Shiny: Getting a user input into a future function - shiny

Question

I am building an shiny app, where users upload a bunch of data and than choose which should be computed. The computation itself is rather timeconsuming and should be stored in a list. To keep shiny responsive during the calculation (for the user and other users) I tried to utilize promises and future. The problem is that I am not able to get an input into future function. I always get Warning: Error in $: Can't access reactive value 'mem_pos' outside of reactive consumer. i Do you need to wrap inside reactive() or observe()? [No stack trace available]. I tried to read about reactive but I am simply stuck.
Here is a minimal example of the problem (to display it, the list has only one value each):
library(shiny)
library(promises)
library(future)
plan(multisession)
# example function
subfct = function(n) {
Sys.sleep(3)
return(n*2)
}
# shiny page
ui <- fluidPage(
sidebarLayout(
sidebarPanel(
numericInput("mem_pos", min = 1, max = 30, value = 1, label="mem pos"),
actionButton("mem_button", label="set mem value")
),
mainPanel(
tableOutput("result")
)
)
)
server <- function(input, output) {
superval = reactiveValues(mem = rep(list(0), 10))
# set the future calculations
observeEvent(input$mem_button, {future({return(subfct( input$mem_pos ))}) %...>% {superval$mem[[input$mem_pos]] = .}}) # here lies the problem
# show result table
observe( {output$result = renderTable({unlist(superval$mem)})})
}
# Run the application
shinyApp(ui = ui, server = server)
If the problematic line is exchanged by observeEvent(input$mem_button, {future({return(subfct( 5 ))}) %...>% {superval$mem[[input$mem_pos]] = .}}) it basically works. But I am not able to get the user input into the function. I am grateful for a direct help or an explanation of reactive for my specific problem.

Answer 1

I solved it. Not enterily sure why, but isolate does the trick.
This code works for me:
library(shiny)
library(promises)
library(future)
plan(multisession)
# example function
subfct = function(n) {
Sys.sleep(3)
return(n*2)
}
# shiny page
ui <- fluidPage(
sidebarLayout(
sidebarPanel(
numericInput("mem_pos", min = 1, max = 30, value = 1, label="mem pos"),
actionButton("mem_button", label="set mem value")
),
mainPanel(
tableOutput("result")
)
)
)
server <- function(input, output) {
superval = reactiveValues(mem = rep(list(0), 10))
# set the future calculations
observeEvent(input$mem_button, {future({return(subfct( isolate(input$mem_pos) ))}) %...>% {superval$mem[[input$mem_pos]] = .}}) # here lied the problem
# show result table
observe( {output$result = renderTable({unlist(superval$mem)})})
}
# Run the application
shinyApp(ui = ui, server = server)

Answer 2

You may try to use delay() function from shinyjs package. This enables the code to run only after the times set lapses. e.g.
library(shinyjs)
ui<-fluidPage(useShinyjs(),
...)
server<-function(input,output, session){
delay (ms=,
...)
}
shinyApp(ui,server)