Within an observer button event, I’m trying to render an update message to the Shiny UI upon completion. In the code below, each task is timestamped correctly however they aren’t rendered to the UI until the end.

library(shiny)

ui <- fluidPage(
  titlePanel("Task Renderer"),
  mainPanel(
    h3("Completed Tasks:"),
    verbatimTextOutput("task_output"),
    actionButton("process_button", "Process")
  )
)

server <- function(input, output, session) {
  milestones <- c("Task 1: Data collection", 
                 "Task 2: Data cleaning", 
                 "Task 3: Data analysis", 
                 "Task 4: Model building", 
                 "Task 5: Model evaluation", 
                 "Task 6: Report generation")
  
  current_task <- reactiveVal(0)
  completed_tasks <- reactiveVal(character())
  
  nextTask <- function() {
    task_index <- current_task() + 1
    if (task_index <= length(milestones)) {
      current_task(task_index)
      current_time <- format(Sys.time(), "%Y-%m-%d %H:%M:%S")
      completed_tasks(c(completed_tasks(), paste(milestones[task_index], "-", current_time)))
      invalidateLater(1000)  # Trigger UI update after 1 second
    }
  }
  
  output$task_output <- renderText({
    task_index <- current_task()
    if (task_index > 0) {
      paste(completed_tasks(), collapse = "n")
    } else {
      "No tasks completed yet."
    }
  })
  
  observeEvent(input$process_button, {
    # Reset current task and completed tasks
    current_task(0)
    completed_tasks(character())

    #simulate next bit of code
    Sys.sleep(5)
    nextTask() #render task complete message as each is finished

    #simulate next bit of code
    Sys.sleep(5)
    nextTask() #render task complete message as each is finished

    #simulate next bit of code
    Sys.sleep(5)
    nextTask() #render task complete message as each is finished
    })
  }


shinyApp(ui = ui, server = server)

I’ve tried moving the function in and out of the button event code block and several tries with different reactive configurations