Move where the default theme is registered

crates/egui/src/context.rs

@@ -372,7 +372,7 @@ impl ViewportRepaintInfo {
 // ----------------------------------------------------------------------------
 
 #[derive(Default)]
-struct ContextImpl {
+pub(crate) struct ContextImpl {
     fonts: Option<Fonts>,
     font_definitions: FontDefinitions,
 
@@ -414,7 +414,7 @@ struct ContextImpl {
 
     loaders: Arc<Loaders>,
 
-    themes: theme::Themes,
+    pub(crate) themes: theme::Themes,
 }
 
 impl ContextImpl {
@@ -753,6 +753,9 @@ impl Default for Context {
         ctx.add_plugin(crate::text_selection::LabelSelectionState::default());
         ctx.add_plugin(crate::DragAndDrop::default());
 
+        // Register the default theme for all built-in widgets:
+        theme::DefaultStyle::register(&ctx);
+
         ctx
     }
 }
@@ -764,7 +767,7 @@ impl Context {
     }
 
     /// Do read-write (exclusive access) transaction on Context
-    fn write<R>(&self, writer: impl FnOnce(&mut ContextImpl) -> R) -> R {
+    pub(crate) fn write<R>(&self, writer: impl FnOnce(&mut ContextImpl) -> R) -> R {
         writer(&mut self.0.write())
     }
 

crates/egui/src/theme/default_style.rs

@@ -2,7 +2,7 @@ use emath::Vec2;
 use epaint::Margin;
 
 use crate::{
-    Button, Frame, Style, TextStyle,
+    Button, Context, Frame, Style, TextStyle,
     style::WidgetVisuals,
     theme::StyleProvider,
     widget_style::{
@@ -16,6 +16,20 @@ use crate::{
 #[derive(Debug, Clone)]
 pub struct DefaultStyle;
 
+impl DefaultStyle {
+    /// Register `Self` as the [`StyleProvider`] of every built-in widget style.
+    ///
+    /// [`Context::default`] does this. Any theme you register yourself
+    /// replaces the default one for that widget style.
+    pub fn register(ctx: &Context) {
+        ctx.write(|ctx| {
+            ctx.themes.register::<ButtonStyle>(Self, false);
+            ctx.themes.register::<SeparatorStyle>(Self, false);
+            ctx.themes.register::<CheckboxStyle>(Self, false);
+        });
+    }
+}
+
 /// The text of a widget, based on the [`WidgetVisuals`] of its current state.
 fn text_visuals(style: &Style, widget_visuals: &WidgetVisuals) -> TextVisuals {
     TextVisuals {

crates/egui/src/theme/themes.rs

@@ -2,52 +2,23 @@ use std::sync::Arc;
 
 use epaint::mutex::Mutex;
 
-use crate::{
-    Id,
-    theme::{StyleProvider, default_style::DefaultStyle},
-    util::IdTypeMap,
-    widget_style::{ButtonStyle, CheckboxStyle, SeparatorStyle, WidgetStyle},
-};
+use crate::{Id, theme::StyleProvider, util::IdTypeMap, widget_style::WidgetStyle};
 
 /// The registry of [`StyleProvider`]s, one per [`WidgetStyle`] type.
 ///
 /// Each widget asks this registry for the provider of its style type
-/// (e.g. [`ButtonStyle`]), and that provider computes the final style from the
+/// (e.g. [`crate::widget_style::ButtonStyle`]), and that provider computes the final style from the
 /// widget's classes and state.
 ///
-/// A default provider is registered for every built-in style; register your
-/// own with [`Context::add_widget_theme`](crate::Context::add_widget_theme) or
-/// [`Context::replace_widget_theme`](crate::Context::replace_widget_theme).
+/// A default provider is registered for every built-in style. Register your
+/// own with [`crate::Context::add_widget_theme`] or [`crate::Context::replace_widget_theme`].
+#[derive(Default)]
 pub struct Themes {
     themes: IdTypeMap,
 }
 
 type ThemeWrap<S> = Arc<Mutex<Box<dyn StyleProvider<S> + Send + Sync>>>;
 
-impl Default for Themes {
-    /// Register the default egui theme
-    fn default() -> Self {
-        let mut themes = IdTypeMap::default();
-
-        themes.insert_temp::<ThemeWrap<ButtonStyle>>(
-            Id::NULL,
-            Arc::new(Mutex::new(Box::new(DefaultStyle))),
-        );
-
-        themes.insert_temp::<ThemeWrap<SeparatorStyle>>(
-            Id::NULL,
-            Arc::new(Mutex::new(Box::new(DefaultStyle))),
-        );
-
-        themes.insert_temp::<ThemeWrap<CheckboxStyle>>(
-            Id::NULL,
-            Arc::new(Mutex::new(Box::new(DefaultStyle))),
-        );
-
-        Self { themes }
-    }
-}
-
 impl Themes {
     /// Register a [`StyleProvider`] for the specified widget [`WidgetStyle`] `S`
     ///