Use custom commands instead of observers for spawning (#242)

Co-authored-by: Ben Frankel <[email protected]>

src/audio/soundtrack.rs

@@ -26,12 +26,12 @@ impl Command for PlaySoundtrack {
     /// This command will despawn the current soundtrack, then spawn a new one
     /// if necessary.
     fn apply(self, world: &mut World) {
-        world.run_system_once_with(self, apply_play_soundtrack);
+        world.run_system_once_with(self, play_soundtrack);
     }
 }
 
-fn apply_play_soundtrack(
-    In(play_soundtrack): In<PlaySoundtrack>,
+fn play_soundtrack(
+    In(config): In<PlaySoundtrack>,
     mut commands: Commands,
     soundtrack_query: Query<Entity, With<IsSoundtrack>>,
     soundtrack_handles: Res<SoundtrackHandles>,
@@ -40,7 +40,7 @@ fn apply_play_soundtrack(
         commands.entity(entity).despawn_recursive();
     }
 
-    let soundtrack_key = match play_soundtrack {
+    let soundtrack_key = match config {
         PlaySoundtrack::Key(key) => key,
         PlaySoundtrack::Disable => return,
     };

src/demo/level.rs

@@ -1,18 +1,26 @@
-//! Spawn the main level by triggering other observers.
+//! Spawn the main level.
 
-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 { 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();
+    }
 }

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 as _, 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 +27,25 @@ 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;
 
+/// A command to spawn the player character.
+#[derive(Debug)]
+pub struct SpawnPlayer {
+    /// See [`MovementController::max_speed`].
+    pub max_speed: f32,
+}
+
+impl Command for SpawnPlayer {
+    fn apply(self, world: &mut World) {
+        world.run_system_once_with(self, spawn_player);
+    }
+}
+
 fn spawn_player(
-    _trigger: Trigger<SpawnPlayer>,
+    In(config): In<SpawnPlayer>,
     mut commands: Commands,
     image_handles: Res<ImageHandles>,
     mut texture_atlas_layouts: ResMut<Assets<TextureAtlasLayout>>,
@@ -59,7 +71,10 @@ fn spawn_player(
             layout: texture_atlas_layout.clone(),
             index: player_animation.get_atlas_index(),
         },
-        MovementController::default(),
+        MovementController {
+            max_speed: config.max_speed,
+            ..default()
+        },
         ScreenWrap,
         player_animation,
         StateScoped(Screen::Playing),

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);
 }