Rework access to ThemeRegistry global (#7023)

This PR reworks how we access the `ThemeRegistry` global.

Previously we were making calls directly on the context, like
`cx.global::<ThemeRegistry>`. However, one problem with this is that it
spreads out the knowledge of exactly what type is stored in the global.

In order to make it easier to adjust the type we store in the context
(e.g., wrapping the `ThemeRegistry` in an `Arc`), we now call methods on
the `ThemeRegistry` itself for accessing the global.

It would also be interesting to see how we could prevent access to the
`ThemeRegistry` without going through one of these dedicated methods 🤔

Release Notes:

- N/A

crates/theme/src/registry.rs

@@ -2,7 +2,7 @@ use std::collections::HashMap;
 use std::sync::Arc;
 
 use anyhow::{anyhow, Context, Result};
-use gpui::{AssetSource, HighlightStyle, SharedString};
+use gpui::{AppContext, AssetSource, HighlightStyle, SharedString};
 use refineable::Refineable;
 use util::ResultExt;
 
@@ -24,6 +24,23 @@ pub struct ThemeRegistry {
 }
 
 impl ThemeRegistry {
+    /// Returns the global [`ThemeRegistry`].
+    pub fn global(cx: &AppContext) -> &Self {
+        cx.global::<ThemeRegistry>()
+    }
+
+    /// Returns a mutable reference to the global [`ThemeRegistry`].
+    pub fn global_mut(cx: &mut AppContext) -> &mut Self {
+        cx.global_mut::<ThemeRegistry>()
+    }
+
+    /// Returns a mutable reference to the global [`ThemeRegistry`].
+    ///
+    /// Inserts a default [`ThemeRegistry`] if one does not yet exist.
+    pub fn default_global(cx: &mut AppContext) -> &mut Self {
+        cx.default_global::<ThemeRegistry>()
+    }
+
     pub fn new(assets: Box<dyn AssetSource>) -> Self {
         let mut registry = Self {
             assets,

crates/theme/src/settings.rs

@@ -164,7 +164,7 @@ impl settings::Settings for ThemeSettings {
         user_values: &[&Self::FileContent],
         cx: &mut AppContext,
     ) -> Result<Self> {
-        let themes = cx.default_global::<ThemeRegistry>();
+        let themes = ThemeRegistry::default_global(cx);
 
         let mut this = Self {
             ui_font_size: defaults.ui_font_size.unwrap().into(),
@@ -230,8 +230,7 @@ impl settings::Settings for ThemeSettings {
         cx: &AppContext,
     ) -> schemars::schema::RootSchema {
         let mut root_schema = generator.root_schema_for::<ThemeSettingsContent>();
-        let theme_names = cx
-            .global::<ThemeRegistry>()
+        let theme_names = ThemeRegistry::global(cx)
             .list_names(params.staff_mode)
             .map(|theme_name| Value::String(theme_name.to_string()))
             .collect();

crates/theme/src/theme.rs

@@ -63,7 +63,7 @@ pub fn init(themes_to_load: LoadThemes, cx: &mut AppContext) {
     cx.set_global(ThemeRegistry::new(assets));
 
     if load_user_themes {
-        cx.global_mut::<ThemeRegistry>().load_user_themes();
+        ThemeRegistry::global_mut(cx).load_user_themes();
     }
 
     ThemeSettings::register(cx);