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.

Aniakudo

I guess you can use withCallingHandlers() to capture messages in an R expression, then print/cat them. – Yamada

Thanks Yihui , I was able to use that, that's a huge help – Aniakudo

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)
      })
    })
  }
))

Aniakudo

I tip my hat to you, sir. – Pronunciation

Thank you very much. I've had a function that printed message() text. With your solution I can edit the shinyjs::html(id = "text", html... line using HTML tags. – Autolysin

This currently outputs the messages without line returns in between. Is this feasible to do? How would one do it? – Ducat

@Ducat Did you manage to find a way to inset a line return so messages are displayed in different lines? Thanks! – Niu

This doesn't seem to work. Has anyone used this recently? – Without

@Aniakudo Something’s wacky on my end then. Thank you for checking! – Without

I'm trying to use this in a module framework... doesn't look like the namespace ns() is working properly with this solution...? – Pathan

That's possible, I haven't tried. Though to be honest, I've yet to see many things that break inside modules, usually it's a user error. – Aniakudo

To add a line break after each message you can adjust to shinyjs::html(id = "text", html = paste0(m$message, '<br>'), add = TRUE) – Trivet

@DeanAttali: This is very impressive, thank you very much. Is there a way, thou, to show the messages only in the Shiny UI, but not in the R console? That is, fully redirecting them to the UI. – Spurtle

Is there a way? Probably :) Is it going to be annoying to play around with code until you find it? Also probably :) I think that would be a different solution than what I came up with here. Look into the sink() function and perhaps have a sort of timer that checks on any new messages in a regular interval, that would be the first thing I'd look into. – Aniakudo

@DeanAttali, Is it possible to capture cat and print to UI in real-time? If someone's function use cat or print, withCallingHandlers or tryCatch is not working. – Rolle

It seems this only works with observeEvent. Am I missing something for this to work on eventReactive? – Palua

I am using sink() with reactiveFileReader() for the generated log file of console output and then using renderPrint() to display output by verbatimTextOutput() but it is not live. Couldn't find an good examples that can do the live line by line display of console to Shiny. @Aniakudo if you have any example of this that you could share, I would really appreciate that. – Runkle

Answer 2

This can now be done with the high-level function shinyCatch from the spsComps package.

Basic usage

library(shiny)
library(spsComps)
ui <- fluidPage(
  actionButton("msg", "msg"),
  actionButton("warn", "warn"),
  actionButton("err", "err"),
)

server <- function(input, output, session) {
    observeEvent(input$msg, {
      shinyCatch({message("a message")}, prefix = '')
    })
    observeEvent(input$warn, {
        shinyCatch({warning("a warning")}, prefix = '')
    })
    observeEvent(input$err, {
        shinyCatch({stop("an error")}, prefix = '')
    })
    
}

shinyApp(ui, server)

Choose blocking level

If exceptions happened, we can choose to continue the code or block downstream code in the reactive context. For example, we want to stop downstream code if an error/warning/message happens:

library(shiny)
library(spsComps)
ui <- fluidPage(
  actionButton("err", "code not blocked after error"),
  actionButton("err_block", "code blocked after error"),
)

server <- function(input, output, session) {
    observeEvent(input$err, {
        shinyCatch({stop("an error")}, prefix = '')
        print("error does not block ")
    })
    
    observeEvent(input$err_block, {
        shinyCatch({stop("an error")}, prefix = '', blocking_level = "error")
        print("you can't see me if error happens")
    })
}

shinyApp(ui, server)

More advanced use

check website and demo

cat and print

There is still no good method to catch real-time info from cat and print. I will come back to update this answer if I found a fix for this.

Rolle

Answer 3

I know this isn't nearly as elegant, but I worked around a bit similar problem using capture.output; sadly sink doesn't allow simultaneous capture of messages and output though. You don't get them in the original order, but you can extract both streams at least (here turned to HTML):

runApp(shinyApp(
  ui = fluidPage(
    uiOutput("test")
  ),
  server = function(input,output, session) {
    output$test <- renderUI({
      HTML(
      paste(capture.output(type = "message", expr = { 
        message(capture.output(type = "output", expr = {
          cat("test cat<br>")
          message("test message")
          cat("test cat2<br>")
          message("test message2")
        }))
      }), collapse="<br>")
  )})
 })
)

Output:

test message
test message2
test cat
test cat2

Perhaps in the case if user wants to capture both but also separate them, this will provide a handy work-around. (Your shinyjs package seems neat, need to take a look at it!)

Carley

The problem with this approach is that all the output gets printed at the end, it doesn't come in live. So if you run a slow function that prints output as it runs, you won't see it until it's done – Aniakudo