R Shiny: Output warning messages to UI

Question

Warning messsages are output in the console, but how do I get these warnings to appear in the UI so the user can see them without looking at the console?

Answer 1

You can use tryCatch to store a warning object. You can then put the message anywhere on your UI.

library(shiny)

ui <- fluidPage(
  actionButton("btn", "click me")
)

server <- function(input, output)
{
  observeEvent(input$btn, {
    #x <- (1:3 * 1:2)  # this generates a warning
    #warning("manually generated warning message")
    #mess <- names(last.warning)
    a <- tryCatch(warning(Sys.time()), warning=function(w) { w })
    mess <- a$message
    showNotification(mess)
  })
}

runApp(list(ui=ui, server=server))

Getting both message and value

If the operation is not heavy, an easy way is to run it twice: once with tryCatch and the other without.

library(shiny)

ui <- fluidPage(
  actionButton("btn", "click me")
)

server <- function(input, output)
{
  observeEvent(input$btn, {

    x <- tryCatch(1:3 * 1:2, warning=function(w) { w })
    if (inherits(x, "simpleWarning")) {
      mess <- x$message
      showNotification(mess)
      x <- 1:3 * 1:2
    }
    print(x)
  })
}

runApp(list(ui=ui, server=server))

If running the same operation twice is not desirable, then use the following trick (See this SO thread)

library(shiny)

ui <- fluidPage(
  actionButton("btn", "click me")
)

server <- function(input, output)
{
  withWarnings <- function(expr) {
    myWarnings <- NULL
    wHandler <- function(w) {
      myWarnings <<- c(myWarnings, list(w))
      invokeRestart("muffleWarning")
    }
    val <- withCallingHandlers(expr, warning = wHandler)
    list(value = val, warnings = myWarnings)
  } 

  observeEvent(input$btn, {
    x <- withWarnings(1:3 * 1:2)
    if (!is.null(x$warnings)) {
      for (w in x$warnings) showNotification(w$message)

    }
  })
}

runApp(list(ui=ui, server=server))