Fixed panicking on and added Observed signal.

Author: mintlu8

examples/ui.rs

@@ -103,7 +103,7 @@ fn setup(mut commands: Commands) {
                 ))
                 .spawn_task(move |entity| async move {
                     let btn = fetch!(#btn_entity);
-                    let mut stream = btn.on::<Pointer<Click>>();
+                    let mut stream = btn.on::<Pointer<Click>>()?;
                     while let Some(item) = stream.next().await {
                         let s =
                             format!("Clicked at {}", item.hit.position.unwrap_or_default().xz());
@@ -115,7 +115,7 @@ fn setup(mut commands: Commands) {
                 })
                 .spawn_task(move |entity| async move {
                     let btn = fetch!(#btn_entity);
-                    let mut stream = btn.on::<Pointer<Pressed>>();
+                    let mut stream = btn.on::<Pointer<Pressed>>()?;
                     while let Some(item) = stream.next().await {
                         let s = format!(
                             "Mouse down at {}",
@@ -129,7 +129,7 @@ fn setup(mut commands: Commands) {
                 })
                 .spawn_task(move |entity| async move {
                     let btn = fetch!(#btn_entity);
-                    let mut stream = btn.on::<Pointer<Over>>();
+                    let mut stream = btn.on::<Pointer<Over>>()?;
                     while let Some(item) = stream.next().await {
                         let s = format!(
                             "Hover entered at {}",
@@ -143,7 +143,7 @@ fn setup(mut commands: Commands) {
                 })
                 .spawn_task(move |entity| async move {
                     let btn = fetch!(#btn_entity);
-                    let mut stream = btn.on::<Pointer<Out>>();
+                    let mut stream = btn.on::<Pointer<Out>>()?;
                     while let Some(item) = stream.next().await {
                         let s = format!(
                             "Hover exited at {}",

src/entity_commands.rs

@@ -1,5 +1,6 @@
 use crate::access::AsyncWorld;
 use crate::executor::{with_world_mut, with_world_ref};
+use crate::signals::Observed;
 use crate::{access::AsyncEntityMut, AccessError, AccessResult};
 use crate::{AsyncAccess, InspectEntity, OwnedQueryState};
 use bevy::ecs::bundle::BundleFromComponents;
@@ -344,17 +345,37 @@ impl AsyncEntityMut {
     /// # Note
     ///
     /// This function spawns an observer.
-    pub fn on<T: Event + Clone>(&self) -> impl Stream<Item = T> + 'static {
+    pub fn on<T: Event + Clone>(&self) -> AccessResult<impl Stream<Item = T> + 'static> {
         let entity = self.id();
         let (sender, receiver) = flume::unbounded();
+        AsyncWorld.run(|world| {
+            world
+                .get_entity_mut(entity)
+                .map(|mut entity| {
+                    entity.observe(move |trigger: Trigger<T>| {
+                        let _ = sender.send(trigger.event().clone());
+                    });
+                })
+                .map_err(|_| AccessError::EntityNotFound(entity))
+        })?;
+        Ok(receiver.into_stream())
+    }
+
+    /// Initialize a signal receiver [`Observed<T>`] on this entity
+    /// and spawn an observer that feeds into that signal receiver.
+    ///
+    /// Call [`AsyncEntityMut::signal_receiver`] to read from that signal.
+    pub fn signal_observe<T: Event + Clone>(&self) -> AccessResult {
+        let entity = self.id();
+        let signal = self.signal_receiver::<Observed<T>>()?;
         AsyncWorld.run(|world| {
             world
                 .entity_mut(entity)
                 .observe(move |trigger: Trigger<T>| {
-                    let _ = sender.send(trigger.event().clone());
+                    signal.write(trigger.event().clone());
                 });
         });
-        receiver.into_stream()
+        Ok(())
     }
 
     /// Returns a future that yields when the entity is despawned.

src/signals/signal_utils.rs

@@ -44,6 +44,13 @@ impl<T: Clone + Default + Send + Sync + 'static> SignalId for Fac<T> {
     type Data = T;
 }
 
+/// Standard [`SignalId`] for observed trigger events.
+pub struct Observed<T: Clone + Event>(PhantomData<T>, std::convert::Infallible);
+
+impl<T: Clone + Event> SignalId for Observed<T> {
+    type Data = T;
+}
+
 mod sealed {
     use std::marker::PhantomData;
 
@@ -135,4 +142,5 @@ mod sealed {
     }
 }
 
+use bevy::ecs::event::Event;
 pub use sealed::{SignalReceiver, SignalSender};