Experiment with styling via extension traits

examples/hello_world_simple/src/main.rs

@@ -2,6 +2,10 @@
 #![allow(rustdoc::missing_crate_level_docs)] // it's an example
 
 use eframe::egui;
+use eframe::egui::style::{WidgetVisuals, Widgets};
+use eframe::egui::util::IdTypeMap;
+use eframe::egui::{Button, Id, Response};
+use std::sync::Arc;
 
 fn main() -> eframe::Result {
     env_logger::init(); // Log to stderr (if you run with `RUST_LOG=debug`).
@@ -28,6 +32,156 @@ fn main() -> eframe::Result {
                 age += 1;
             }
             ui.label(format!("Hello '{name}', age {age}"));
+
+            ui.add(Button::new("Hi").primary().big());
+
+            let mut ui = ImprovedUi::new();
+            let mut ui = &mut ui;
+
+            ui.widget_styles::<Button>().primary();
         });
     })
 }
+
+struct ImprovedUi {
+    styles: Arc<IdTypeMap>,
+}
+
+impl ImprovedUi {
+    fn new() -> Self {
+        Self {
+            styles: Default::default(),
+        }
+    }
+
+    fn widget_styles<T>(&mut self) -> &mut Widgets {
+        Arc::make_mut(&mut self.styles).get_temp_mut_or_default(Id::NULL)
+    }
+
+    fn get_styles<T>(&self, response: &Response) -> WidgetVisuals {
+        // This could be improved if idtypemap had a get_temp fn that returns a reference
+        self.styles
+            .get_temp::<Widgets>(Id::NULL)
+            .unwrap_or_default()
+            .style(response)
+            .clone()
+    }
+}
+
+pub trait WidgetBuilder
+where
+    Self: Sized,
+{
+    fn styles_mut(&mut self) -> &mut Widgets;
+
+    /// We could add helper functions to customize widgets like requested here: <https://github.com/emilk/egui/pull/5203>
+    fn background_color(mut self, color: egui::Color32) -> Self {
+        self.styles_mut().inactive.bg_fill = color;
+        self
+    }
+}
+
+impl<T> WidgetBuilder for &mut T
+where
+    T: WidgetBuilder,
+{
+    fn styles_mut(&mut self) -> &mut Widgets {
+        (*self).styles_mut()
+    }
+}
+
+impl WidgetBuilder for Widgets {
+    fn styles_mut(&mut self) -> &mut Widgets {
+        self
+    }
+}
+
+impl<'a> WidgetBuilder for Button<'a> {
+    fn styles_mut(&mut self) -> &mut Widgets {
+        todo!()
+        // &mut self.visuals
+    }
+}
+
+// Now we could have "classes" as Extension Traits
+
+pub trait MyWidgetStyles: WidgetBuilder {
+    fn primary(mut self) -> Self {
+        self.styles_mut().inactive.bg_fill = egui::Color32::RED;
+        self
+    }
+}
+
+// Instead of implementing this for T we could also just implement it for e.g. Button<'a> to limit it to a specific widget
+impl<T> MyWidgetStyles for T where T: WidgetBuilder {}
+
+struct WidgetStyle<T> {
+    style: Widgets,
+    _marker: std::marker::PhantomData<T>,
+}
+
+/// Should be used like this
+/// ```rust
+/// make_styles! {
+///     MyStyle {
+///         .primary {
+///             :hover, :active {
+///                 background_color: Color32::RED,
+///             }
+///         }
+///     }
+/// }
+/// ```
+macro_rules! make_styles {
+    (
+        $name:ident {
+            $(.$style:ident {
+                $($(:$pseudo:ident $(,)?)* {
+                    $($prop:ident: $value:expr,)*
+                })*
+            })*
+        }
+    ) => {
+        pub trait $name: WidgetBuilder {
+            $(
+                fn $style(mut self) -> Self {
+                    $(
+                        {
+                            let c = |pseudo: &mut WidgetVisuals| {
+                                *pseudo = WidgetVisuals {
+                                    $($prop: $value,)*
+                                    ..*pseudo
+                                };
+                            };
+
+                            $(
+                                c(&mut self.styles_mut().$pseudo);
+                            )*
+                        }
+                    )*
+                    self
+                }
+            )*
+        }
+    };
+}
+
+make_styles! {
+    MyStyleExt {
+        .primary {
+            :hovered {
+                bg_fill: egui::Color32::RED,
+                expansion: 1.1,
+            }
+            :inactive, :active {
+                bg_fill: egui::Color32::GREEN,
+            }
+        }
+
+        .secondary {
+            :hovered {
+
+            }
+        }
+    }
+}