Start example with multiple levels and asset unloading

bevy_asset_loader/assets/levels/desert.assets.ron

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+({
+    "image.background": File (
+        path: "levels/backgroundColorDesert.png",
+    )
+})

bevy_asset_loader/assets/levels/forest.assets.ron

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+({
+    "image.background": File (
+        path: "levels/backgroundColorForest.png",
+    )
+})

bevy_asset_loader/examples/levels.rs

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+use bevy::prelude::*;
+
+fn main() {
+    App::new()
+        .add_plugins(DefaultPlugins)
+        .init_state::<AppState>()
+        .add_systems(Startup, spawn_camera)
+        .add_systems(OnEnter(AppState::Menu), spawn_menu)
+        .add_systems(
+            Update,
+            listen_for_menu_buttons.run_if(in_state(AppState::Menu)),
+        )
+        .enable_state_scoped_entities::<AppState>()
+        .run();
+}
+
+#[derive(Debug, Clone, Copy, Default, Eq, PartialEq, Hash, States)]
+enum AppState {
+    #[default]
+    Menu,
+    Loading,
+    InGame,
+}
+
+#[derive(Debug, Clone, Copy, Default, Eq, PartialEq, Hash, SubStates)]
+#[source(AppState = AppState::InGame)]
+enum Level {
+    #[default]
+    Forest,
+    Desert,
+}
+
+#[derive(Component)]
+struct PrepareLevel(Level);
+
+fn spawn_menu(mut commands: Commands) {
+    commands
+        .spawn((
+            NodeBundle {
+                style: Style {
+                    width: Val::Percent(100.0),
+                    height: Val::Percent(100.0),
+                    flex_direction: FlexDirection::Column,
+                    align_items: AlignItems::Center,
+                    justify_content: JustifyContent::Center,
+                    row_gap: Val::Px(15.),
+                    ..default()
+                },
+                background_color: Color::BLACK.into(),
+                ..default()
+            },
+            StateScoped(AppState::Menu),
+        ))
+        .with_children(|parent| {
+            parent
+                .spawn((menu_button(), PrepareLevel(Level::Forest)))
+                .with_children(|children| {
+                    children.spawn(TextBundle::from_section("Forest", Default::default()));
+                });
+            parent
+                .spawn((menu_button(), PrepareLevel(Level::Desert)))
+                .with_children(|children| {
+                    children.spawn(TextBundle::from_section("Desert", Default::default()));
+                });
+        });
+}
+
+fn menu_button() -> impl Bundle {
+    ButtonBundle {
+        background_color: BackgroundColor(Color::srgb(0.5, 0.5, 0.5)),
+        style: Style {
+            align_items: AlignItems::Center,
+            justify_content: JustifyContent::Center,
+            width: Val::Px(200.),
+            height: Val::Px(40.),
+            ..default()
+        },
+        ..default()
+    }
+}
+
+fn listen_for_menu_buttons(
+    clicks: Query<(&Interaction, &PrepareLevel)>,
+    mut next_state: ResMut<NextState<AppState>>,
+) {
+    for (interaction, _prepare_level) in &clicks {
+        if matches!(interaction, Interaction::Pressed) {
+            next_state.set(AppState::Loading);
+        }
+    }
+}
+
+fn spawn_camera(mut commands: Commands) {
+    commands.spawn(Camera2dBundle::default());
+}