How to make an operation uninterruptible in R shiny

Question

In my shiny app I have a output which should update itself continuously. But whenever I execute a long-running calculation, the output is just paused. My question is: how to mak

Answer 1

Thanks @Shree for pointing out the solution. After reading the response from Joe Cheng. It seems like the key is to:

Hide the async operation from Shiny by not having the promise be the last expression.

The problem is resolved by creating a reactive value and assign the promise to it in observeEvent as the side effect.

server <- function(input, output, session) {

  output$time <- renderText({
    invalidateLater(1000)
    as.character(Sys.time())
  })

  process <- reactiveVal()

  observeEvent(input$button,{
    output$isbusy <- renderText("busy") # a simple busy indicator
    future({
      Sys.sleep(5)
      runif(1)
    }) %...>%
      process()
    # Hide the async operation from Shiny by not having the promise be the last expression
    NULL # important
  })

  output$result <- renderText({
    output$isbusy <- renderText("") # a simple busy indicator
    process()
  })
}

Answer 2

Your problem is that Sys.sleep() suspends the execution of R expression. You can use things like the delay() function from shinyjs

library(shiny)
library(shinyjs)

ui <- fluidPage(
  useShinyjs(), #You have to add this.
  actionButton("button","Expensive calcualtion(takes 5 seconds)"),
  tags$p("Current Time:"),
  textOutput("time"),
  tags$p("Result from clicking button:"),
  textOutput("result")
)

server <- function(input, output, session) {
  timer <- reactiveTimer(500)
  current_time <- reactive({
  timer()
  as.character(Sys.time())
 })
 output$time <- renderText(current_time())

 observeEvent(input$button,{
 delay(5000, output$result <- renderText("result"))
 })
}

shinyApp(ui, server)