Second ProgressBar can't be cleared if set_position before added to MultiProgress

[zhangkaizhao]

I have no idea if this is a bug. Tested with indicatif == 0.17.2.

The good one which can be cleared when set_position after added to MultiProgress:

use indicatif::{MultiProgress, ProgressBar, ProgressStyle};

fn main() {
    let mp = MultiProgress::new();
    let pb = ProgressBar::new(0);
    let style = ProgressStyle::with_template("{msg}").unwrap();
    pb.set_style(style);
    mp.add(pb.clone());

    let pb2 = ProgressBar::new(100);
    let style2 = ProgressStyle::with_template("{bar}").unwrap();
    pb2.set_style(style2);
    mp.add(pb2.clone());
    pb2.set_position(0);
    pb2.finish_and_clear();

    pb.set_message("Good!");
    pb.finish();
}

Output:

Good!

The bad one which can't be cleared when set_position before added to MultiProgress:

use indicatif::{MultiProgress, ProgressBar, ProgressStyle};

fn main() {
    let mp = MultiProgress::new();
    let pb = ProgressBar::new(0);
    let style = ProgressStyle::with_template("{msg}").unwrap();
    pb.set_style(style);
    mp.add(pb.clone());

    let pb2 = ProgressBar::new(100);
    let style2 = ProgressStyle::with_template("{bar}").unwrap();
    pb2.set_style(style2);
    pb2.set_position(0);
    mp.add(pb2.clone());
    pb2.finish_and_clear();

    pb.set_message("Bad!");
    pb.finish();
}

Output:

░░░░░░░░░░░░░░░░░░░░
Bad!

[chris-laplante]

The problem here is that we cannot make any guarantees about what happens when you add a ProgressBar to a MultiProgress after that ProgressBar has already been drawn. In other words, MultiProgress assumes that ProgressBars that have been added to it have never been drawn before. So I am inclined to close this as "not a bug".

[djc]

In other words, MultiProgress assumes that ProgressBars that have been added to it have never been drawn before.

Hmm, I'm not sure this is all that obvious? Why is this a necessary requirement? And how does it influence this particular behavior?