reactive debounce for Shiny

debounce.R

# Returns a reactive that debounces the given expression by the given time in
# milliseconds.
#
# This is not a true debounce in that it will not prevent \code{expr} from being
# called many times (in fact it may be called more times than usual), but
# rather, the reactive invalidation signal that is produced by expr is debounced
# instead. This means that this function should be used when \code{expr} is
# cheap but the things it will trigger (outputs and reactives that use
# \code{expr}) are expensive.
debounce <- function(expr, millis, env = parent.frame(), quoted = FALSE,
  domain = getDefaultReactiveDomain()) {
  
  force(millis)
  
  f <- exprToFunction(expr, env, quoted)
  label <- sprintf("debounce(%s)", paste(deparse(body(f)), collapse = "\n"))

  v <- reactiveValues(
    trigger = NULL,
    when = NULL # the deadline for the timer to fire; NULL if not scheduled
  )  

  # Responsible for tracking when f() changes.
  observeEvent(f(), {
    # The value changed. Start or reset the timer.
    v$when <- Sys.time() + millis/1000
  }, ignoreNULL = FALSE)

  # This observer is the timer. It rests until v$when elapses, then touches
  # v$trigger.
  observe({
    if (is.null(v$when))
      return()
    
    now <- Sys.time()
    if (now >= v$when) {
      v$trigger <- runif(1)
      v$when <- NULL
    } else {
      invalidateLater((v$when - now) * 1000, domain)
    }
  })

  # This is the actual reactive that is returned to the user. It returns the
  # value of f(), but only invalidates/updates when v$trigger is touched.
  eventReactive(v$trigger, {
    f()
  }, ignoreNULL = FALSE)
}


#' @examples
#' library(shiny)
#' 
#' ui <- fluidPage(
#'   numericInput("val", "Change this rapidly, then pause", 5),
#'   textOutput("out")
#' )
#' 
#' server <- function(input, output, session) {
#'   debounced <- debounce(input$val, 1000)
#'   output$out <- renderText(
#'     debounced()
#'   )
#' }
#' 
#' shinyApp(ui, server)