egui_extras: Add Table::vertical_scroll_offset (#1946)

* egui_extras: Add Table::vertical_scroll_offset

* Added example for TableBuilder::vertical_scroll_offset

* Format code

* Add link to PR in the changelog

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/egui_demo_lib/src/demo/table_demo.rs

@@ -12,6 +12,8 @@ pub struct TableDemo {
     demo: DemoType,
     resizable: bool,
     num_rows: usize,
+    row_to_scroll_to: i32,
+    vertical_scroll_offset: Option<f32>,
 }
 
 impl Default for TableDemo {
@@ -20,6 +22,8 @@ impl Default for TableDemo {
             demo: DemoType::Manual,
             resizable: true,
             num_rows: 10_000,
+            row_to_scroll_to: 0,
+            vertical_scroll_offset: None,
         }
     }
 }
@@ -41,6 +45,12 @@ impl super::Demo for TableDemo {
     }
 }
 
+fn scroll_offset_for_row(ui: &egui::Ui, row: i32) -> f32 {
+    let text_height = egui::TextStyle::Body.resolve(ui.style()).size;
+    let row_item_spacing = ui.spacing().item_spacing.y;
+    row as f32 * (text_height + row_item_spacing)
+}
+
 impl super::View for TableDemo {
     fn ui(&mut self, ui: &mut egui::Ui) {
         ui.vertical(|ui| {
@@ -66,6 +76,19 @@ impl super::View for TableDemo {
                         .text("Num rows"),
                 );
             }
+
+            if self.demo == DemoType::ManyHomogenous {
+                ui.add(
+                    egui::Slider::new(&mut self.row_to_scroll_to, 0..=self.num_rows as i32)
+                        .logarithmic(true)
+                        .text("Row to scroll to"),
+                );
+
+                if ui.button("Scroll to row").clicked() {
+                    self.vertical_scroll_offset
+                        .replace(scroll_offset_for_row(ui, self.row_to_scroll_to));
+                }
+            }
         });
 
         ui.separator();
@@ -94,13 +117,19 @@ impl TableDemo {
 
         let text_height = egui::TextStyle::Body.resolve(ui.style()).size;
 
-        TableBuilder::new(ui)
+        let mut table = TableBuilder::new(ui)
             .striped(true)
             .cell_layout(egui::Layout::left_to_right(egui::Align::Center))
             .column(Size::initial(60.0).at_least(40.0))
             .column(Size::initial(60.0).at_least(40.0))
             .column(Size::remainder().at_least(60.0))
-            .resizable(self.resizable)
+            .resizable(self.resizable);
+
+        if let Some(y_scroll) = self.vertical_scroll_offset.take() {
+            table = table.vertical_scroll_offset(y_scroll);
+        }
+
+        table
             .header(20.0, |mut header| {
                 header.col(|ui| {
                     ui.heading("Row");

crates/egui_extras/CHANGELOG.md

@@ -3,7 +3,7 @@ All notable changes to the `egui_extras` integration will be noted in this file.
 
 
 ## Unreleased
-
+* Added `TableBuilder::vertical_scroll_offset`: method to set vertical scroll offset position for a table ([#1946](https://github.com/emilk/egui/pull/1946)).
 
 ## 0.19.0 - 2022-08-20
 * MSRV (Minimum Supported Rust Version) is now `1.61.0` ([#1846](https://github.com/emilk/egui/pull/1846)).

crates/egui_extras/src/table.rs

@@ -57,6 +57,7 @@ pub struct TableBuilder<'a> {
     resizable: bool,
     clip: bool,
     stick_to_bottom: bool,
+    scroll_offset_y: Option<f32>,
     cell_layout: egui::Layout,
 }
 
@@ -71,6 +72,7 @@ impl<'a> TableBuilder<'a> {
             resizable: false,
             clip: true,
             stick_to_bottom: false,
+            scroll_offset_y: None,
             cell_layout,
         }
     }
@@ -115,6 +117,12 @@ impl<'a> TableBuilder<'a> {
         self
     }
 
+    /// Set the vertical scroll offset position.
+    pub fn vertical_scroll_offset(mut self, offset: f32) -> Self {
+        self.scroll_offset_y = Some(offset);
+        self
+    }
+
     /// What layout should we use for the individual cells?
     pub fn cell_layout(mut self, cell_layout: egui::Layout) -> Self {
         self.cell_layout = cell_layout;
@@ -156,6 +164,7 @@ impl<'a> TableBuilder<'a> {
             resizable,
             clip,
             stick_to_bottom,
+            scroll_offset_y,
             cell_layout,
         } = self;
 
@@ -189,6 +198,7 @@ impl<'a> TableBuilder<'a> {
             striped,
             clip,
             stick_to_bottom,
+            scroll_offset_y,
             cell_layout,
         }
     }
@@ -208,6 +218,7 @@ impl<'a> TableBuilder<'a> {
             resizable,
             clip,
             stick_to_bottom,
+            scroll_offset_y,
             cell_layout,
         } = self;
 
@@ -229,6 +240,7 @@ impl<'a> TableBuilder<'a> {
             striped,
             clip,
             stick_to_bottom,
+            scroll_offset_y,
             cell_layout,
         }
         .body(body);
@@ -268,6 +280,7 @@ pub struct Table<'a> {
     striped: bool,
     clip: bool,
     stick_to_bottom: bool,
+    scroll_offset_y: Option<f32>,
     cell_layout: egui::Layout,
 }
 
@@ -288,6 +301,7 @@ impl<'a> Table<'a> {
             striped,
             clip,
             stick_to_bottom,
+            scroll_offset_y,
             cell_layout,
         } = self;
 
@@ -295,10 +309,14 @@ impl<'a> Table<'a> {
 
         let mut new_widths = widths.clone();
 
-        let scroll_area = egui::ScrollArea::new([false, scroll])
+        let mut scroll_area = egui::ScrollArea::new([false, scroll])
             .auto_shrink([true; 2])
             .stick_to_bottom(stick_to_bottom);
 
+        if let Some(scroll_offset_y) = scroll_offset_y {
+            scroll_area = scroll_area.vertical_scroll_offset(scroll_offset_y);
+        }
+
         scroll_area.show(ui, move |ui| {
             let layout = StripLayout::new(ui, CellDirection::Horizontal, clip, cell_layout);