Simplify commands a bit (#268)

src/demo/level.rs

@@ -10,17 +10,11 @@ pub(super) fn plugin(_app: &mut App) {
     // later if needed.
 }
 
-#[derive(Debug)]
-pub struct SpawnLevel;
-
-impl Command for SpawnLevel {
-    fn apply(self, world: &mut World) {
-        // The only thing we have in our level is a player,
-        // but add things like walls etc. here.
-        world.commands().add(SpawnPlayer { max_speed: 400.0 });
-
-        // Flush the commands we just added so that they are
-        // all executed now, as part of this command.
-        world.flush_commands();
-    }
+/// A [`Command`] to spawn the level.
+/// Functions that accept only `&mut World` as their parameter implement [`Command`].
+/// We use this style when a command requires no configuration.
+pub fn spawn_level(world: &mut World) {
+    // The only thing we have in our level is a player,
+    // but add things like walls etc. here.
+    SpawnPlayer { max_speed: 400.0 }.apply(world);
 }

src/screens/playing.rs

@@ -3,7 +3,9 @@
 use bevy::{input::common_conditions::input_just_pressed, prelude::*};
 
 use super::Screen;
-use crate::{asset_tracking::LoadResource, audio::Music, demo::level::SpawnLevel};
+use crate::{
+    asset_tracking::LoadResource, audio::Music, demo::level::spawn_level as spawn_level_command,
+};
 
 pub(super) fn plugin(app: &mut App) {
     app.load_resource::<GameplayMusic>();
@@ -35,7 +37,7 @@ impl FromWorld for GameplayMusic {
 }
 
 fn spawn_level(mut commands: Commands, mut music: ResMut<GameplayMusic>) {
-    commands.add(SpawnLevel);
+    commands.add(spawn_level_command);
     music.entity = Some(
         commands
             .spawn((