Allow indeterminate progress bars and spinners to have their text updated

[mattrayner]

Hi, first of all, thanks for this library, it's really useful - I was wondering if there were any plans to add, or if it was currently possible to update the text on a spinner or indeterminate progress bar to provide context to a user.

I've got an indeterminate action that have a few stages, it would be cool to update a user on what is happening behind the scenes

[henrik]

Hi @mattrayner! Thank you :)

To be honest I haven't really touched this lib in ages :/

I agree that'd be nice functionality to have. If anyone wants to make a PR I'll try to take the time to look and merge!