116 lines
3.3 KiB
Rust
116 lines
3.3 KiB
Rust
/// Generic scroll/selection state for a vertical list menu.
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///
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/// Encapsulates the common behavior of a selectable list that supports:
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/// - Optional selection (None when list is empty)
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/// - Wrap-around navigation on Up/Down
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/// - Maintaining a scroll window (`scroll_top`) so the selected row stays visible
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#[derive(Debug, Default, Clone, Copy)]
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pub(crate) struct ScrollState {
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pub selected_idx: Option<usize>,
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pub scroll_top: usize,
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}
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impl ScrollState {
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pub fn new() -> Self {
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Self {
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selected_idx: None,
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scroll_top: 0,
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}
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}
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/// Reset selection and scroll.
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pub fn reset(&mut self) {
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self.selected_idx = None;
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self.scroll_top = 0;
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}
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/// Clamp selection to be within the [0, len-1] range, or None when empty.
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pub fn clamp_selection(&mut self, len: usize) {
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self.selected_idx = match len {
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0 => None,
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_ => Some(self.selected_idx.unwrap_or(0).min(len - 1)),
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};
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if len == 0 {
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self.scroll_top = 0;
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}
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}
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/// Move selection up by one, wrapping to the bottom when necessary.
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pub fn move_up_wrap(&mut self, len: usize) {
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if len == 0 {
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self.selected_idx = None;
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self.scroll_top = 0;
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return;
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}
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self.selected_idx = Some(match self.selected_idx {
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Some(idx) if idx > 0 => idx - 1,
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Some(_) => len - 1,
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None => 0,
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});
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}
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/// Move selection down by one, wrapping to the top when necessary.
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pub fn move_down_wrap(&mut self, len: usize) {
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if len == 0 {
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self.selected_idx = None;
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self.scroll_top = 0;
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return;
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}
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self.selected_idx = Some(match self.selected_idx {
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Some(idx) if idx + 1 < len => idx + 1,
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_ => 0,
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});
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}
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/// Adjust `scroll_top` so that the current `selected_idx` is visible within
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/// the window of `visible_rows`.
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pub fn ensure_visible(&mut self, len: usize, visible_rows: usize) {
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if len == 0 || visible_rows == 0 {
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self.scroll_top = 0;
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return;
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}
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if let Some(sel) = self.selected_idx {
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if sel < self.scroll_top {
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self.scroll_top = sel;
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} else {
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let bottom = self.scroll_top + visible_rows - 1;
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if sel > bottom {
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self.scroll_top = sel + 1 - visible_rows;
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}
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}
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} else {
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self.scroll_top = 0;
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}
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}
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}
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#[cfg(test)]
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mod tests {
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use super::ScrollState;
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#[test]
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fn wrap_navigation_and_visibility() {
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let mut s = ScrollState::new();
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let len = 10;
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let vis = 5;
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s.clamp_selection(len);
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assert_eq!(s.selected_idx, Some(0));
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s.ensure_visible(len, vis);
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assert_eq!(s.scroll_top, 0);
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s.move_up_wrap(len);
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s.ensure_visible(len, vis);
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assert_eq!(s.selected_idx, Some(len - 1));
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match s.selected_idx {
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Some(sel) => assert!(s.scroll_top <= sel),
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None => panic!("expected Some(selected_idx) after wrap"),
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}
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s.move_down_wrap(len);
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s.ensure_visible(len, vis);
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assert_eq!(s.selected_idx, Some(0));
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assert_eq!(s.scroll_top, 0);
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}
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}
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