feat(ecs): implement fallible observer systems

This commit builds on top of the work done in bevyengine#16589 and bevyengine#17051, by
adding support for fallible observer systems.

As with the previous work, the actual results of the observer system
are suppressed by default, but the intention is to provide a way to
handle errors in a global way.

Until then, you can use a `PipeSystem` to manually handle results.

Signed-off-by: Jean Mertz <[email protected]>

Author: JeanMertz

crates/bevy_ecs/src/observer/runner.rs

@@ -386,7 +386,8 @@ fn observer_system_runner<E: Event, B: Bundle, S: ObserverSystem<E, B>>(
     unsafe {
         (*system).update_archetype_component_access(world);
         if (*system).validate_param_unsafe(world) {
-            (*system).run_unsafe(trigger, world);
+            // TODO: implement an error-handling API instead of suppressing a possible failure.
+            let _ = (*system).run_unsafe(trigger, world);
             (*system).queue_deferred(world.into_deferred());
         }
     }

crates/bevy_ecs/src/system/observer_system.rs

@@ -1,22 +1,27 @@
+use alloc::{borrow::Cow, vec::Vec};
+use core::marker::PhantomData;
+
 use crate::{
+    archetype::ArchetypeComponentId,
+    component::{ComponentId, Tick},
     prelude::{Bundle, Trigger},
-    system::System,
+    query::Access,
+    result::Result,
+    schedule::{Fallible, Infallible},
+    system::{input::SystemIn, System},
+    world::{unsafe_world_cell::UnsafeWorldCell, DeferredWorld, World},
 };
 
 use super::IntoSystem;
 
 /// Implemented for [`System`]s that have a [`Trigger`] as the first argument.
-pub trait ObserverSystem<E: 'static, B: Bundle, Out = ()>:
+pub trait ObserverSystem<E: 'static, B: Bundle, Out = Result>:
     System<In = Trigger<'static, E, B>, Out = Out> + Send + 'static
 {
 }
 
-impl<
-        E: 'static,
-        B: Bundle,
-        Out,
-        T: System<In = Trigger<'static, E, B>, Out = Out> + Send + 'static,
-    > ObserverSystem<E, B, Out> for T
+impl<E: 'static, B: Bundle, Out, T> ObserverSystem<E, B, Out> for T where
+    T: System<In = Trigger<'static, E, B>, Out = Out> + Send + 'static
 {
 }
 
@@ -32,31 +37,158 @@ impl<
     label = "the trait `IntoObserverSystem` is not implemented",
     note = "for function `ObserverSystem`s, ensure the first argument is a `Trigger<T>` and any subsequent ones are `SystemParam`"
 )]
-pub trait IntoObserverSystem<E: 'static, B: Bundle, M, Out = ()>: Send + 'static {
+pub trait IntoObserverSystem<E: 'static, B: Bundle, M, Out = Result>: Send + 'static {
     /// The type of [`System`] that this instance converts into.
     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>, Out, M> + Send + 'static,
-        M,
-        Out,
-        E: 'static,
-        B: Bundle,
-    > IntoObserverSystem<E, B, M, Out> for S
+impl<E, B, M, Out, S> IntoObserverSystem<E, B, (Fallible, M), Out> for S
 where
+    S: IntoSystem<Trigger<'static, E, B>, Out, M> + Send + 'static,
     S::System: ObserverSystem<E, B, Out>,
+    E: 'static,
+    B: Bundle,
 {
-    type System = <S as IntoSystem<Trigger<'static, E, B>, Out, M>>::System;
+    type System = S::System;
 
     fn into_system(this: Self) -> Self::System {
         IntoSystem::into_system(this)
     }
 }
 
+impl<E, B, M, S> IntoObserverSystem<E, B, (Infallible, M), Result> for S
+where
+    S: IntoSystem<Trigger<'static, E, B>, (), M> + Send + 'static,
+    S::System: ObserverSystem<E, B, ()>,
+    E: Send + Sync + 'static,
+    B: Bundle,
+{
+    type System = InfallibleObserverWrapper<E, B, S::System>;
+
+    fn into_system(this: Self) -> Self::System {
+        InfallibleObserverWrapper::new(IntoSystem::into_system(this))
+    }
+}
+
+/// A wrapper that converts an observer system that returns `()` into one that returns `Ok(())`.
+pub struct InfallibleObserverWrapper<E, B, S> {
+    observer: S,
+    _marker: PhantomData<(E, B)>,
+}
+
+impl<E, B, S> InfallibleObserverWrapper<E, B, S> {
+    /// Create a new `InfallibleObserverWrapper`.
+    pub fn new(observer: S) -> Self {
+        Self {
+            observer,
+            _marker: PhantomData,
+        }
+    }
+}
+
+impl<E, B, S> System for InfallibleObserverWrapper<E, B, S>
+where
+    S: ObserverSystem<E, B, ()>,
+    E: Send + Sync + 'static,
+    B: Bundle,
+{
+    type In = Trigger<'static, E, B>;
+    type Out = Result;
+
+    #[inline]
+    fn name(&self) -> Cow<'static, str> {
+        self.observer.name()
+    }
+
+    #[inline]
+    fn component_access(&self) -> &Access<ComponentId> {
+        self.observer.component_access()
+    }
+
+    #[inline]
+    fn archetype_component_access(&self) -> &Access<ArchetypeComponentId> {
+        self.observer.archetype_component_access()
+    }
+
+    #[inline]
+    fn is_send(&self) -> bool {
+        self.observer.is_send()
+    }
+
+    #[inline]
+    fn is_exclusive(&self) -> bool {
+        self.observer.is_exclusive()
+    }
+
+    #[inline]
+    fn has_deferred(&self) -> bool {
+        self.observer.has_deferred()
+    }
+
+    #[inline]
+    unsafe fn run_unsafe(
+        &mut self,
+        input: SystemIn<'_, Self>,
+        world: UnsafeWorldCell,
+    ) -> Self::Out {
+        self.observer.run_unsafe(input, world);
+        Ok(())
+    }
+
+    #[inline]
+    fn run(&mut self, input: SystemIn<'_, Self>, world: &mut World) -> Self::Out {
+        self.observer.run(input, world);
+        Ok(())
+    }
+
+    #[inline]
+    fn apply_deferred(&mut self, world: &mut World) {
+        self.observer.apply_deferred(world);
+    }
+
+    #[inline]
+    fn queue_deferred(&mut self, world: DeferredWorld) {
+        self.observer.queue_deferred(world);
+    }
+
+    #[inline]
+    unsafe fn validate_param_unsafe(&mut self, world: UnsafeWorldCell) -> bool {
+        self.observer.validate_param_unsafe(world)
+    }
+
+    #[inline]
+    fn initialize(&mut self, world: &mut World) {
+        self.observer.initialize(world);
+    }
+
+    #[inline]
+    fn update_archetype_component_access(&mut self, world: UnsafeWorldCell) {
+        self.observer.update_archetype_component_access(world);
+    }
+
+    #[inline]
+    fn check_change_tick(&mut self, change_tick: Tick) {
+        self.observer.check_change_tick(change_tick);
+    }
+
+    #[inline]
+    fn get_last_run(&self) -> Tick {
+        self.observer.get_last_run()
+    }
+
+    #[inline]
+    fn set_last_run(&mut self, last_run: Tick) {
+        self.observer.set_last_run(last_run);
+    }
+
+    fn default_system_sets(&self) -> Vec<crate::schedule::InternedSystemSet> {
+        self.observer.default_system_sets()
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use crate::{