Inside R6 class definition: 'object not found' (or: how to define 'local' objects in R6 classes)

Question

I want to define an R6 class that sets up, updates and closes a progress bar. For these 3 tasks, I have 3 functions. The first, setup_progressbar(), calls R's txtProgressbar() which returns an object (say, pb) which needs to be passed on to the second and third functions, update_progressbar() and close_progressbar(). But the object pb is not found by the latter two functions.

library(R6)
myprogressbar <- R6Class("my_progress_bar",
                         public = list(
                             n = numeric(1),
                             initialize = function(n) {
                                 stopifnot(n >= 1)
                                 self$n <- n
                             },
                             setup_progressbar = function() {
                                 pb <- txtProgressBar(max = self$n)
                             },
                             update_progressbar = function(i) {
                                 setTxtProgressBar(pb, i)
                             },
                             close_progressbar = function () {
                                 close(pb)
                                 cat("\n")
                             }
                         ))
mypb <- myprogressbar$new(10)
mypb$setup_progressbar()
mypb$update_progressbar(3) # Error in setTxtProgressBar(pb, i) : object 'pb' not found

I tried to add pb to self in the hope it would be found, but then I obtain "cannot add bindings to a locked environment".

Note: In my actual (non-minimal) example, the i is found/provided/visible, so that's not an additional problem (most likely this is just a problem in the above minimal working example once fixed beyond the 'pb' not found error).

Answer 1

The following works:

library(R6)
myprogressbar <- R6Class("my_progress_bar",
                         public = list(
                             n = numeric(1),
                             pb = NULL, # provide as argument
                             initialize = function(n, pb = NULL) { # provide with default so that $new() doesn't require 'pb'
                                 stopifnot(n >= 1)
                                 self$n <- n
                             },
                             setup_progressbar = function() {
                                 self$pb <- txtProgressBar(max = self$n)
                             },
                             update_progressbar = function(i) {
                                 setTxtProgressBar(self$pb, i)
                             },
                             close_progressbar = function () {
                                 close(self$pb)
                                 cat("\n")
                             }
                         ))

mypb <- myprogressbar$new(10)
mypb$setup_progressbar()
mypb$update_progressbar(3)