="", title="Logs herunterladen", icon = icon("book-open"))
)
server <- function(input, output, session) {
autoInvalidate <- reactiveTimer(1000)
observe({
autoInvalidate()
logfile <- list.files(path = ".", pattern = basename("logs.log"))
if(length(logfile)){
runjs("$('#showLogs').off('click.x')")
}else{
runjs("$('#showLogs').off('click.x').on('click.x', function(e){alert('No log file'); e.preventDefault();})")
}
})
output$showLogs <- downloadHandler(
filename = function() {
paste('logs-', Sys.Date(), '.csv', sep='')
},
content = function(file) {
logfile <- list.files(path = ".", pattern = basename("logs.log"))
logfile <- fread(logfile, sep = ";", header = F)
fwrite(logfile, file, sep = ";", row.names = FALSE)
}
)
}
shinyApp(ui, server)