callback Callback function to call when the progress bar finishes.
The progress bar object itself is passed to it as the single
parameter.
and in the tests you do use a function of the form function(self) ... but when callback is called by progress bar it's called as self$callback() not self$callback(self)
In the docs you say:
and in the tests you do use a function of the form
function(self) ...
but when callback is called by progress bar it's called asself$callback()
notself$callback(self)