Add option to select collapsing headers (#623)

* Add collapsing header select as selectable label

* Modified Tree demo adding selectable example

* Update egui/src/containers/collapsing_header.rs

Selected is not linked to selectable

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

* Update egui/src/containers/collapsing_header.rs

Description example

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

* Changing example without name clashing

* Fixing merge issue (ah I miss P4 sometimes)

* Fixing doctest example

* Add possibility to show background to a single one

* Fixing clippy test

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

egui/src/containers/collapsing_header.rs

@@ -140,6 +140,9 @@ pub struct CollapsingHeader {
     default_open: bool,
     id_source: Id,
     enabled: bool,
+    selectable: bool,
+    selected: bool,
+    show_background: bool,
 }
 
 impl CollapsingHeader {
@@ -157,6 +160,9 @@ impl CollapsingHeader {
             default_open: false,
             id_source,
             enabled: true,
+            selectable: false,
+            selected: false,
+            show_background: false,
         }
     }
 
@@ -188,6 +194,44 @@ impl CollapsingHeader {
         self.enabled = enabled;
         self
     }
+
+    /// Can the `CollapsingHeader` be selected by clicking it? Default: `false`.
+    ///
+    pub fn selectable(mut self, selectable: bool) -> Self {
+        self.selectable = selectable;
+        self
+    }
+
+    /// If you set this to 'true', the `CollapsingHeader` will be shown as selected.
+    ///
+    /// Example:
+    /// ```
+    /// # let ui = &mut egui::Ui::__test();
+    /// let mut selected = false;
+    /// let response = egui::CollapsingHeader::new("Select and open me")
+    ///     .selectable(true)
+    ///     .selected(selected)
+    ///     .show(ui, |ui| ui.label("Content"));
+    /// if response.header_response.clicked() {
+    ///     selected = true;
+    /// }
+    /// ```
+    pub fn selected(mut self, selected: bool) -> Self {
+        self.selected = selected;
+        self
+    }
+
+    /// Should the `CollapsingHeader` show a background behind it? Default: `false`.
+    ///
+    /// To show it behind all `CollapsingHeader` you can just use:
+    /// ```
+    /// # let ui = &mut egui::Ui::__test();
+    /// ui.visuals_mut().collapsing_header_frame = true;
+    /// ```
+    pub fn show_background(mut self, show_background: bool) -> Self {
+        self.show_background = show_background;
+        self
+    }
 }
 
 struct Prepared {
@@ -207,6 +251,9 @@ impl CollapsingHeader {
             default_open,
             id_source,
             enabled: _,
+            selectable: _,
+            selected: _,
+            show_background: _,
         } = self;
 
         label.text_style = label
@@ -247,10 +294,16 @@ impl CollapsingHeader {
         header_response
             .widget_info(|| WidgetInfo::labeled(WidgetType::CollapsingHeader, &galley.text));
 
-        let visuals = ui.style().interact(&header_response);
+        let visuals = ui
-        let text_color = visuals.text_color();
+            .style()
+            .interact_selectable(&header_response, self.selected);
+        let text_color = ui
+            .style()
+            .visuals
+            .override_text_color
+            .unwrap_or_else(|| visuals.text_color());
 
-        if ui.visuals().collapsing_header_frame {
+        if ui.visuals().collapsing_header_frame || self.show_background {
             ui.painter().add(Shape::Rect {
                 rect: header_response.rect.expand(visuals.expansion),
                 corner_radius: visuals.corner_radius,
@@ -260,6 +313,16 @@ impl CollapsingHeader {
             });
         }
 
+        if self.selected
+            || self.selectable && (header_response.hovered() || header_response.has_focus())
+        {
+            let rect = rect.expand(visuals.expansion);
+
+            let corner_radius = 2.0;
+            ui.painter()
+                .rect(rect, corner_radius, visuals.bg_fill, visuals.bg_stroke);
+        }
+
         {
             let (mut icon_rect, _) = ui.spacing().icon_rectangles(header_response.rect);
             icon_rect.set_center(pos2(

egui_demo_lib/src/apps/demo/misc_demo_window.rs

@@ -339,28 +339,53 @@ enum Action {
 
 #[derive(Clone, Default)]
 #[cfg_attr(feature = "persistence", derive(serde::Deserialize, serde::Serialize))]
-struct Tree(Vec<Tree>);
+struct Tree(String, SubTree);
 
 impl Tree {
     pub fn demo() -> Self {
-        Self(vec![
+        Self(
-            Tree(vec![Tree::default(); 4]),
+            String::from("root"),
-            Tree(vec![Tree(vec![Tree::default(); 2]); 3]),
+            SubTree(vec![
-        ])
+                SubTree(vec![SubTree::default(); 4]),
+                SubTree(vec![SubTree(vec![SubTree::default(); 2]); 3]),
+            ]),
+        )
     }
     pub fn ui(&mut self, ui: &mut Ui) -> Action {
-        self.ui_impl(ui, 0, "root")
+        self.1.ui(ui, 0, "root", &mut self.0)
     }
+}
 
-    fn ui_impl(&mut self, ui: &mut Ui, depth: usize, name: &str) -> Action {
+#[derive(Clone, Default)]
-        CollapsingHeader::new(name)
+#[cfg_attr(feature = "persistence", derive(serde::Deserialize, serde::Serialize))]
+struct SubTree(Vec<SubTree>);
+
+impl SubTree {
+    pub fn ui(
+        &mut self,
+        ui: &mut Ui,
+        depth: usize,
+        name: &str,
+        selected_name: &mut String,
+    ) -> Action {
+        let response = CollapsingHeader::new(name)
             .default_open(depth < 1)
-            .show(ui, |ui| self.children_ui(ui, depth))
+            .selectable(true)
-            .body_returned
+            .selected(selected_name.as_str() == name)
-            .unwrap_or(Action::Keep)
+            .show(ui, |ui| self.children_ui(ui, name, depth, selected_name));
+        if response.header_response.clicked() {
+            *selected_name = name.to_string();
+        }
+        response.body_returned.unwrap_or(Action::Keep)
     }
 
-    fn children_ui(&mut self, ui: &mut Ui, depth: usize) -> Action {
+    fn children_ui(
+        &mut self,
+        ui: &mut Ui,
+        parent_name: &str,
+        depth: usize,
+        selected_name: &mut String,
+    ) -> Action {
         if depth > 0
             && ui
                 .add(Button::new("delete").text_color(Color32::RED))
@@ -374,7 +399,13 @@ impl Tree {
             .into_iter()
             .enumerate()
             .filter_map(|(i, mut tree)| {
-                if tree.ui_impl(ui, depth + 1, &format!("child #{}", i)) == Action::Keep {
+                if tree.ui(
+                    ui,
+                    depth + 1,
+                    &format!("{}/{}", parent_name, i),
+                    selected_name,
+                ) == Action::Keep
+                {
                     Some(tree)
                 } else {
                     None
@@ -383,7 +414,7 @@ impl Tree {
             .collect();
 
         if ui.button("+").clicked() {
-            self.0.push(Tree::default());
+            self.0.push(SubTree::default());
         }
 
         Action::Keep