Reactive monitoring of a file in Shiny - r

Question

I have an app where I need to monitor a file for changes. However I am struggling to do this.
Consider the following example:
library(shiny)
A function that returns the modified date of a file:
file_info <- function(){
if(file.exists("example.txt")){
return(as.character(as.Date(file.info("example.txt")[1,4])))
} else {
return("File not present")
}
}
Shiny part:
ui <- shinyUI(fluidPage(
textOutput("file_status"),
actionButton("create_file", "Create file"),
actionButton("delete_file", "Delete file")
))
server <- shinyServer(function(input, output, session) {
file_st <- reactive(file_info()) #what is the correct approach here?
output$file_status <- renderText({
file_st()
})
observeEvent(input$create_file,{
file.create("example.txt")
})
observeEvent(input$delete_file, {
unlink("example.txt")
})
})
I would like the file_status text field to update each time the example.txt file changes - if possible even if this happens outside of the app.
I have tried various combinations of reactive, observe and reactiveValues without finding the adequate combo.
Thank you

Answer 1

I was able to solve this using reactivePoll which is practically tailored for this type of task
server <- shinyServer(function(input, output, session) {
file_st <- reactivePoll(500, session,
checkFunc = function() {
if (file.exists("example.txt"))
file.info("example.txt")$mtime[1]
else
""},
valueFunc = function(){
if(file.exists("example.txt")){
return(as.character(as.Date(file.info("example.txt")[1,4])))
} else {
return("File not present")
}
}
)
output$file_status <- renderText({
file_st()
})
observeEvent(input$create_file,{
file.create("example.txt")
})
observeEvent(input$delete_file, {
unlink("example.txt")
})
})
ui <- shinyUI(fluidPage(
textOutput("file_status"),
actionButton("create_file", "Create file"),
actionButton("delete_file", "Delete file")
))