impl<F> Widget for F where F: FnOnce(&mut Ui) -> Response

This enables functions that return `impl Widget`, so that you can
create a widget by just returning a lambda from a function.

For instance: `ui.add(toggle(bool))` (instead of `toggle(ui, bool)`)

egui/src/style.rs

@@ -506,10 +506,10 @@ impl Spacing {
             tooltip_width,
         } = self;
 
-        ui_slider_vec2(ui, item_spacing, 0.0..=10.0, "item_spacing");
-        ui_slider_vec2(ui, window_padding, 0.0..=10.0, "window_padding");
-        ui_slider_vec2(ui, button_padding, 0.0..=10.0, "button_padding");
-        ui_slider_vec2(ui, interact_size, 0.0..=60.0, "interact_size")
+        ui.add(slider_vec2(item_spacing, 0.0..=10.0, "item_spacing"));
+        ui.add(slider_vec2(window_padding, 0.0..=10.0, "window_padding"));
+        ui.add(slider_vec2(button_padding, 0.0..=10.0, "button_padding"));
+        ui.add(slider_vec2(interact_size, 0.0..=60.0, "interact_size"))
             .on_hover_text("Minimum size of an interactive widget");
         ui.add(Slider::f32(indent, 0.0..=100.0).text("indent"));
         ui.add(Slider::f32(slider_width, 0.0..=1000.0).text("slider_width"));
@@ -694,38 +694,20 @@ impl Visuals {
     }
 }
 
-// TODO: improve and standardize ui_slider_vec2
-fn ui_slider_vec2(
-    ui: &mut Ui,
-    value: &mut Vec2,
+// TODO: improve and standardize `slider_vec2`
+fn slider_vec2<'a>(
+    value: &'a mut Vec2,
     range: std::ops::RangeInclusive<f32>,
-    text: &str,
-) -> Response {
+    text: &'a str,
+) -> impl Widget + 'a {
+    move |ui: &mut crate::Ui| {
         ui.horizontal(|ui| {
-        /*
-        let fsw = full slider_width
-        let ssw = small slider_width
-        let space = item_spacing.x
-        let value = interact_size.x;
-
-        fsw + space + value = ssw + space + value + space + ssw + space + value
-        fsw + space + value = 2 * ssw + 3 * space + 2 * value
-        fsw + space - value = 2 * ssw + 3 * space
-        fsw - 2 * space - value = 2 * ssw
-        ssw = fsw / 2 - space - value / 2
-        */
-        // let spacing = &ui.spacing();
-        // let space = spacing.item_spacing.x;
-        // let value_w = spacing.interact_size.x;
-        // let full_slider_width = spacing.slider_width;
-        // let small_slider_width = full_slider_width / 2.0 - space - value_w / 2.0;
-        // ui.spacing_mut().slider_width = small_slider_width;
-
             ui.add(Slider::f32(&mut value.x, range.clone()).text("w"));
             ui.add(Slider::f32(&mut value.y, range.clone()).text("h"));
             ui.label(text);
         })
         .response
+    }
 }
 
 fn ui_color(ui: &mut Ui, srgba: &mut Color32, text: &str) {

egui/src/widgets/mod.rs

@@ -35,6 +35,31 @@ pub trait Widget {
     fn ui(self, ui: &mut Ui) -> Response;
 }
 
+/// This enables functions that return `impl Widget`, so that you can
+/// create a widget by just returning a lambda from a function.
+///
+/// For instance: `ui.add(slider_vec2(&mut vec2));` with:
+///
+/// ```
+/// pub fn slider_vec2(value: &mut egui::Vec2) -> impl egui::Widget + '_ {
+///    move |ui: &mut egui::Ui| {
+///        ui.horizontal(|ui| {
+///            ui.add(egui::Slider::f32(&mut value.x, 0.0..=1.0).text("x"));
+///            ui.add(egui::Slider::f32(&mut value.y, 0.0..=1.0).text("y"));
+///        })
+///        .response
+///    }
+/// }
+/// ```
+impl<F> Widget for F
+where
+    F: FnOnce(&mut Ui) -> Response,
+{
+    fn ui(self, ui: &mut Ui) -> Response {
+        self(ui)
+    }
+}
+
 // ----------------------------------------------------------------------------
 
 /// Show a button to reset a value to its default.

egui_demo_lib/src/apps/demo/toggle_switch.rs

@@ -9,7 +9,12 @@
 ///    |       |.......|
 ///     \_______\_____/
 /// ```
-pub fn toggle(ui: &mut egui::Ui, on: &mut bool) -> egui::Response {
+///
+/// ## Example:
+/// ``` ignore
+/// toggle_ui(ui, &mut my_bool);
+/// ```
+pub fn toggle_ui(ui: &mut egui::Ui, on: &mut bool) -> egui::Response {
     // Widget code can be broken up in four steps:
     //  1. Decide a size for the widget
     //  2. Allocate space for it
@@ -58,7 +63,7 @@ pub fn toggle(ui: &mut egui::Ui, on: &mut bool) -> egui::Response {
 
 /// Here is the same code again, but a bit more compact:
 #[allow(dead_code)]
-fn toggle_compact(ui: &mut egui::Ui, on: &mut bool) -> egui::Response {
+fn toggle_ui_compact(ui: &mut egui::Ui, on: &mut bool) -> egui::Response {
     let desired_size = ui.spacing().interact_size.y * egui::vec2(2.0, 1.0);
     let (rect, response) = ui.allocate_exact_size(desired_size, egui::Sense::click());
     *on ^= response.clicked(); // toggle if clicked
@@ -77,14 +82,13 @@ fn toggle_compact(ui: &mut egui::Ui, on: &mut bool) -> egui::Response {
     response
 }
 
-pub fn demo(ui: &mut egui::Ui, on: &mut bool) {
-    ui.horizontal_wrapped_for_text(egui::TextStyle::Button, |ui| {
-        toggle(ui, on).on_hover_text("Click to toggle");
-        ui.add(crate::__egui_github_link_file!());
-    })
-    .response
-    .on_hover_text(
-        "It's easy to create your own widgets!\n\
-        This toggle switch is just one function and 15 lines of code.",
-    );
+// A wrapper that allows the more idiomatic usage pattern: `ui.add(toggle(&mut my_bool))`
+/// iOS-style toggle switch.
+///
+/// ## Example:
+/// ``` ignore
+/// ui.add(toggle(&mut my_bool));
+/// ```
+pub fn toggle(on: &mut bool) -> impl egui::Widget + '_ {
+    move |ui: &mut egui::Ui| toggle_ui(ui, on)
 }

egui_demo_lib/src/apps/demo/widget_gallery.rs

@@ -156,7 +156,10 @@ impl super::View for WidgetGallery {
             ui.end_row();
 
             ui.label("Custom widget:");
-            super::toggle_switch::demo(ui, boolean);
+            ui.add(super::toggle_switch::toggle(boolean)).on_hover_text(
+                "It's easy to create your own widgets!\n\
+                This toggle switch is just 15 lines of code.",
+            );
             ui.end_row();
 
             ui.label("Plot:");