Grid layout and widget gallery (#106)

* Wrap Layout and Region into a new struct Placer

* [egui] Add a simple grid layout

* Refactor CollapsingHeader code (simplify header painting)

* Fix: allow putting a CollapsingHeader inside of a grid layout

* [demo] Add a widget gallery

Closes https://github.com/emilk/egui/issues/88

* Add optional striped grid background

egui_demo_lib/src/apps/demo/demo_windows.rs

@@ -13,6 +13,7 @@ struct Demos {
 impl Default for Demos {
     fn default() -> Self {
         let demos: Vec<Box<dyn super::Demo>> = vec![
+            Box::new(super::WidgetGallery::default()),
             Box::new(super::FontBook::default()),
             Box::new(super::Painting::default()),
             Box::new(super::DancingStrings::default()),

egui_demo_lib/src/apps/demo/mod.rs

@@ -17,13 +17,14 @@ mod scrolls;
 mod sliders;
 mod tests;
 pub mod toggle_switch;
+mod widget_gallery;
 mod widgets;
 mod window_options;
 
 pub use {
     app::*, dancing_strings::DancingStrings, demo_window::DemoWindow, demo_windows::*,
     drag_and_drop::*, font_book::FontBook, painting::Painting, scrolls::Scrolls, sliders::Sliders,
-    tests::Tests, widgets::Widgets, window_options::WindowOptions,
+    tests::Tests, widget_gallery::*, widgets::Widgets, window_options::WindowOptions,
 };
 
 // ----------------------------------------------------------------------------

egui_demo_lib/src/apps/demo/painting.rs

@@ -75,7 +75,7 @@ impl super::Demo for Painting {
 
 impl super::View for Painting {
     fn ui(&mut self, ui: &mut Ui) {
-        ui.add(crate::__egui_github_link_file!("(source code)"));
+        ui.add(crate::__egui_github_link_file!());
         self.ui_control(ui);
         ui.label("Paint with your mouse/touch!");
         Frame::dark_canvas(ui.style()).show(ui, |ui| {

egui_demo_lib/src/apps/demo/widget_gallery.rs

@@ -0,0 +1,117 @@
+#[derive(Debug, PartialEq)]
+#[cfg_attr(feature = "persistence", derive(serde::Deserialize, serde::Serialize))]
+enum Enum {
+    First,
+    Second,
+    Third,
+}
+
+#[cfg_attr(feature = "persistence", derive(serde::Deserialize, serde::Serialize))]
+pub struct WidgetGallery {
+    boolean: bool,
+    radio: Enum,
+    scalar: f32,
+    string: String,
+    color: egui::Color32,
+}
+
+impl Default for WidgetGallery {
+    fn default() -> Self {
+        Self {
+            boolean: false,
+            radio: Enum::First,
+            scalar: 42.0,
+            string: "Hello World!".to_owned(),
+            color: egui::Color32::LIGHT_BLUE,
+        }
+    }
+}
+
+impl super::Demo for WidgetGallery {
+    fn name(&self) -> &str {
+        "🗄 Widget Gallery"
+    }
+
+    fn show(&mut self, ctx: &egui::CtxRef, open: &mut bool) {
+        egui::Window::new(self.name()).open(open).show(ctx, |ui| {
+            use super::View;
+            self.ui(ui);
+        });
+    }
+}
+
+impl super::View for WidgetGallery {
+    fn ui(&mut self, ui: &mut egui::Ui) {
+        let Self {
+            boolean,
+            radio,
+            scalar,
+            string,
+            color,
+        } = self;
+
+        egui::Grid::new("my_grid").striped(true).show(ui, |ui| {
+            ui.label("Label:");
+            ui.label("Welcome to the widget gallery!");
+            ui.end_row();
+
+            ui.label("Text Input:");
+            ui.text_edit_singleline(string);
+            ui.end_row();
+
+            ui.label("Checkbox:");
+            ui.checkbox(boolean, "Checkbox");
+            ui.end_row();
+
+            ui.label("Radio buttons:");
+            ui.horizontal(|ui| {
+                ui.radio_value(radio, Enum::First, "First");
+                ui.radio_value(radio, Enum::Second, "Second");
+                ui.radio_value(radio, Enum::Third, "Third");
+            });
+            ui.end_row();
+
+            ui.label("ComboBox:");
+            egui::combo_box_with_label(ui, "Take your pick", format!("{:?}", radio), |ui| {
+                ui.selectable_value(radio, Enum::First, "First");
+                ui.selectable_value(radio, Enum::Second, "Second");
+                ui.selectable_value(radio, Enum::Third, "Third");
+            });
+            ui.end_row();
+
+            ui.label("Slider:");
+            ui.add(egui::Slider::f32(scalar, 0.0..=100.0).text("value"));
+            ui.end_row();
+
+            ui.label("DragValue:");
+            ui.add(egui::DragValue::f32(scalar).speed(1.0));
+            ui.end_row();
+
+            ui.label("Color picker:");
+            ui.color_edit_button_srgba(color);
+            ui.end_row();
+
+            ui.label("Image:");
+            ui.image(egui::TextureId::Egui, [24.0, 16.0])
+                .on_hover_text("The font texture");
+            ui.end_row();
+
+            ui.label("Button:");
+            if ui.button("Toggle boolean").clicked {
+                *boolean = !*boolean;
+            }
+            ui.end_row();
+
+            ui.label("ImageButton:");
+            if ui
+                .add(egui::ImageButton::new(egui::TextureId::Egui, [24.0, 16.0]))
+                .clicked
+            {
+                *boolean = !*boolean;
+            }
+            ui.end_row();
+        });
+
+        ui.add(crate::__egui_github_link_file!());
+    }
+}