Add Spinner widget (#1013)

* add spinner widget

* implement requested changes to spinner

- removed enabled field
- cleaned up math
- improved docs

* improve spinner docs & make spinner in demo always active

egui/src/widgets/mod.rs

@@ -17,6 +17,7 @@ mod progress_bar;
 mod selected_label;
 mod separator;
 mod slider;
+mod spinner;
 pub mod text_edit;
 
 pub use button::*;
@@ -28,6 +29,7 @@ pub use progress_bar::ProgressBar;
 pub use selected_label::SelectableLabel;
 pub use separator::Separator;
 pub use slider::*;
+pub use spinner::*;
 pub use text_edit::{TextBuffer, TextEdit};
 
 // ----------------------------------------------------------------------------

egui/src/widgets/spinner.rs

@@ -0,0 +1,56 @@
+use epaint::{emath::lerp, vec2, Pos2, Shape, Stroke};
+
+use crate::{Response, Sense, Ui, Widget};
+
+/// A spinner widget used to indicate loading.
+#[must_use = "You should put this widget in an ui with `ui.add(widget);`"]
+#[derive(Default)]
+pub struct Spinner {
+    /// Uses the style's `interact_size` if `None`.
+    size: Option<f32>,
+}
+
+impl Spinner {
+    /// Create a new spinner that uses the style's `interact_size` unless changed.
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    /// Sets the spinner's size. The size sets both the height and width, as the spinner is always
+    /// square. If the size isn't set explicitly, the active style's `interact_size` is used.
+    pub fn size(mut self, size: f32) -> Self {
+        self.size = Some(size);
+        self
+    }
+}
+
+impl Widget for Spinner {
+    fn ui(self, ui: &mut Ui) -> Response {
+        let size = self
+            .size
+            .unwrap_or_else(|| ui.style().spacing.interact_size.y);
+        let (rect, response) = ui.allocate_exact_size(vec2(size, size), Sense::hover());
+
+        if ui.is_rect_visible(rect) {
+            ui.ctx().request_repaint();
+
+            let radius = (rect.height() / 2.0) - 2.0;
+            let n_points = 20;
+            let start_angle = ui.input().time as f64 * 360f64.to_radians();
+            let end_angle = start_angle + 240f64.to_radians() * ui.input().time.sin();
+            let points: Vec<Pos2> = (0..n_points)
+                .map(|i| {
+                    let angle = lerp(start_angle..=end_angle, i as f64 / n_points as f64);
+                    let (sin, cos) = angle.sin_cos();
+                    rect.center() + radius * vec2(cos as f32, sin as f32)
+                })
+                .collect();
+            ui.painter().add(Shape::line(
+                points,
+                Stroke::new(3.0, ui.visuals().strong_text_color()),
+            ));
+        }
+
+        response
+    }
+}

egui_demo_lib/src/apps/demo/widget_gallery.rs

@@ -223,6 +223,10 @@ impl WidgetGallery {
             This toggle switch is just 15 lines of code.",
         );
         ui.end_row();
+
+        ui.add(doc_link_label("Spinner", "spinner"));
+        ui.add(egui::Spinner::new());
+        ui.end_row();
     }
 }