Using ExtendedTask together with fileInput()

[arangaca]

I'm working on an app that relies on user-supplied data that can be quite large (up to a few GB). It takes a bit of time to upload and process (essentially uncompress) the data. I'm trying to take advantage of the ExtendedTask class to perform these long-running tasks in a separate R process.

I use an R6 class (Handler) to handle and uncompress the input file in a temporary directory. For the purpose of the example, I made the Handler class very minimalistic and used Sys.sleep() to emulate the time of uncompressing the file. I use a module for the upload button to keep the example as close as my actual app.

I'm a bit clueless how I should use the ExtendedTask for this purpose. Any help is welcome!

Here's an attempt at reproducing my problem:

library(shiny)
library(bslib)

future::plan(future::multisession)

Handler <- R6::R6Class(
  "Handler",
  private = list(
    .file = NULL
  ),
  public = list(
    read = function(file) {
      Sys.sleep(5)
      private$.file <- file
    },
    file = function() {
      private$.file$datapath
    }
  )
)


mod_upload_ui <- function(id, ...) {
  ns <- NS(id)
  fileInput(
    ns("file"),
    label = "Upload a file",
    buttonLabel = "Browse files",
    placeholder = "No files selected",
    ...
  )
}

mod_upload_server <- function(id, handler) {
  moduleServer(id, function(input, output, session) {
    promise <- ExtendedTask$new(function(file) {
      promises::future_promise(handler$read(file))
    })
    
    observeEvent(input$file, {
      promise$invoke(input$file)
      promise$result()
    })
  })
}

ui <- fluidPage(
  mod_upload_ui("upload"),
  input_task_button("btn", "Click me!"),
  verbatimTextOutput("text"),
)

server <- function(input, output, session) {
  r6 <- Handler$new()
  
  mod_upload_server("upload", r6)
  
  observeEvent(input$btn, {
    output$text <- renderPrint(r6$file())
  })
}

shinyApp(ui, server)