Allow observer systems to have outputs (#14159)

# Objective

Fixes #14157

## Solution

- Update the ObserverSystem traits to accept an `Out` parameter

## Testing

- Added a test where an observer system has a non-empty output which is
piped into another system

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>

crates/bevy_ecs/src/system/observer_system.rs

@@ -10,31 +10,40 @@ use super::IntoSystem;
 /// Implemented for systems that have an [`Observer`] as the first argument.
 ///
 /// [`Observer`]: crate::observer::Observer
-pub trait ObserverSystem<E: 'static, B: Bundle>:
-    System<In = Trigger<'static, E, B>, Out = ()> + Send + 'static
+pub trait ObserverSystem<E: 'static, B: Bundle, Out = ()>:
+    System<In = Trigger<'static, E, B>, Out = Out> + Send + 'static
 {
 }
 
-impl<E: 'static, B: Bundle, T: System<In = Trigger<'static, E, B>, Out = ()> + Send + 'static>
-    ObserverSystem<E, B> for T
+impl<
+        E: 'static,
+        B: Bundle,
+        Out,
+        T: System<In = Trigger<'static, E, B>, Out = Out> + Send + 'static,
+    > ObserverSystem<E, B, Out> for T
 {
 }
 
 /// Implemented for systems that convert into [`ObserverSystem`].
-pub trait IntoObserverSystem<E: 'static, B: Bundle, M>: Send + 'static {
+pub trait IntoObserverSystem<E: 'static, B: Bundle, M, Out = ()>: Send + 'static {
     /// The type of [`System`] that this instance converts into.
-    type System: ObserverSystem<E, B>;
+    type System: ObserverSystem<E, B, Out>;
 
     /// Turns this value into its corresponding [`System`].
     fn into_system(this: Self) -> Self::System;
 }
 
-impl<S: IntoSystem<Trigger<'static, E, B>, (), M> + Send + 'static, M, E: 'static, B: Bundle>
-    IntoObserverSystem<E, B, M> for S
+impl<
+        S: IntoSystem<Trigger<'static, E, B>, Out, M> + Send + 'static,
+        M,
+        Out,
+        E: 'static,
+        B: Bundle,
+    > IntoObserverSystem<E, B, M, Out> for S
 where
-    S::System: ObserverSystem<E, B>,
+    S::System: ObserverSystem<E, B, Out>,
 {
-    type System = <S as IntoSystem<Trigger<'static, E, B>, (), M>>::System;
+    type System = <S as IntoSystem<Trigger<'static, E, B>, Out, M>>::System;
 
     fn into_system(this: Self) -> Self::System {
         IntoSystem::into_system(this)
@@ -44,23 +53,23 @@ where
 macro_rules! impl_system_function {
     ($($param: ident),*) => {
         #[allow(non_snake_case)]
-        impl<E: 'static, B: Bundle, Func: Send + Sync + 'static, $($param: SystemParam),*> SystemParamFunction<fn(Trigger<E, B>, $($param,)*)> for Func
+        impl<E: 'static, B: Bundle, Out, Func: Send + Sync + 'static, $($param: SystemParam),*> SystemParamFunction<fn(Trigger<E, B>, $($param,)*)> for Func
         where
         for <'a> &'a mut Func:
-                FnMut(Trigger<E, B>, $($param),*) +
-                FnMut(Trigger<E, B>, $(SystemParamItem<$param>),*)
+                FnMut(Trigger<E, B>, $($param),*) -> Out +
+                FnMut(Trigger<E, B>, $(SystemParamItem<$param>),*) -> Out, Out: 'static
         {
             type In = Trigger<'static, E, B>;
-            type Out = ();
+            type Out = Out;
             type Param = ($($param,)*);
             #[inline]
-            fn run(&mut self, input: Trigger<'static, E, B>, param_value: SystemParamItem< ($($param,)*)>) {
+            fn run(&mut self, input: Trigger<'static, E, B>, param_value: SystemParamItem< ($($param,)*)>) -> Out {
                 #[allow(clippy::too_many_arguments)]
-                fn call_inner<E: 'static, B: Bundle, $($param,)*>(
-                    mut f: impl FnMut(Trigger<'static, E, B>, $($param,)*),
+                fn call_inner<E: 'static, B: Bundle, Out, $($param,)*>(
+                    mut f: impl FnMut(Trigger<'static, E, B>, $($param,)*) -> Out,
                     input: Trigger<'static, E, B>,
                     $($param: $param,)*
-                ){
+                ) -> Out{
                     f(input, $($param,)*)
                 }
                 let ($($param,)*) = param_value;
@@ -71,3 +80,37 @@ macro_rules! impl_system_function {
 }
 
 all_tuples!(impl_system_function, 0, 16, F);
+
+#[cfg(test)]
+mod tests {
+    use crate::{
+        self as bevy_ecs,
+        event::Event,
+        observer::Trigger,
+        system::{In, IntoSystem},
+        world::World,
+    };
+
+    #[derive(Event)]
+    struct TriggerEvent;
+
+    #[test]
+    fn test_piped_observer_systems_no_input() {
+        fn a(_: Trigger<TriggerEvent>) {}
+        fn b() {}
+
+        let mut world = World::new();
+        world.observe(a.pipe(b));
+    }
+
+    #[test]
+    fn test_piped_observer_systems_with_inputs() {
+        fn a(_: Trigger<TriggerEvent>) -> u32 {
+            3
+        }
+        fn b(_: In<u32>) {}
+
+        let mut world = World::new();
+        world.observe(a.pipe(b));
+    }
+}