feat(widgets): add offset() and offset_mut() for table and list state

[orhun]

Upstream: #497

Description

Adds offset access and manipulation. Fixes #495.

Testing guidelines

I have a few structs that keep the table state. I use this trait ScrollableTable for each of these to attach actions like scroll up/down, etc. I've added this fn recenter to test offset manipulation. ListState should be pretty much analogously.

Note that I haven't added tests per se, because I think that these kinds of accessor/mutator functions are quite obvious.

use tui::layout::Rect;
use tui::widgets::TableState;

pub trait ScrollableTable {
    fn size(&self) -> Rect;
    fn state(&self) -> &TableState;
    fn state_mut(&mut self) -> &mut TableState;

    fn recenter(&mut self) {
        if let Some(selected) = self.state().selected() {
            let height = self.size().height as usize;

            let offset = self.state_mut().offset_mut();
            *offset = selected.saturating_sub(height / 2);
        }
    }
}

Checklist

• I have read the contributing guidelines.
• I have added relevant tests.
• I have documented all new additions.

[orhun]

wookietreiber said: cargo make does not seem to be compatible with rustc 1.56.1:

error: failed to compile `cargo-make v0.35.8`, intermediate artifacts can be found at `/tmp/cargo-install52QPLl`

Caused by:
  package `os_str_bytes v6.2.0` cannot be built because it requires rustc 1.57.0 or newer, while the currently active rustc version is 1.56.1
Error: Process completed with exit code 101.

[joshka]

I'd suggest adding the with_offset / with_selected methods to TableState to make building state in a single statement easy, but this is fine without it.

[orhun]

Updated the implementation so that Table and List state implementations are identical. Makes one think if we should have one struct such as e.g. WidgetState for both and deprecate the separate usage. 🤔

[joshka]

That's kinda what the scroll RFC talks about.