Add data-driven group_by API for DataTable row grouping

TableState::group_by(&[col_ix]) scans delegate.cell_text() to
automatically build a group tree from flat data, rendering group
header rows with expand/collapse toggles on the right side.

- Multi-level grouping via group_by(&[1, 0]) with nested children
- Group tree is cached and only recomputed when row count or columns
  change, avoiding per-frame cell_text() scanning
- Delegate can customize toggle icon (render_row_toggle) and group
  header row (render_group_tr)
- TableEvent::ToggleGroup emitted on expand/collapse
- groups() accessor returns all group info

Author: panzhifu

crates/story/src/stories/data_table_story.rs

@@ -742,6 +742,7 @@ pub struct DataTableStory {
     num_extra_cols_input: Entity<InputState>,
     stripe: bool,
     refresh_data: bool,
+    group_by_enabled: bool,
     size: Size,
 
     _subscriptions: Vec<Subscription>,
@@ -841,6 +842,7 @@ impl DataTableStory {
             num_extra_cols_input,
             stripe: false,
             refresh_data: false,
+            group_by_enabled: false,
             size: Size::default(),
             _subscriptions,
             _load_task,
@@ -964,6 +966,22 @@ impl DataTableStory {
         cx.notify();
     }
 
+    fn toggle_group_by(&mut self, checked: &bool, _: &mut Window, cx: &mut Context<Self>) {
+        self.group_by_enabled = *checked;
+        self.table.update(cx, |table, cx| {
+            if *checked {
+                table
+                    .delegate_mut()
+                    .stocks
+                    .sort_by(|a, b| a.counter.market.cmp(&b.counter.market));
+                table.set_group_by(&[1], cx);
+            } else {
+                table.set_group_by(&[], cx);
+            }
+        });
+        cx.notify();
+    }
+
     fn on_change_size(&mut self, a: &ChangeSize, _: &mut Window, cx: &mut Context<Self>) {
         self.size = a.0;
         cx.notify();
@@ -1011,6 +1029,9 @@ impl DataTableStory {
             TableEvent::ClearSelection => {
                 println!("Selection cleared");
             }
+            TableEvent::ToggleGroup(group_ix, expanded) => {
+                println!("Toggle group: {}, expanded={}", group_ix, expanded);
+            }
         }
     }
 
@@ -1166,6 +1187,12 @@ impl Render for DataTableStory {
                             .label("Group Headers")
                             .checked(self.table.read(cx).delegate().show_group_headers)
                             .on_click(cx.listener(Self::toggle_group_headers)),
+                    )
+                    .child(
+                        Checkbox::new("group-by-market")
+                            .label("Group by Market")
+                            .selected(self.group_by_enabled)
+                            .on_click(cx.listener(Self::toggle_group_by)),
                     ),
             )
             .child(

crates/ui/src/table/data_table.rs

@@ -98,7 +98,10 @@ where
 {
     /// Create a new DataTable element with the given [`TableState`].
     pub fn new(state: &Entity<TableState<D>>) -> Self {
-        Self { state: state.clone(), options: TableOptions::default() }
+        Self {
+            state: state.clone(),
+            options: TableOptions::default(),
+        }
     }
 
     /// Set to use stripe style of the table, default to false.
@@ -115,8 +118,11 @@ where
 
     /// Set scrollbar visibility.
     pub fn scrollbar_visible(mut self, vertical: bool, horizontal: bool) -> Self {
-        self.options.scrollbar_visible =
-            Edges { right: vertical, bottom: horizontal, ..Default::default() };
+        self.options.scrollbar_visible = Edges {
+            right: vertical,
+            bottom: horizontal,
+            ..Default::default()
+        };
         self
     }
 }
@@ -158,7 +164,9 @@ where
             .on_action(window.listener_for(&self.state, TableState::action_select_page_down))
             .bg(cx.theme().table)
             .when(bordered, |this| {
-                this.rounded(cx.theme().radius).border_1().border_color(cx.theme().border)
+                this.rounded(cx.theme().radius)
+                    .border_1()
+                    .border_color(cx.theme().border)
             })
             .child(self.state)
     }

crates/ui/src/table/delegate.rs

@@ -226,4 +226,41 @@ pub trait TableDelegate: Sized + 'static {
     fn cell_text(&self, row_ix: usize, col_ix: usize, cx: &App) -> String {
         String::new()
     }
+
+    // ── Row Grouping ────────────────────────────────────────────────
+
+    /// Render the toggle icon (chevron) for a group header row.
+    ///
+    /// The default implementation draws a `ChevronRight` / `ChevronDown`
+    /// icon. Override to customise the icon or spacing.
+    fn render_row_toggle(
+        &mut self,
+        is_expanded: bool,
+        _window: &mut Window,
+        cx: &mut Context<TableState<Self>>,
+    ) -> impl IntoElement {
+        let icon = if is_expanded {
+            IconName::ChevronDown
+        } else {
+            IconName::ChevronRight
+        };
+        div()
+            .flex()
+            .items_center()
+            .justify_center()
+            .child(Icon::new(icon).size_3())
+    }
+
+    /// Render a group header row.  Called for each group row
+    /// produced by [`TableState::group_by`].
+    ///
+    /// The default renders a label showing the group value and count.
+    fn render_group_tr(
+        &mut self,
+        group: &crate::table::GroupInfo,
+        _window: &mut Window,
+        _cx: &mut Context<TableState<Self>>,
+    ) -> Stateful<Div> {
+        div().id(group.key.clone()).child(group.label.clone())
+    }
 }