An example of interrupting a prolonged, iterative computation in Shiny.

iterativeTask.R

library(shiny)

appUI <- pageWithSidebar(
  
  # Application title
  headerPanel("New Application"),
  
  sidebarPanel(
    "Progress: ",
    textOutput("counter"),
    hr(),
    "Elapsed Time (seconds):",
    textOutput("elapsed"),
    actionButton("stop", "Stop")
  ),
  
  mainPanel(
    textOutput("x")
  )
)

appServer <- function(input, output, session) {
  # The number of iterations to perform
  maxIter <- 50
  
  # Track the start and elapsed time
  startTime <- Sys.time()  
  output$elapsed <- renderText({
    vals$x
    round(Sys.time() - startTime)
  })
  
  # Create a reactiveValues object where we can track some extra elements
  # reactively.
  vals <- reactiveValues(x = 0, counter = 0, stop = FALSE)
  
  # Update the percentage complete
  output$counter <- renderText({
    paste0(round(vals$counter/maxIter * 100, 1), "%")
  })
  
  # Show the value of x
  output$x <- renderText({
    round(vals$x,2)
  })
  
  # Set up a progress indicator
  progress <- Progress$new()
  progress$set(message = "Counting...", value = 0)
  
  # Do the actual computation here.
  observe({
    isolate({
      # This is where we do the expensive computing
      sum <- 0
      for (i in 1:100000){
        sum <- sum + rnorm(1)
      }
      vals$x <- vals$x + sum
      
      # Increment the counter
      vals$counter <- vals$counter + 1
      progress$set(value=vals$counter/maxIter)
    })
    
    # If we're not done yet, then schedule this block to execute again ASAP.
    # Note that we can be interrupted by other reactive updates to, for
    # instance, update a text output.
    if (isolate(vals$counter) < maxIter && !vals$stop){
      invalidateLater(0, session)
    } else {
      progress$close()
    }
  })
  
  observeEvent(input$stop, {
    vals$stop <- TRUE
  })
  
}

shinyApp(appUI, appServer)