Possible to show console messages (written with `message`) in a shiny ui?

Question

I don't understand R's message vs cat vs print vs etc. too deeply, but I'm wondering if it's possible to capture messages and show them in a shiny app?

Example: the following app can capture cat statements (and print statements as well) but not message statements

runApp(shinyApp(
  ui = fluidPage(
    textOutput("test")
  ),
  server = function(input,output, session) {
    output$test <- renderPrint({
      cat("test cat")
      message("test message")
    })
  }
))

Cross post from the shiny-discuss Google group since I got 0 answers.

Answer 1

Yihui suggested I use withCallingHandlers, and that indeed let me to a solution. I wasn't quite sure how to use that function in a way that would do exactly what I needed because my problem was that I had a function that printed out several messages one at a time and using a naive approach only printed the last message. Here is the my first attempt (which works if you only have one message to show):

foo <- function() {
  message("one")
  message("two")
}

runApp(shinyApp(
  ui = fluidPage(
    actionButton("btn","Click me"),
    textOutput("text")
  ),
  server = function(input,output, session) {
    observeEvent(input$btn, {
      withCallingHandlers(
        foo(),
        message = function(m) output$text <- renderPrint(m$message)
      )
    })
  }
))

Notice how only two\n gets outputted. So my final solution was to use the html function from shinyjs package (disclaimer: I wrote that package), which lets me change or append to the HTML inside an element. It worked perfectly - now both messages got printed out in real-time.

foo <- function() {
  message("one")
  Sys.sleep(0.5)
  message("two")
}

runApp(shinyApp(
  ui = fluidPage(
    shinyjs::useShinyjs(),
    actionButton("btn","Click me"),
    textOutput("text")
  ),
  server = function(input,output, session) {
    observeEvent(input$btn, {
      withCallingHandlers({
        shinyjs::html("text", "")
        foo()
      },
        message = function(m) {
          shinyjs::html(id = "text", html = m$message, add = TRUE)
      })
    })
  }
))