Rename App/World::observe to add_observer, `EntityWorldMut::obser…

…ve_entity` to `observe`. (bevyengine#15754)

# Objective

- Closes bevyengine#15752

Calling the functions `App::observe` and `World::observe` doesn't make
sense because you're not "observing" the `App` or `World`, you're adding
an observer that listens for an event that occurs *within* the `World`.
We should rename them to better fit this.

## Solution

Renames:
- `App::observe` -> `App::add_observer`
- `World::observe` -> `World::add_observer`
- `Commands::observe` -> `Commands::add_observer`
- `EntityWorldMut::observe_entity` -> `EntityWorldMut::observe`

(Note this isn't a breaking change as the original rename was introduced
earlier this cycle.)

## Testing

Reusing current tests.

benches/benches/bevy_ecs/observers/propagation.rs

@@ -106,15 +106,15 @@ fn add_listeners_to_hierarchy<const DENSITY: usize, const N: usize>(
     world: &mut World,
 ) {
     for e in roots.iter() {
-        world.entity_mut(*e).observe_entity(empty_listener::<N>);
+        world.entity_mut(*e).observe(empty_listener::<N>);
     }
     for e in leaves.iter() {
-        world.entity_mut(*e).observe_entity(empty_listener::<N>);
+        world.entity_mut(*e).observe(empty_listener::<N>);
     }
     let mut rng = deterministic_rand();
     for e in nodes.iter() {
         if rng.gen_bool(DENSITY as f64 / 100.0) {
-            world.entity_mut(*e).observe_entity(empty_listener::<N>);
+            world.entity_mut(*e).observe(empty_listener::<N>);
         }
     }
 }

benches/benches/bevy_ecs/observers/simple.rs

@@ -17,7 +17,7 @@ pub fn observe_simple(criterion: &mut Criterion) {
 
     group.bench_function("trigger_simple", |bencher| {
         let mut world = World::new();
-        world.observe(empty_listener_base);
+        world.add_observer(empty_listener_base);
         bencher.iter(|| {
             for _ in 0..10000 {
                 world.trigger(EventBase)
@@ -29,7 +29,7 @@ pub fn observe_simple(criterion: &mut Criterion) {
         let mut world = World::new();
         let mut entities = vec![];
         for _ in 0..10000 {
-            entities.push(world.spawn_empty().observe_entity(empty_listener_base).id());
+            entities.push(world.spawn_empty().observe(empty_listener_base).id());
         }
         entities.shuffle(&mut deterministic_rand());
         bencher.iter(|| {

crates/bevy_app/src/app.rs

@@ -1267,19 +1267,19 @@ impl App {
     /// # struct Friend;
     /// #
     /// // An observer system can be any system where the first parameter is a trigger
-    /// app.observe(|trigger: Trigger<Party>, friends: Query<Entity, With<Friend>>, mut commands: Commands| {
+    /// app.add_observer(|trigger: Trigger<Party>, friends: Query<Entity, With<Friend>>, mut commands: Commands| {
     ///     if trigger.event().friends_allowed {
     ///         for friend in friends.iter() {
     ///             commands.trigger_targets(Invite, friend);
     ///         }
     ///     }
     /// });
     /// ```
-    pub fn observe<E: Event, B: Bundle, M>(
+    pub fn add_observer<E: Event, B: Bundle, M>(
         &mut self,
         observer: impl IntoObserverSystem<E, B, M>,
     ) -> &mut Self {
-        self.world_mut().observe(observer);
+        self.world_mut().add_observer(observer);
         self
     }
 }

crates/bevy_dev_tools/src/ci_testing/systems.rs

@@ -25,7 +25,7 @@ pub(crate) fn send_events(world: &mut World, mut current_frame: Local<u32>) {
                 let path = format!("./screenshot-{}.png", *current_frame);
                 world
                     .spawn(Screenshot::primary_window())
-                    .observe_entity(save_to_disk(path));
+                    .observe(save_to_disk(path));
                 info!("Took a screenshot at frame {}.", *current_frame);
             }
             // Custom events are forwarded to the world.

crates/bevy_ecs/README.md

@@ -315,7 +315,7 @@ struct MyEvent {
 
 let mut world = World::new();
 
-world.observe(|trigger: Trigger<MyEvent>| {
+world.add_observer(|trigger: Trigger<MyEvent>| {
     println!("{}", trigger.event().message);
 });
 
@@ -339,7 +339,7 @@ struct Explode;
 let mut world = World::new();
 let entity = world.spawn_empty().id();
 
-world.observe(|trigger: Trigger<Explode>, mut commands: Commands| {
+world.add_observer(|trigger: Trigger<Explode>, mut commands: Commands| {
     println!("Entity {:?} goes BOOM!", trigger.entity());
     commands.entity(trigger.entity()).despawn();
 });