Use custom commands instead of observers for spawning

src/demo/level.rs

@@ -1,18 +1,22 @@
 //! Spawn the main level by triggering other observers.
 
-use bevy::prelude::*;
+use bevy::{ecs::world::Command, prelude::*};
 
 use super::player::SpawnPlayer;
 
-pub(super) fn plugin(app: &mut App) {
-    app.observe(spawn_level);
+pub(super) fn plugin(_app: &mut App) {
+    // No setup required for this plugin.
+    // It's still good to have a function here so that we can add some setup
+    // later if needed.
 }
 
-#[derive(Event, Debug)]
+#[derive(Debug)]
 pub struct SpawnLevel;
 
-fn spawn_level(_trigger: Trigger<SpawnLevel>, mut commands: Commands) {
-    // The only thing we have in our level is a player,
-    // but add things like walls etc. here.
-    commands.trigger(SpawnPlayer);
+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);
+    }
 }

src/demo/player.rs

@@ -2,7 +2,10 @@
 //! Note that this is separate from the `movement` module as that could be used
 //! for other characters as well.
 
-use bevy::prelude::*;
+use bevy::{
+    ecs::{system::RunSystemOnce, world::Command},
+    prelude::*,
+};
 
 use crate::{
     assets::ImageHandles,
@@ -15,7 +18,6 @@ use crate::{
 };
 
 pub(super) fn plugin(app: &mut App) {
-    app.observe(spawn_player);
     app.register_type::<Player>();
 
     // Record directional input as movement controls.
@@ -25,15 +27,22 @@ pub(super) fn plugin(app: &mut App) {
     );
 }
 
-#[derive(Event, Debug)]
-pub struct SpawnPlayer;
-
 #[derive(Component, Debug, Clone, Copy, PartialEq, Eq, Default, Reflect)]
 #[reflect(Component)]
 pub struct Player;
 
+/// Command to spawn the player character.
+/// We can add fields to this struct if we need data for spawning.
+#[derive(Debug)]
+pub struct SpawnPlayer;
+
+impl Command for SpawnPlayer {
+    fn apply(self, world: &mut World) {
+        world.run_system_once(spawn_player);
+    }
+}
+
 fn spawn_player(
-    _trigger: Trigger<SpawnPlayer>,
     mut commands: Commands,
     image_handles: Res<ImageHandles>,
     mut texture_atlas_layouts: ResMut<Assets<TextureAtlasLayout>>,

src/screens/playing.rs

@@ -19,7 +19,7 @@ pub(super) fn plugin(app: &mut App) {
 }
 
 fn spawn_level(mut commands: Commands) {
-    commands.trigger(SpawnLevel);
+    commands.add(SpawnLevel);
     commands.play_soundtrack(SoundtrackHandles::KEY_GAMEPLAY);
 }