How to get keyboard input with Termion in Rust?

Question

I'm trying out Rust and I really like it so far. I'm working on a tool that needs to get arrow key input from the user. So far, I've got something half-working: if I hold a key for a while, the relevant function gets called. However, it's far from instantaneous.

What I've got so far:

let mut stdout = io::stdout().into_raw_mode();
let mut stdin = termion::async_stdin();
// let mut stdin = io::stdin();
let mut it = stdin.keys(); //iterator object

loop {
    //copied straight from GitLab: https://gitlab.redox-os.org/redox-os/termion/-/issues/168
    let b = it.next(); 
    match b {
        Some(x) => match x {
            Ok(k) => {
                match k {
                    Key::Left => move_cursor(&mut cursor_char, -1, &enc_chars, &mpt, &status),
                    Key::Right => move_cursor(&mut cursor_char, 1, &enc_chars, &mpt, &status),
                    Key::Ctrl('c') => break,
                    _ => {}
                }
            },  
            _ => {}
        },
        None => {}
    }
    //this loop might do nothing if no recognized key was pressed.
}

I don't quite understand it myself. I'm using the terminal raw mode, if that has anything to do with it. I've looked at the rustyline crate, but that's really no good as it's more of an interactive shell-thing, and I just want to detect keypresses.

score 4 · Accepted Answer · answered May 18 '21 at 20:26

If you're using raw input mode and reading key by key, you'll need to manually buffer the character keys using the same kind of match loop you already have. The Key::Char(ch) enum variant can be used to match regular characters. You can then use either a mutable String or an array like [u8; MAX_SIZE] to store the character data and append characters as they're typed. If the user moves the cursor, you'd need to keep track of the current position within your input buffer and make sure to insert the newly typed characters into the correct spot, moving the existing characters if needed. It is a lot of work, which is why there are crates that will do it for you, but you will have less chance to control how the input behaves. If you want to use an existing crate, then tui-rs might be a good one to check out for a complete solution, or linefeed for something much simpler.

As for the delay, I think it might be because you're using AsyncReader, which according to the docs is using a secondary thread to do blocking reads

How to get keyboard input with Termion in Rust?

1 Answers1