capturing cat output periodically for R shiny output (renderPrint)

Question

Hope someone can help me with this.

Let's say there is a function "example" which is something like

##function from a package

example<-function(f){
         #does something
         cat("step 1 done....")
         # etc etc
         cat("step 2 done....")
         return(some_data_frame)
}

##server ui code
example2<-reactive({
         if(input$some_action_button==0)
           return()
         result<-isolate(example(input$f1))
         return(result)
})

output$f2<-renderPrint({
         example2()
})

Is there some way to capture the "cat" outputs from the function into renderPrint, periodically? Assuming that this is a long function to process and it would be nice for the user to get some feedbabk. invalidateLater does not work for things that are already within a function (at least it seems that way when I tried it here).

Also, as a secondary problem, writing the code in the above manner would cause renderPrint to capture both the "cat" and the data.frame together, possibly because of the "return".

If anyone could point me in the right direction, it would be most helpful! Thanks!

Answer 1

First of, great question I've been thinking a lot about this.

Since shiny is single threaded it's a bit tricky capturing function output and displaying it in shiny from what i know.

A work around for this would be using a non blocking file connection and running the function you want to capture the output from in the background while reading the file for the function output (Check the edit history to see how to do this).

Another way of doing this would be overriding the cat function to write to stderr (simply switching cat with message) and capture the function output like this:

library(shiny)
library(shinyjs)

myPeriodicFunction <- function(){
  for(i in 1:5){
    msg <- paste(sprintf("Step %d done.... \n",i))
    cat(msg)
    Sys.sleep(1)
  }
}

# Override cat function
cat <- message

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

This example is mostly based on this question by daattali.