Asynchronous Programming in R with the Future Package - r

Question

I am new to asynchronous programming in R with the Future Package so needed some help. I am trying to build a simple application with rshiny which supports asynchronous programming. So my code as a histogram plot, a slider, a simple text print and read.csv function to read a large CSV file. So my plan is before my read.csv function runs in the background using the future package in R, I would like to have control over my other application.
But my code waits for the CSV file to read. Any help will be appreciated. The code sample is below.
library(promises)
library(future)
library(shinydashboard)
library(shiny)
library(tidyverse)
plan(multiprocess)
#UI parts
ui <- dashboardBody(fluidRow(box(tableOutput("input1")),
box(textOutput("input2"))),
fluidRow(box(
sliderInput(
inputId = "bins",
label = "Number of bins:",
min = 1,
max = 5,
value = 2
)
),
box(plotOutput(outputId = "distPlot"))),
fluidRow(box(
sliderInput(
"slider2",
label = h3("Slider Range"),
min = 0,
max = 100,
value = c(40, 60)
)
),
box(verbatimTextOutput("range"))))
#server part
server <- function(input, output, session) {
output$input1 <- renderTable({
promise <- future((read.csv("data/sample_large.csv")))
promise %...>% head() %...>% print()
})
output$input2 <- renderText({
print("hello")
})
output$distPlot <- renderPlot({
dist <- rnorm(input$bins)
hist(dist)
})
output$range <- renderPrint({
input$slider2
})
}
shinyApp(ui = dashboardPage(dashboardHeader(),
dashboardSidebar(),
ui),
server = server)

Answer 1

The behaviour you're experiencing where the rest of the UI is not loading until the promise is evaluated is expected behaviour. It is explained in the promises package as part of what they call the 'shiny flush cycle' and is described in more detail here and here.
Only after all of the outputs have completed executing are they sent back to Shiny to update the UI. You may expect/prefer outputs to be rendered as soon as they are ready but unfortunately that's not how Shiny operates.
As noted in the second link you can 'trick' shiny into thinking all outputs are executed and then use a reactive value to trigger the final update once the promise has evaluated:
#server part
server <- function(input, output, session) {
data <- reactiveVal()
# Return NULL from this operation so Shiny 'thinks' the output is evaluated
observe({
data(NULL)
future({read.csv("data/sample_large.csv")}) %...>%
data() #Assign to data
NULL
})
# When data() is updated as a side effect of our promise the table will be updated
output$input1 <- renderTable({
req(data()) %>%
head(5) %>%
print()
})
# in the mean time all outputs will be judged to be complete so can be rendered
output$input2 <- renderText({
print("hello")
})
output$distPlot <- renderPlot({
dist <- rnorm(input$bins)
hist(dist)
})
output$range <- renderPrint({
input$slider2
})
}
shinyApp(ui = dashboardPage(dashboardHeader(),
dashboardSidebar(),
ui),
server = server)