print console output on shiny synchronously during the computing process

[Eric Hung]

Hello, I recently ran into a problem that as my computing took lots of time, I'd like to show the text output on shiny rather than progress bar or loading message. My function looked like

printText <- function() {
  for(i in 1:10){
    Sys.sleep(0.1)
    print(paste("My text", i))
    y = i + 1
  }
  return(y)
}

I can print it with verbatimTextOutput but I also need the returned value of y. For example,

runApp(list(
  ui = shinyUI(fluidPage(
    titlePanel("Print consol output"),
    sidebarLayout(
      sidebarPanel(actionButton("go", "Go")),
      mainPanel(
        verbatimTextOutput("text")
      )
    )
  )),
  server = shinyServer(function(input, output, session){
    observeEvent(input$go, {
      output$text <- renderPrint({
          y <- printText()
      })
    })
  })
))

The problem is that if I want to use the returned y I need to create a reactive object, which may take me 2 times longer because I execute printText() twice, while printing and pass to reactive object.

How could I get the value of y and print the text onto shiny without duplicated work? Notice that I'm not gonna use progress bar because my real function is not a loop actually. I don't know how many times it may repeat. What I want is to capture the text output during the process and pass the returned value to y without creating a reactive object that causes duplicated work.

Many thanks in advance!

[Joe Cheng]

Yeah, print() doesn't work inside renderPrint. However, cat(file=stderr(), message) should work.

--
You received this message because you are subscribed to the Google Groups "Shiny - Web Framework for R" group.
To unsubscribe from this group and stop receiving emails from it, send an email to shiny-discus...@googlegroups.com.
To view this discussion on the web visit https://groups.google.com/d/msgid/shiny-discuss/718b948f-89f9-4bce-9054-7ccbe6a6f049%40googlegroups.com.
For more options, visit https://groups.google.com/d/optout.

[Dean Attali]

I posted a solution to something similar to SO a while ago http://stackoverflow.com/questions/30474538/possible-to-show-console-messages-written-with-message-in-a-shiny-ui

In your case, something like this would work

printText <- function() {

  for(i in 1:10){

    Sys.sleep(0.1)

    shinyjs::html("text", paste("My text", i), add = TRUE)

    y = i + 1

  }

  return(y)

}

runApp(list(

  ui = shinyUI(fluidPage(

    shinyjs::useShinyjs(),

    titlePanel("Print consol output"),

    sidebarLayout(

      sidebarPanel(actionButton("go", "Go")),

      mainPanel(

        div(id = "text")

      )

    )

  )),

  server = shinyServer(function(input, output, session){

    observeEvent(input$go, {

      shinyjs::html("text", "")

      y <- printText()

    })

  })

))

Basically, it's using the shinyjs package to manually change the HTML inside the element, instead of using shiny's reactivity.  If you can get Joe's solution to work, that'd be better though

[Eric Hung]

Thanks Dean. It works perfect in this case. But what should I do if I don't change my function? Because in my real function, I cannot find where

paste("My text", i)

is. Is it possible to make it without hacking the function? My real function is actually `applyStrategy` in quantstrat package. I can't find the code that prints the texts so I prefer not to change the function.

Many thanks again!


Dean Attali於 2016年4月3日星期日 UTC+8下午2時49分10秒寫道：