Add manual scrolling

egui/src/containers/scroll_area.rs

@@ -153,6 +153,10 @@ impl Prepared {
 
         let content_size = content_ui.min_size();
 
+        if let Some(offset_y) = content_ui.ctx().frame_state().scroll_offset_y() {
+            state.offset.y = offset_y;
+        }
+
         let inner_rect = Rect::from_min_size(
             inner_rect.min,
             vec2(

egui/src/context.rs

@@ -39,6 +39,7 @@ pub(crate) struct FrameState {
     /// How much space is used by panels.
     used_by_panels: Rect,
     // TODO: move some things from `Memory` to here
+    scroll_offset: Option<f32>
 }
 
 impl Default for FrameState {
@@ -47,6 +48,7 @@ impl Default for FrameState {
             available_rect: Rect::invalid(),
             unused_rect: Rect::invalid(),
             used_by_panels: Rect::invalid(),
+            scroll_offset: None
         }
     }
 }
@@ -56,6 +58,7 @@ impl FrameState {
         self.available_rect = input.screen_rect();
         self.unused_rect = input.screen_rect();
         self.used_by_panels = Rect::nothing();
+        self.scroll_offset = None;
     }
 
     /// How much space is still available after panels has been added.
@@ -97,6 +100,14 @@ impl FrameState {
         self.unused_rect = Rect::nothing(); // Nothing left unused after this
         self.used_by_panels = self.used_by_panels.union(panel_rect);
     }
+
+    pub(crate) fn set_scroll_offset_y(&mut self, offset: f32) {
+        self.scroll_offset = Some(offset);
+    }
+
+    pub(crate) fn scroll_offset_y(&self) -> Option<f32> {
+        self.scroll_offset
+    }
 }
 
 // ----------------------------------------------------------------------------

egui/src/demos/demo_window.rs

@@ -11,12 +11,14 @@ pub struct DemoWindow {
     layout: LayoutDemo,
     tree: Tree,
     box_painting: BoxPainting,
+    ratio: f32,
 }
 
 impl Default for DemoWindow {
     fn default() -> DemoWindow {
         DemoWindow {
             num_columns: 2,
+            ratio: 0.0,
 
             widgets: Default::default(),
             colors: Default::default(),
@@ -69,7 +71,28 @@ impl DemoWindow {
             .default_open(false)
             .show(ui, |ui| {
                 ScrollArea::from_max_height(200.0).show(ui, |ui| {
+                    let middle = ui.button("Go middle").clicked;
+                    let bottom = ui.button("Go bottom").clicked;
+                    let not_working = ui.button("Go middle (not working)").clicked;
+                    ui.add(Slider::f32(&mut self.ratio, 0.0..=1.0).text("ratio"));
                     ui.label(LOREM_IPSUM_LONG);
+                    ui.label(LOREM_IPSUM_LONG);
+                    if middle {
+                        ui.scroll_to_here(self.ratio);
+                    }
+
+                    // FIXME: inside the children Ui, scroll_to_here does not work.
+                    ui.horizontal(|ui| {
+                        if not_working {
+                            ui.scroll_to_here(0.5);
+                        }
+                    });
+                    ui.label("Middle");
+                    ui.label(LOREM_IPSUM_LONG);
+                    ui.label(LOREM_IPSUM_LONG);
+                    if bottom {
+                        ui.scroll_to_here(1.0);
+                    }
                 });
             });
 

egui/src/ui.rs

@@ -526,6 +526,14 @@ impl Ui {
         let clip_rect = self.clip_rect().intersect(rect); // Make sure we don't paint out of bounds
         Painter::new(self.ctx().clone(), self.layer_id(), clip_rect)
     }
+
+    pub fn scroll_to_here(&mut self, center_ratio: f32) {
+        // This is the imgui aproach to set the scroll relative to the position 
+        // 0.0: top of widget / 0.5: midle of widget / 1.0: bottom of widget
+        assert!(center_ratio >= 0.0 && center_ratio <= 1.0);
+        let offset = self.min_rect().height() - self.clip_rect().height() * center_ratio;
+        self.ctx().frame_state().set_scroll_offset_y(offset);
+    }
 }
 
 /// # Adding widgets