Collapsing header with custom header (#1538)

* Returns openness in CollapsingResponse
* Make CollapsingState a building block for custom collapsing headers
* Add a demo of the custom collapsing header
* Revert to much simpler tree demo
* Add CollapsingState::is_open and CollapsingState::set_open

egui_demo_lib/src/apps/demo/misc_demo_window.rs

@@ -14,6 +14,7 @@ pub struct MiscDemoWindow {
 
     widgets: Widgets,
     colors: ColorWidgets,
+    custom_collapsing_header: CustomCollapsingHeader,
     tree: Tree,
     box_painting: BoxPainting,
 
@@ -32,6 +33,7 @@ impl Default for MiscDemoWindow {
 
             widgets: Default::default(),
             colors: Default::default(),
+            custom_collapsing_header: Default::default(),
             tree: Tree::demo(),
             box_painting: Default::default(),
 
@@ -82,6 +84,10 @@ impl View for MiscDemoWindow {
                 self.colors.ui(ui);
             });
 
+        CollapsingHeader::new("Custom Collapsing Header")
+            .default_open(false)
+            .show(ui, |ui| self.custom_collapsing_header.ui(ui));
+
         CollapsingHeader::new("Tree")
             .default_open(false)
             .show(ui, |ui| self.tree.ui(ui));
@@ -351,6 +357,44 @@ impl BoxPainting {
 
 // ----------------------------------------------------------------------------
 
+#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
+struct CustomCollapsingHeader {
+    selected: bool,
+    radio_value: bool,
+}
+
+impl Default for CustomCollapsingHeader {
+    fn default() -> Self {
+        Self {
+            selected: true,
+            radio_value: false,
+        }
+    }
+}
+
+impl CustomCollapsingHeader {
+    pub fn ui(&mut self, ui: &mut egui::Ui) {
+        ui.label("Example of a collapsing header with custom header:");
+
+        let id = ui.make_persistent_id("my_collapsing_header");
+        egui::collapsing_header::CollapsingState::load_with_default_open(ui.ctx(), id, true)
+            .show_header(ui, |ui| {
+                ui.toggle_value(&mut self.selected, "Click to select/unselect");
+                ui.radio_value(&mut self.radio_value, false, "");
+                ui.radio_value(&mut self.radio_value, true, "");
+            })
+            .body(|ui| {
+                ui.label("The body is always custom");
+            });
+
+        CollapsingHeader::new("Normal collapsing header for comparison").show(ui, |ui| {
+            ui.label("Nothing exciting here");
+        });
+    }
+}
+
+// ----------------------------------------------------------------------------
+
 #[derive(Clone, Copy, PartialEq)]
 enum Action {
     Keep,
@@ -359,53 +403,30 @@ enum Action {
 
 #[derive(Clone, Default)]
 #[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
-struct Tree(String, SubTree);
+struct Tree(Vec<Tree>);
 
 impl Tree {
     pub fn demo() -> Self {
-        Self(
-            String::from("root"),
-            SubTree(vec![
-                SubTree(vec![SubTree::default(); 4]),
-                SubTree(vec![SubTree(vec![SubTree::default(); 2]); 3]),
-            ]),
-        )
+        Self(vec![
+            Tree(vec![Tree::default(); 4]),
+            Tree(vec![Tree(vec![Tree::default(); 2]); 3]),
+        ])
     }
     pub fn ui(&mut self, ui: &mut Ui) -> Action {
-        self.1.ui(ui, 0, "root", &mut self.0)
+        self.ui_impl(ui, 0, "root")
     }
 }
 
-#[derive(Clone, Default)]
-#[cfg_attr(feature = "serde", 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)
+impl Tree {
+    fn ui_impl(&mut self, ui: &mut Ui, depth: usize, name: &str) -> Action {
+        CollapsingHeader::new(name)
             .default_open(depth < 1)
-            .selectable(true)
-            .selected(selected_name.as_str() == name)
-            .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)
+            .show(ui, |ui| self.children_ui(ui, depth))
+            .body_returned
+            .unwrap_or(Action::Keep)
     }
 
-    fn children_ui(
-        &mut self,
-        ui: &mut Ui,
-        parent_name: &str,
-        depth: usize,
-        selected_name: &mut String,
-    ) -> Action {
+    fn children_ui(&mut self, ui: &mut Ui, depth: usize) -> Action {
         if depth > 0
             && ui
                 .button(RichText::new("delete").color(Color32::RED))
@@ -419,13 +440,7 @@ impl SubTree {
             .into_iter()
             .enumerate()
             .filter_map(|(i, mut tree)| {
-                if tree.ui(
-                    ui,
-                    depth + 1,
-                    &format!("{}/{}", parent_name, i),
-                    selected_name,
-                ) == Action::Keep
-                {
+                if tree.ui_impl(ui, depth + 1, &format!("child #{}", i)) == Action::Keep {
                     Some(tree)
                 } else {
                     None
@@ -434,7 +449,7 @@ impl SubTree {
             .collect();
 
         if ui.button("+").clicked() {
-            self.0.push(SubTree::default());
+            self.0.push(Tree::default());
         }
 
         Action::Keep