Generalised ECS reactivity with Observers (#10839) (#13873)

# Objective

- Fixes #13825 

## Solution

- Cherry picked and fixed non-trivial conflicts to be able to merge
#10839 into the 0.14 release branch.

Link to PR: #10839

Co-authored-by: James O'Brien <[email protected]>
Co-authored-by: Alice Cecile <[email protected]>
Co-authored-by: MiniaczQ <[email protected]>
Co-authored-by: Carter Anderson <[email protected]>

Cargo.toml

@@ -2534,6 +2534,17 @@ description = "Systems run in parallel, but their order isn't always determinist
 category = "ECS (Entity Component System)"
 wasm = false
 
+[[example]]
+name = "observers"
+path = "examples/ecs/observers.rs"
+doc-scrape-examples = true
+
+[package.metadata.example.observers]
+name = "Observers"
+description = "Demonstrates observers that react to events (both built-in life-cycle events and custom events)"
+category = "ECS (Entity Component System)"
+wasm = true
+
 [[example]]
 name = "3d_rotation"
 path = "examples/transforms/3d_rotation.rs"

crates/bevy_app/src/app.rs

@@ -8,7 +8,7 @@ use bevy_ecs::{
     intern::Interned,
     prelude::*,
     schedule::{ScheduleBuildSettings, ScheduleLabel},
-    system::SystemId,
+    system::{IntoObserverSystem, SystemId},
 };
 #[cfg(feature = "trace")]
 use bevy_utils::tracing::info_span;
@@ -829,6 +829,15 @@ impl App {
 
         None
     }
+
+    /// Spawns an [`Observer`] entity, which will watch for and respond to the given event.
+    pub fn observe<E: Event, B: Bundle, M>(
+        &mut self,
+        observer: impl IntoObserverSystem<E, B, M>,
+    ) -> &mut Self {
+        self.world_mut().observe(observer);
+        self
+    }
 }
 
 type RunnerFn = Box<dyn FnOnce(App) -> AppExit>;

crates/bevy_ecs/README.md

@@ -307,4 +307,52 @@ fn reader(mut reader: EventReader<MyEvent>) {
 
 A minimal set up using events can be seen in [`events.rs`](examples/events.rs).
 
+### Observers
+
+Observers are systems that listen for a "trigger" of a specific `Event`:
+
+```rust
+use bevy_ecs::prelude::*;
+
+#[derive(Event)]
+struct MyEvent {
+    message: String
+}
+
+let mut world = World::new();
+
+world.observe(|trigger: Trigger<MyEvent>| {
+    println!("{}", trigger.event().message);
+});
+
+world.flush();
+
+world.trigger(MyEvent {
+    message: "hello!".to_string(),
+});
+```
+
+These differ from `EventReader` and `EventWriter` in that they are "reactive". Rather than happening at a specific point in a schedule, they happen _immediately_ whenever a trigger happens. Triggers can trigger other triggers, and they all will be evaluated at the same time!
+
+Events can also be triggered to target specific entities:
+
+```rust
+use bevy_ecs::prelude::*;
+
+#[derive(Event)]
+struct Explode;
+
+let mut world = World::new();
+let entity = world.spawn_empty().id();
+
+world.observe(|trigger: Trigger<Explode>, mut commands: Commands| {
+    println!("Entity {:?} goes BOOM!", trigger.entity());
+    commands.entity(trigger.entity()).despawn();
+});
+
+world.flush();
+
+world.trigger_targets(Explode, entity);
+```
+
 [bevy]: https://bevyengine.org/

crates/bevy_ecs/macros/src/component.rs

@@ -18,6 +18,10 @@ pub fn derive_event(input: TokenStream) -> TokenStream {
     TokenStream::from(quote! {
         impl #impl_generics #bevy_ecs_path::event::Event for #struct_name #type_generics #where_clause {
         }
+
+        impl #impl_generics #bevy_ecs_path::component::Component for #struct_name #type_generics #where_clause {
+            const STORAGE_TYPE: #bevy_ecs_path::component::StorageType = #bevy_ecs_path::component::StorageType::SparseSet;
+        }
     })
 }
 

crates/bevy_ecs/macros/src/lib.rs

@@ -74,6 +74,7 @@ pub fn derive_bundle(input: TokenStream) -> TokenStream {
         .collect::<Vec<_>>();
 
     let mut field_component_ids = Vec::new();
+    let mut field_get_component_ids = Vec::new();
     let mut field_get_components = Vec::new();
     let mut field_from_components = Vec::new();
     for (((i, field_type), field_kind), field) in field_type
@@ -87,6 +88,9 @@ pub fn derive_bundle(input: TokenStream) -> TokenStream {
                 field_component_ids.push(quote! {
                 <#field_type as #ecs_path::bundle::Bundle>::component_ids(components, storages, &mut *ids);
                 });
+                field_get_component_ids.push(quote! {
+                    <#field_type as #ecs_path::bundle::Bundle>::get_component_ids(components, &mut *ids);
+                });
                 match field {
                     Some(field) => {
                         field_get_components.push(quote! {
@@ -133,6 +137,13 @@ pub fn derive_bundle(input: TokenStream) -> TokenStream {
                 #(#field_component_ids)*
             }
 
+            fn get_component_ids(
+                components: &#ecs_path::component::Components,
+                ids: &mut impl FnMut(Option<#ecs_path::component::ComponentId>)
+            ){
+                #(#field_get_component_ids)*
+            }
+
             #[allow(unused_variables, non_snake_case)]
             unsafe fn from_components<__T, __F>(ctx: &mut __T, func: &mut __F) -> Self
             where
@@ -435,6 +446,10 @@ pub fn derive_system_param(input: TokenStream) -> TokenStream {
                     <#fields_alias::<'_, '_, #punctuated_generic_idents> as #path::system::SystemParam>::apply(&mut state.state, system_meta, world);
                 }
 
+                fn queue(state: &mut Self::State, system_meta: &#path::system::SystemMeta, world: #path::world::DeferredWorld) {
+                    <#fields_alias::<'_, '_, #punctuated_generic_idents> as #path::system::SystemParam>::queue(&mut state.state, system_meta, world);
+                }
+
                 unsafe fn get_param<'w, 's>(
                     state: &'s mut Self::State,
                     system_meta: &#path::system::SystemMeta,

crates/bevy_ecs/src/archetype.rs

@@ -23,6 +23,7 @@ use crate::{
     bundle::BundleId,
     component::{ComponentId, Components, StorageType},
     entity::{Entity, EntityLocation},
+    observer::Observers,
     storage::{ImmutableSparseSet, SparseArray, SparseSet, SparseSetIndex, TableId, TableRow},
 };
 use std::{
@@ -119,6 +120,7 @@ pub(crate) struct AddBundle {
     /// For each component iterated in the same order as the source [`Bundle`](crate::bundle::Bundle),
     /// indicate if the component is newly added to the target archetype or if it already existed
     pub bundle_status: Vec<ComponentStatus>,
+    pub added: Vec<ComponentId>,
 }
 
 /// This trait is used to report the status of [`Bundle`](crate::bundle::Bundle) components
@@ -202,12 +204,14 @@ impl Edges {
         bundle_id: BundleId,
         archetype_id: ArchetypeId,
         bundle_status: Vec<ComponentStatus>,
+        added: Vec<ComponentId>,
     ) {
         self.add_bundle.insert(
             bundle_id,
             AddBundle {
                 archetype_id,
                 bundle_status,
+                added,
             },
         );
     }
@@ -314,6 +318,9 @@ bitflags::bitflags! {
         const ON_ADD_HOOK    = (1 << 0);
         const ON_INSERT_HOOK = (1 << 1);
         const ON_REMOVE_HOOK = (1 << 2);
+        const ON_ADD_OBSERVER = (1 << 3);
+        const ON_INSERT_OBSERVER = (1 << 4);
+        const ON_REMOVE_OBSERVER = (1 << 5);
     }
 }
 
@@ -335,6 +342,7 @@ pub struct Archetype {
 impl Archetype {
     pub(crate) fn new(
         components: &Components,
+        observers: &Observers,
         id: ArchetypeId,
         table_id: TableId,
         table_components: impl Iterator<Item = (ComponentId, ArchetypeComponentId)>,
@@ -348,6 +356,7 @@ impl Archetype {
             // SAFETY: We are creating an archetype that includes this component so it must exist
             let info = unsafe { components.get_info_unchecked(component_id) };
             info.update_archetype_flags(&mut flags);
+            observers.update_archetype_flags(component_id, &mut flags);
             archetype_components.insert(
                 component_id,
                 ArchetypeComponentInfo {
@@ -580,21 +589,45 @@ impl Archetype {
 
     /// Returns true if any of the components in this archetype have `on_add` hooks
     #[inline]
-    pub(crate) fn has_on_add(&self) -> bool {
+    pub fn has_add_hook(&self) -> bool {
         self.flags().contains(ArchetypeFlags::ON_ADD_HOOK)
     }
 
     /// Returns true if any of the components in this archetype have `on_insert` hooks
     #[inline]
-    pub(crate) fn has_on_insert(&self) -> bool {
+    pub fn has_insert_hook(&self) -> bool {
         self.flags().contains(ArchetypeFlags::ON_INSERT_HOOK)
     }
 
     /// Returns true if any of the components in this archetype have `on_remove` hooks
     #[inline]
-    pub(crate) fn has_on_remove(&self) -> bool {
+    pub fn has_remove_hook(&self) -> bool {
         self.flags().contains(ArchetypeFlags::ON_REMOVE_HOOK)
     }
+
+    /// Returns true if any of the components in this archetype have at least one [`OnAdd`] observer
+    ///
+    /// [`OnAdd`]: crate::world::OnAdd
+    #[inline]
+    pub fn has_add_observer(&self) -> bool {
+        self.flags().contains(ArchetypeFlags::ON_ADD_OBSERVER)
+    }
+
+    /// Returns true if any of the components in this archetype have at least one [`OnInsert`] observer
+    ///
+    /// [`OnInsert`]: crate::world::OnInsert
+    #[inline]
+    pub fn has_insert_observer(&self) -> bool {
+        self.flags().contains(ArchetypeFlags::ON_INSERT_OBSERVER)
+    }
+
+    /// Returns true if any of the components in this archetype have at least one [`OnRemove`] observer
+    ///
+    /// [`OnRemove`]: crate::world::OnRemove
+    #[inline]
+    pub fn has_remove_observer(&self) -> bool {
+        self.flags().contains(ArchetypeFlags::ON_REMOVE_OBSERVER)
+    }
 }
 
 /// The next [`ArchetypeId`] in an [`Archetypes`] collection.
@@ -681,6 +714,7 @@ impl Archetypes {
         unsafe {
             archetypes.get_id_or_insert(
                 &Components::default(),
+                &Observers::default(),
                 TableId::empty(),
                 Vec::new(),
                 Vec::new(),
@@ -782,6 +816,7 @@ impl Archetypes {
     pub(crate) unsafe fn get_id_or_insert(
         &mut self,
         components: &Components,
+        observers: &Observers,
         table_id: TableId,
         table_components: Vec<ComponentId>,
         sparse_set_components: Vec<ComponentId>,
@@ -808,6 +843,7 @@ impl Archetypes {
                     (sparse_start..*archetype_component_count).map(ArchetypeComponentId);
                 archetypes.push(Archetype::new(
                     components,
+                    observers,
                     id,
                     table_id,
                     table_components.into_iter().zip(table_archetype_components),
@@ -832,6 +868,20 @@ impl Archetypes {
             archetype.clear_entities();
         }
     }
+
+    pub(crate) fn update_flags(
+        &mut self,
+        component_id: ComponentId,
+        flags: ArchetypeFlags,
+        set: bool,
+    ) {
+        // TODO: Refactor component index to speed this up.
+        for archetype in &mut self.archetypes {
+            if archetype.contains(component_id) {
+                archetype.flags.set(flags, set);
+            }
+        }
+    }
 }
 
 impl Index<RangeFrom<ArchetypeGeneration>> for Archetypes {