Add CollapsingHeader::open to control if it is open or collapsed (#1006)

Closes https://github.com/emilk/egui/issues/978

CHANGELOG.md

@@ -19,6 +19,7 @@ NOTE: [`epaint`](epaint/CHANGELOG.md), [`eframe`](eframe/CHANGELOG.md), [`egui_w
 * When using a custom font you can now specify a font index ([#873](https://github.com/emilk/egui/pull/873)).
 * Add vertical sliders with `Slider::new(…).vertical()` ([#875](https://github.com/emilk/egui/pull/875)).
 * Add `Button::image_and_text` ([#832](https://github.com/emilk/egui/pull/832)).
+* Add `CollapsingHeader::open` to control if it is open or collapsed ([#1006](https://github.com/emilk/egui/pull/1006)).
 
 ### Changed 🔧
 * MSRV (Minimum Supported Rust Version) is now `1.56.0`.

egui/src/containers/collapsing_header.rs

@@ -144,6 +144,7 @@ pub(crate) fn paint_icon(ui: &mut Ui, openness: f32, response: &Response) {
 pub struct CollapsingHeader {
     text: WidgetText,
     default_open: bool,
+    open: Option<bool>,
     id_source: Id,
     enabled: bool,
     selectable: bool,
@@ -164,6 +165,7 @@ impl CollapsingHeader {
         Self {
             text,
             default_open: false,
+            open: None,
             id_source,
             enabled: true,
             selectable: false,
@@ -179,6 +181,16 @@ impl CollapsingHeader {
         self
     }
 
+    /// Calling `.open(Some(true))` will make the collapsing header open this frame (or stay open).
+    ///
+    /// Calling `.open(Some(false))` will make the collapsing header close this frame (or stay closed).
+    ///
+    /// Calling `.open(None)` has no effect (default).
+    pub fn open(mut self, open: Option<bool>) -> Self {
+        self.open = open;
+        self
+    }
+
     /// Explicitly set the source of the `Id` of this widget, instead of using title label.
     /// This is useful if the title label is dynamic or not unique.
     pub fn id_source(mut self, id_source: impl Hash) -> Self {
@@ -256,6 +268,7 @@ impl CollapsingHeader {
         let Self {
             text,
             default_open,
+            open,
             id_source,
             enabled: _,
             selectable: _,
@@ -291,10 +304,16 @@ impl CollapsingHeader {
         );
 
         let mut state = State::from_memory_with_default_open(ui.ctx(), id, default_open);
-        if header_response.clicked() {
+        if let Some(open) = open {
+            if open != state.open {
                 state.toggle(ui);
                 header_response.mark_changed();
             }
+        } else if header_response.clicked() {
+            state.toggle(ui);
+            header_response.mark_changed();
+        }
+
         header_response
             .widget_info(|| WidgetInfo::labeled(WidgetType::CollapsingHeader, text.text()));