onFlush: Add callbacks for Shiny session events

View source: R/shiny.R

onFlushR Documentation

Add callbacks for Shiny session events


These functions are for registering callbacks on Shiny session events. onFlush registers a function that will be called before Shiny flushes the reactive system. onFlushed registers a function that will be called after Shiny flushes the reactive system. onUnhandledError registers a function to be called when an unhandled error occurs before the session is closed. onSessionEnded registers a function to be called after the client has disconnected.

These functions should be called within the application's server function.

All of these functions return a function which can be called with no arguments to cancel the registration.


onFlush(fun, once = TRUE, session = getDefaultReactiveDomain())

onFlushed(fun, once = TRUE, session = getDefaultReactiveDomain())

onSessionEnded(fun, session = getDefaultReactiveDomain())

onUnhandledError(fun, session = getDefaultReactiveDomain())



A callback function.


Should the function be run once, and then cleared, or should it re-run each time the event occurs. (Only for onFlush and onFlushed.)


A shiny session object.

Unhandled Errors

Unhandled errors are errors that aren't otherwise handled by Shiny or by the application logic. In other words, they are errors that will either cause the application to crash or will result in "Error" output in the UI.

You can use onUnhandledError() to register a function that will be called when an unhandled error occurs. This function will be called with the error object as its first argument. If the error is fatal and will result in the session closing, the error condition will have the shiny.error.fatal class.

Note that the onUnhandledError() callbacks cannot be used to prevent the app from closing or to modify the error condition. Instead, they are intended to give you an opportunity to log the error or perform other cleanup operations.

See Also

onStop() for registering callbacks that will be invoked when the application exits, or when a session ends.



ui <- fixedPage(
    "Set the number to 8 or higher to cause an error",
    "in the `renderText()` output."
  sliderInput("number", "Number", 0, 10, 4),
    "Click the button below to crash the app with an unhandled error",
    "in an `observe()` block."
  actionButton("crash", "Crash the app!")

log_event <- function(level, ...) {
  ts <- strftime(Sys.time(), " [%F %T] ")
  message(level, ts, ...)

server <- function(input, output, session) {
  log_event("INFO", "Session started")

  onUnhandledError(function(err) {
    # log the unhandled error
    level <- if (inherits(err, "shiny.error.fatal")) "FATAL" else "ERROR"
    log_event(level, conditionMessage(err))

  onStop(function() {
    log_event("INFO", "Session ended")

  observeEvent(input$crash, stop("Oops, an unhandled error happened!"))

  output$text <- renderText({
    if (input$number > 7) {
      stop("that's too high!")
    sprintf("You picked number %d.", input$number)

shinyApp(ui, server)

shiny documentation built on May 29, 2024, 8:47 a.m.