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).

What exactly do you mean by "isn't quite natural"? I'm afraid that's just how R6 objects work. You can read more about them at https://adv-r.hadley.nz/r6.html. It's unclear exactly what your requirements are. — MrFlick, Jan 03 '21 at 19:33
I found it strange that `initialize()` needs all such type of arguments (as `pb`) as argument (and thus some sort of default so that it isn't required to be provided by `$new()`). There could be many such arguments and thus they all need to be part of `initialize()`. Thanks for the reference, I read it but that particular case (passing arguments between functions) is not covered, at least I haven't seen it. I saw from https://homerhanumat.github.io/r-notes/reference-classes-with-package-r6.html that all functions have access to `self` which is why I then 'forced' `pb` to be part of it. — Marius Hofert, Jan 03 '21 at 19:39

score 2 · Accepted Answer · answered Jan 03 '21 at 21:07

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)

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

1 Answers1