Same as `httr:::progress_bar()` but with capability to talk to `shinyWidgets::progressBar()`.
progress_bar(type, con, session, id, title = NULL, status = NULL,
range_value = NULL, unit_mark = "%")(from `httr::progress()` doc) Type of progress to display: either number of bytes uploaded or downloaded.
(from `httr::progress()` doc) Connection to send output too. Usually stdout() or stderr.
(from `shinyWidgets::updateProgressBar()` doc) The 'session' object passed to function given to shinyServer.
(from `shinyWidgets::updateProgressBar()` doc) An id used to update the progress bar.
(from `shinyWidgets::updateProgressBar()` doc) character, optional title.
(from `shinyWidgets::updateProgressBar()` doc) Color, must be a valid Bootstrap status : primary, info, success, warning, danger.
(from `shinyWidgets::updateProgressBar()` doc) Default is to display percentage ([0, 100]), but you can specify a custom range, e.g. -50, 50.
(from `shinyWidgets::updateProgressBar()` doc) Unit for value displayed on the progress bar, default to "%".
a function with rules to print out the progress.