New bevy_settings crate

Cargo.toml

@@ -407,6 +407,9 @@ bevy_input_focus = ["bevy_internal/bevy_input_focus"]
 # Headless widget collection for Bevy UI.
 bevy_ui_widgets = ["bevy_internal/bevy_ui_widgets"]
 
+# Load and save user preferences
+bevy_settings = ["bevy_internal/bevy_settings"]
+
 # Feathers widget collection.
 experimental_bevy_feathers = ["bevy_internal/bevy_feathers", "bevy_ui_widgets"]
 
@@ -5201,6 +5204,30 @@ description = "Demonstrates use of core scrollbar in Bevy UI"
 category = "UI (User Interface)"
 wasm = true
 
+[[example]]
+name = "persisting_preferences"
+path = "examples/app/persisting_preferences.rs"
+doc-scrape-examples = true
+required-features = ["bevy_settings"]
+
+[package.metadata.example.persisting_preferences]
+name = "User Preferences"
+description = "Demonstrates persistence of user preferences"
+category = "Application"
+wasm = true
+
+[[example]]
+name = "persisting_window_settings"
+path = "examples/window/persisting_window_settings.rs"
+doc-scrape-examples = true
+required-features = ["bevy_settings"]
+
+[package.metadata.example.persisting_window_settings]
+name = "Save Window Position"
+description = "Demonstrates saving window position in preferences"
+category = "Application"
+wasm = false
+
 [[example]]
 name = "system_fonts"
 path = "examples/ui/text/system_fonts.rs"

crates/bevy_ecs/macros/src/lib.rs

@@ -15,7 +15,9 @@ use crate::{
     component::map_entities, query_data::derive_query_data_impl,
     query_filter::derive_query_filter_impl,
 };
-use bevy_macro_utils::{derive_label, ensure_no_collision, get_struct_fields, BevyManifest};
+use bevy_macro_utils::{
+    derive_label, ensure_no_collision, get_struct_fields, pascal_to_snake_case, BevyManifest,
+};
 use proc_macro::TokenStream;
 use proc_macro2::{Ident, Span};
 use quote::{format_ident, quote, ToTokens};
@@ -567,6 +569,62 @@ pub fn derive_resource(input: TokenStream) -> TokenStream {
     component::derive_resource(input)
 }
 
+/// Implement the `SettingsGroup` trait.
+#[proc_macro_derive(SettingsGroup, attributes(settings_group))]
+pub fn derive_settings_group(input: TokenStream) -> TokenStream {
+    let input = parse_macro_input!(input as DeriveInput);
+
+    let name = &input.ident;
+
+    let (override_name, override_file) = {
+        let mut override_name: Option<String> = None;
+        let mut override_file: Option<String> = None;
+
+        input
+            .attrs
+            .iter()
+            .find(|attr| attr.path().is_ident("settings_group"))
+            .and_then(|attr| {
+                attr.parse_nested_meta(|meta| {
+                    if meta.path.is_ident("group") {
+                        let value = meta.value()?;
+                        let s: syn::LitStr = value.parse()?;
+                        override_name = Some(s.value());
+                        Ok(())
+                    } else if meta.path.is_ident("file") {
+                        let value = meta.value()?;
+                        let s: syn::LitStr = value.parse()?;
+                        override_file = Some(s.value());
+                        Ok(())
+                    } else {
+                        Err(meta.error("unsupported attribute"))
+                    }
+                })
+                .ok()
+            });
+
+        (override_name, override_file)
+    };
+
+    let group_name = override_name.unwrap_or(pascal_to_snake_case(&name.to_string()));
+    let file_name = override_file
+        .map(|f| quote! { Some(#f) })
+        .unwrap_or(quote! { None });
+
+    let expanded = quote! {
+        impl SettingsGroup for #name {
+            fn settings_group_name() -> &'static str {
+                #group_name
+            }
+            fn settings_source() -> Option<&'static str> {
+                #file_name
+            }
+        }
+    };
+
+    TokenStream::from(expanded)
+}
+
 /// Cheat sheet for derive syntax,
 /// see full explanation and examples on the `Component` trait doc.
 ///

crates/bevy_internal/Cargo.toml

@@ -528,6 +528,7 @@ bevy_input_focus = { path = "../bevy_input_focus", optional = true, version = "0
 ] }
 bevy_pbr = { path = "../bevy_pbr", optional = true, version = "0.19.0-dev" }
 bevy_picking = { path = "../bevy_picking", optional = true, version = "0.19.0-dev" }
+bevy_settings = { path = "../bevy_settings", optional = true, version = "0.19.0-dev" }
 bevy_remote = { path = "../bevy_remote", optional = true, version = "0.19.0-dev" }
 bevy_render = { path = "../bevy_render", optional = true, version = "0.19.0-dev" }
 bevy_scene = { path = "../bevy_scene", optional = true, version = "0.19.0-dev" }

crates/bevy_internal/src/lib.rs

@@ -78,6 +78,8 @@ pub use bevy_remote as remote;
 pub use bevy_render as render;
 #[cfg(feature = "bevy_scene")]
 pub use bevy_scene as scene;
+#[cfg(feature = "bevy_settings")]
+pub use bevy_settings as settings;
 #[cfg(feature = "bevy_shader")]
 pub use bevy_shader as shader;
 #[cfg(feature = "bevy_solari")]

crates/bevy_macro_utils/src/label.rs

@@ -93,3 +93,36 @@ pub fn derive_label(
     }
     .into()
 }
+
+/// Convert a string from ``PascalCase`` to ``snake_case``.
+pub fn pascal_to_snake_case(s: &str) -> String {
+    let mut out = String::new();
+    let chars: Vec<char> = s.chars().collect();
+
+    for (i, &ch) in chars.iter().enumerate() {
+        if ch.is_uppercase() {
+            let prev_is_lower = i > 0 && chars[i - 1].is_lowercase();
+            let next_is_lower = chars.get(i + 1).is_some_and(|c| c.is_lowercase());
+
+            if i > 0 && (prev_is_lower || next_is_lower) {
+                out.push('_');
+            }
+            out.push(ch.to_lowercase().next().unwrap());
+        } else {
+            out.push(ch);
+        }
+    }
+
+    out
+}
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_pascal_to_snake_case() {
+        assert_eq!(pascal_to_snake_case("PascalCase"), "pascal_case");
+        assert_eq!(pascal_to_snake_case("lowercase"), "lowercase");
+        assert_eq!(pascal_to_snake_case("HTTPServer"), "http_server");
+    }
+}

crates/bevy_settings/Cargo.toml

@@ -0,0 +1,40 @@
+[package]
+name = "bevy_settings"
+version = "0.19.0-dev"
+edition = "2024"
+description = "User settings framework for Bevy Engine"
+homepage = "https://bevy.org"
+repository = "https://github.com/bevyengine/bevy"
+license = "MIT OR Apache-2.0"
+keywords = ["bevy"]
+
+[dependencies]
+# bevy
+bevy_app = { path = "../bevy_app", version = "0.19.0-dev" }
+bevy_ecs = { path = "../bevy_ecs", version = "0.19.0-dev" }
+bevy_ecs_macros = { path = "../bevy_ecs/macros", version = "0.19.0-dev" }
+bevy_log = { path = "../bevy_log", version = "0.19.0-dev" }
+bevy_platform = { path = "../bevy_platform", version = "0.19.0-dev" }
+bevy_reflect = { path = "../bevy_reflect", version = "0.19.0-dev" }
+bevy_tasks = { path = "../bevy_tasks", version = "0.19.0-dev" }
+bevy_time = { path = "../bevy_time", version = "0.19.0-dev" }
+
+serde = "1.0.217"
+thiserror = "2.0.18"
+toml = { version = "0.8.19" }
+
+[target.'cfg(target_arch = "wasm32")'.dependencies]
+web-sys = { version = "0.3", default-features = false, features = [
+  "Window",
+  "Storage",
+] }
+
+[features]
+default = []
+
+[lints]
+workspace = true
+
+[package.metadata.docs.rs]
+rustdoc-args = ["-Zunstable-options", "--generate-link-to-definition"]
+all-features = true