IDEA: Transform progress::progress_bar() updates to progression updates on the fly

[HenrikBengtsson]

Would it be possible to listen to progress::progress_bar() update messages and translate those into proper progression conditions? It looks tricky and would rely on knowing what output to expect from progress_bar(). For example,

> library(progress)
> pb <- progress_bar$new(total = 10)
> msg <- NULL
> withCallingHandlers(invisible(pb$tick()), message = function(m) msg <<- m)
> str(msg)
 NULL
> withCallingHandlers(invisible(pb$tick()), message = function(m) msg <<- m)
[=============>---------------------------------------------------------]  20%> 
> str(msg)
List of 2
 $ message: chr "[=============>---------------------------------------------------------]  20%"
 $ call   : NULL
 - attr(*, "class")= chr [1:2] "message" "condition"

As a starter, if all progress_bar() calls would make use of the new message_class, then it would possible to identify progress_bar message from regular messages, e.g.

> library(progress)
> pb <- progress_bar$new(total = 10, message_class = "progress_bar")
> msg <- NULL
> withCallingHandlers(invisible(pb$tick()), message = function(m) msg <<- m)
> str(msg)
 NULL
> withCallingHandlers(invisible(pb$tick()), message = function(m) msg <<- m)
[=============>---------------------------------------------------------]  20%> 
> str(msg)
List of 2
 $ message: chr "[=============>---------------------------------------------------------]  20%"
 $ call   : NULL
 - attr(*, "class")= chr [1:3] "progress_bar" "message" "condition"

Yet, it'll be hard to parse those message strings to reverse engineer the underlying progress state, especially since we don't know what progress_bar(format = ...) has been set. Although, we could possibly be able to infer it after having received a few updates.