Add ability to override collapsing icons

CHANGELOG.md

@@ -10,6 +10,7 @@ NOTE: [`epaint`](epaint/CHANGELOG.md), [`eframe`](eframe/CHANGELOG.md), [`egui_w
 ### Added ⭐
 * `Context::load_texture` to convert an image into a texture which can be displayed using e.g. `ui.image(texture, size)` ([#1110](https://github.com/emilk/egui/pull/1110)).
 * Added `Ui::add_visible` and `Ui::add_visible_ui`.
+* Added `CollapsingHeader::icon` to override the default open/close icon using a custom function. ([1147](https://github.com/emilk/egui/pull/1147))
 
 ### Changed 🔧
 * ⚠️ `Context::input` and `Ui::input` now locks a mutex. This can lead to a dead-lock is used in an `if let` binding!
@@ -26,6 +27,7 @@ NOTE: [`epaint`](epaint/CHANGELOG.md), [`eframe`](eframe/CHANGELOG.md), [`egui_w
 
 ### Contributors 🙏
 * [danielkeller](https://github.com/danielkeller): [#1050](https://github.com/emilk/egui/pull/1050).
+* [juancampa](https://github.com/juancampa): [#1147](https://github.com/emilk/egui/pull/1147).
 
 
 ## 0.16.1 - 2021-12-31 - Add back `CtxRef::begin_frame,end_frame`

egui/src/containers/collapsing_header.rs

@@ -98,7 +98,7 @@ impl State {
 }
 
 /// Paint the arrow icon that indicated if the region is open or not
-pub(crate) fn paint_icon(ui: &mut Ui, openness: f32, response: &Response) {
+pub(crate) fn paint_default_icon(ui: &mut Ui, openness: f32, response: &Response) {
     let visuals = ui.style().interact(response);
     let stroke = visuals.fg_stroke;
 
@@ -117,6 +117,9 @@ pub(crate) fn paint_icon(ui: &mut Ui, openness: f32, response: &Response) {
     ui.painter().add(Shape::closed_line(points, stroke));
 }
 
+/// A function that paints an icon indicating if the region is open or not
+pub type IconPainter = Box<dyn FnOnce(&mut Ui, f32, &Response)>;
+
 /// A header which can be collapsed/expanded, revealing a contained [`Ui`] region.
 ///
 ///
@@ -141,6 +144,7 @@ pub struct CollapsingHeader {
     selectable: bool,
     selected: bool,
     show_background: bool,
+    icon: Option<IconPainter>,
 }
 
 impl CollapsingHeader {
@@ -162,6 +166,7 @@ impl CollapsingHeader {
             selectable: false,
             selected: false,
             show_background: false,
+            icon: None,
         }
     }
 
@@ -236,6 +241,28 @@ impl CollapsingHeader {
         self.show_background = show_background;
         self
     }
+
+    /// Use the provided function to render a different `CollapsingHeader` icon.
+    /// Defaults to a triangle that animates as the `CollapsingHeader` opens and closes.
+    ///
+    /// For example:
+    /// ```
+    /// # egui::__run_test_ui(|ui| {
+    /// fn circle_icon(ui: &mut egui::Ui, openness: f32, response: &egui::Response) {
+    ///     let stroke = ui.style().interact(&response).fg_stroke;
+    ///     let radius = egui::lerp(2.0..=3.0, openness);
+    ///     ui.painter().circle_filled(response.rect.center(), radius, stroke.color);
+    /// }
+    ///
+    /// egui::CollapsingHeader::new("Circles")
+    ///   .icon(circle_icon)
+    ///   .show(ui, |ui| { ui.label("Hi!"); });
+    /// # });
+    /// ```
+    pub fn icon(mut self, icon_fn: impl FnOnce(&mut Ui, f32, &Response) + 'static) -> Self {
+        self.icon = Some(Box::new(icon_fn));
+        self
+    }
 }
 
 struct Prepared {
@@ -251,6 +278,7 @@ impl CollapsingHeader {
             "Horizontal collapsing is unimplemented"
         );
         let Self {
+            icon,
             text,
             default_open,
             open,
@@ -341,7 +369,11 @@ impl CollapsingHeader {
                     ..header_response.clone()
                 };
                 let openness = state.openness(ui.ctx(), id);
-                paint_icon(ui, openness, &icon_response);
+                if let Some(icon) = icon {
+                    icon(ui, openness, &icon_response);
+                } else {
+                    paint_default_icon(ui, openness, &icon_response);
+                }
             }
 
             text.paint_with_visuals(ui.painter(), text_pos, &visuals);

egui/src/containers/window.rs

@@ -782,7 +782,7 @@ fn show_title_bar(
                 collapsing.toggle(ui);
             }
             let openness = collapsing.openness(ui.ctx(), collapsing_id);
-            collapsing_header::paint_icon(ui, openness, &collapse_button_response);
+            collapsing_header::paint_default_icon(ui, openness, &collapse_button_response);
         }
 
         let title_galley = title.into_galley(ui, Some(false), f32::INFINITY, TextStyle::Heading);