shiny app with long running calculation using promises

shinypromise.R

# notice the flush on the single user holds up the user updating, as the promise and reactive variable is there per user

library(shiny)
library(DT)
library(dplyr)
library(future)
library(promises)
plan(multiprocess)

ui <- fluidPage(
    verbatimTextOutput("timer"),
    DTOutput("myTable")
)

observe({
  invalidateLater(1000, NULL)
  print(Sys.time())
})

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

#initial value
    rlist <- reactiveValues()
    rlist$tbl_vals <- tibble(Value = rnorm(5))


#second timer
    output$timer <- renderText({
      invalidateLater(1000, session)
      as.character(Sys.time())
    })

    observe({
      print("observer running")
        future ({get_data()}) %>%
            then(function(got_data){
                isolate({rlist$tbl_vals <<- got_data})
                invalidateLater(3000, session)
                print("now returning data from future")
            })
    })


    output$myTable <- renderDT({
      rlist$tbl_vals
    })

}

get_data <- function(){
    expensive_operation()
    tibble(Value = rnorm(20))
}

expensive_operation <- function(){
    invisible(rnorm(100000000))
}

shinyApp(ui, server)