Asynchronous Shiny calculations with package Future

withFuture.R

# Author: Zygmunt Zawadzki
# http://www.blog.zstat.pl/2017/04/24/make-asynchronous-call-in-shiny./

library(future)
library(shiny)
plan(multiprocess)

ui <- fluidPage(
  
  # Application title
  titlePanel("Old Faithful Geyser Data"),
  
  # Sidebar with a slider input for number of bins 
  sidebarLayout(
    sidebarPanel(
      sliderInput("bins",
                  "Number of bins:",
                  min = 1,
                  max = 50,
                  value = 30),
      actionButton("MakePlot", "MakePlot")
    ),
    
    # Show a plot of the generated distribution
    mainPanel(
      fluidRow(
        column(6, plotOutput("distPlot")),
        column(6, plotOutput("FuturePlot")))
    )
  )
)

# Define server logic required to draw a histogram
server <- function(input, output) {
  
  output$distPlot <- renderPlot({
    # generate bins based on input$bins from ui.R
    x    <- faithful[, 2] 
    bins <- seq(min(x), max(x), length.out = input$bins + 1)
    
    # draw the histogram with the specified number of bins
    hist(x, breaks = bins, col = 'darkgray', border = 'white')
  })
  
  future <- reactiveValues(future = NULL)
  
  output$FuturePlot <- renderPlot({
    input$MakePlot
    
    x <- NULL
    
    isolate({
      if(is.null(future$future)) {
        future$future <- future({
          Sys.sleep(5)
          cbind(rnorm(1000), rnorm(1000))
        })
        cat("Work scheduled\n")
      } else if(!resolved(future$future)) {
        cat("Waiting\n")
      } else {
        x <- value(future$future)
        future$future <- NULL
        cat("Done!\n")
      }
    })
    
    if(is.null(x)) invalidateLater(1000) # Wait 1s
    
    req(x) # Stop there if x is still null
    plot(x)
  })
  
}

shinyApp(ui = ui, server = server)