How to make an operation uninterruptible in R shiny - r

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 make the output runs continuously and uninterruptible?
Please see the short demo below:
The clock refreshes every one second, but if I click the button which runs for 5 seconds, the clock is paused.
library(shiny)
ui <- fluidPage(
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) {
output$time <- renderText({
invalidateLater(1000)
as.character(Sys.time())
})
observeEvent(input$button,{
Sys.sleep(5)
output$result <- renderText(runif(1))
})
}
shinyApp(ui, server)
I tried to use future and promises to make the long-running process runs asynchronously, but it doesn't work. Where is wrong? And is there a better way for achieving this purpose?
library(shiny)
library(future)
library(promises)
plan("multisession")
ui <- fluidPage(
actionButton("button","Expensive calcualtion(takes 5 seconds)"),
tags$p("Current Time:"),
textOutput("time"),
tags$p("Result from clicking button:"),
textOutput("result")
)
server <- function(input, output, session) {
output$time <- renderText({
invalidateLater(1000)
as.character(Sys.time())
})
process <- eventReactive(input$button,{
future({
Sys.sleep(5)
runif(1)
})
})
output$result <- renderText(process())
}
shinyApp(ui, server)
Any help is appreciated!

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)