Simplify command application

src/audio/sound_effects.rs

@@ -61,6 +61,8 @@ struct PlaySoundEffect {
 
 impl Command for PlaySoundEffect {
     fn apply(self, world: &mut World) {
+        // If you need more complex behavior, use `world.run_system_once_with`,
+        // as demonstrated with `PlaySoundtrack`.
         world.resource_scope(|world, mut sound_effects: Mut<SoundEffectHandles>| {
             sound_effects.play(self.name, world, self.settings);
         });

src/audio/soundtrack.rs

@@ -1,4 +1,8 @@
-use bevy::{audio::PlaybackMode, ecs::world::Command, prelude::*};
+use bevy::{
+    audio::PlaybackMode,
+    ecs::{system::RunSystemOnce as _, world::Command},
+    prelude::*,
+};
 
 use crate::assets::SoundtrackHandles;
 
@@ -22,29 +26,35 @@ impl Command for PlaySoundtrack {
     /// This command will despawn the current soundtrack, then spawn a new one
     /// if necessary.
     fn apply(self, world: &mut World) {
-        let mut soundtrack_query = world.query_filtered::<Entity, With<IsSoundtrack>>();
-        let soundtracks: Vec<_> = soundtrack_query.iter(world).collect();
-        for entity in soundtracks {
-            world.entity_mut(entity).despawn_recursive();
-        }
+        world.run_system_once_with(self, apply_play_soundtrack);
+    }
+}
 
-        let soundtrack_key = match &self {
-            PlaySoundtrack::Key(key) => key.clone(),
-            PlaySoundtrack::Disable => return,
-        };
+fn apply_play_soundtrack(
+    In(play_soundtrack): In<PlaySoundtrack>,
+    mut commands: Commands,
+    soundtrack_query: Query<Entity, With<IsSoundtrack>>,
+    soundtrack_handles: Res<SoundtrackHandles>,
+) {
+    for entity in soundtrack_query.iter() {
+        commands.entity(entity).despawn_recursive();
+    }
 
-        let soundtrack_handles = world.resource::<SoundtrackHandles>();
-        world.spawn((
-            AudioSourceBundle {
-                source: soundtrack_handles[&soundtrack_key].clone_weak(),
-                settings: PlaybackSettings {
-                    mode: PlaybackMode::Loop,
-                    ..default()
-                },
+    let soundtrack_key = match play_soundtrack {
+        PlaySoundtrack::Key(key) => key,
+        PlaySoundtrack::Disable => return,
+    };
+
+    commands.spawn((
+        AudioSourceBundle {
+            source: soundtrack_handles[&soundtrack_key].clone_weak(),
+            settings: PlaybackSettings {
+                mode: PlaybackMode::Loop,
+                ..default()
             },
-            IsSoundtrack,
-        ));
-    }
+        },
+        IsSoundtrack,
+    ));
 }
 
 /// An extension trait with convenience methods for soundtrack commands.

src/theme/interaction.rs

@@ -37,10 +37,10 @@ pub struct InteractionPalette {
 pub struct OnPress(#[reflect(ignore)] pub SystemId);
 
 fn apply_on_press(
-    interactions: Query<(&Interaction, &OnPress), Changed<Interaction>>,
+    interaction_query: Query<(&Interaction, &OnPress), Changed<Interaction>>,
     mut commands: Commands,
 ) {
-    for (interaction, &OnPress(system_id)) in &interactions {
+    for (interaction, &OnPress(system_id)) in &interaction_query {
         if matches!(interaction, Interaction::Pressed) {
             commands.run_system(system_id);
         }
@@ -64,10 +64,10 @@ fn apply_interaction_palette(
 }
 
 fn trigger_interaction_sfx(
-    interactions: Query<&Interaction, Changed<Interaction>>,
+    interaction_query: Query<&Interaction, Changed<Interaction>>,
     mut commands: Commands,
 ) {
-    for interaction in &interactions {
+    for interaction in &interaction_query {
         match interaction {
             Interaction::Hovered => {
                 commands.play_sound_effect(SoundEffectHandles::KEY_BUTTON_HOVER)