Initial attempt

Cargo.toml

@@ -3852,3 +3852,8 @@ doc-scrape-examples = true
 
 [package.metadata.example.testbed_3d]
 hidden = true
+
+[[example]]
+name = "playground"
+path = "examples/playground.rs"
+doc-scrape-examples = false

crates/bevy_ecs/src/archetype.rs

@@ -352,6 +352,7 @@ bitflags::bitflags! {
         const ON_INSERT_OBSERVER = (1 << 5);
         const ON_REPLACE_OBSERVER = (1 << 6);
         const ON_REMOVE_OBSERVER = (1 << 7);
+        const ON_MUTATE_OBSERVER = (1 << 8);
     }
 }
 
@@ -697,6 +698,11 @@ impl Archetype {
     pub fn has_remove_observer(&self) -> bool {
         self.flags().contains(ArchetypeFlags::ON_REMOVE_OBSERVER)
     }
+
+    #[inline]
+    pub fn has_mutate_observer(&self) -> bool {
+        self.flags().contains(ArchetypeFlags::ON_MUTATE_OBSERVER)
+    }
 }
 
 /// The next [`ArchetypeId`] in an [`Archetypes`] collection.

crates/bevy_ecs/src/bundle.rs

@@ -18,7 +18,7 @@ use crate::{
     prelude::World,
     query::DebugCheckedUnwrap,
     storage::{SparseSetIndex, SparseSets, Storages, Table, TableRow},
-    world::{unsafe_world_cell::UnsafeWorldCell, ON_ADD, ON_INSERT, ON_REPLACE},
+    world::{unsafe_world_cell::UnsafeWorldCell, ON_ADD, ON_INSERT, ON_MUTATE, ON_REPLACE},
 };
 use bevy_ptr::{ConstNonNull, OwningPtr};
 use bevy_utils::{all_tuples, HashMap, HashSet, TypeIdMap};
@@ -1093,6 +1093,13 @@ impl<'w> BundleInserter<'w> {
                             add_bundle.iter_inserted(),
                         );
                     }
+                    if new_archetype.has_mutate_observer() {
+                        deferred_world.trigger_observers(
+                            ON_MUTATE,
+                            entity,
+                            add_bundle.iter_inserted(),
+                        );
+                    }
                 }
                 InsertMode::Keep => {
                     // insert triggers only for new components if we're not replacing them (since
@@ -1109,6 +1116,13 @@ impl<'w> BundleInserter<'w> {
                             add_bundle.iter_added(),
                         );
                     }
+                    if new_archetype.has_mutate_observer() {
+                        deferred_world.trigger_observers(
+                            ON_MUTATE,
+                            entity,
+                            add_bundle.iter_added(),
+                        );
+                    }
                 }
             }
         }
@@ -1249,6 +1263,13 @@ impl<'w> BundleSpawner<'w> {
                     bundle_info.iter_contributed_components(),
                 );
             }
+            if archetype.has_mutate_observer() {
+                deferred_world.trigger_observers(
+                    ON_MUTATE,
+                    entity,
+                    bundle_info.iter_contributed_components(),
+                );
+            }
         };
 
         location

crates/bevy_ecs/src/lib.rs

@@ -64,7 +64,7 @@ pub mod prelude {
         },
         world::{
             Command, EntityMut, EntityRef, EntityWorldMut, FilteredResources, FilteredResourcesMut,
-            FromWorld, OnAdd, OnInsert, OnRemove, OnReplace, World,
+            FromWorld, OnAdd, OnInsert, OnRemove, OnReplace, OnMutate, World,
         },
     };
 

crates/bevy_ecs/src/observer/mod.rs

@@ -247,6 +247,7 @@ pub struct Observers {
     on_insert: CachedObservers,
     on_replace: CachedObservers,
     on_remove: CachedObservers,
+    on_mutate: CachedObservers,
     // Map from trigger type to set of observers
     cache: HashMap<ComponentId, CachedObservers>,
 }
@@ -258,6 +259,7 @@ impl Observers {
             ON_INSERT => &mut self.on_insert,
             ON_REPLACE => &mut self.on_replace,
             ON_REMOVE => &mut self.on_remove,
+            ON_MUTATE => &mut self.on_mutate,
             _ => self.cache.entry(event_type).or_default(),
         }
     }
@@ -268,6 +270,7 @@ impl Observers {
             ON_INSERT => Some(&self.on_insert),
             ON_REPLACE => Some(&self.on_replace),
             ON_REMOVE => Some(&self.on_remove),
+            ON_MUTATE => Some(&self.on_mutate),
             _ => self.cache.get(&event_type),
         }
     }
@@ -342,6 +345,7 @@ impl Observers {
             ON_INSERT => Some(ArchetypeFlags::ON_INSERT_OBSERVER),
             ON_REPLACE => Some(ArchetypeFlags::ON_REPLACE_OBSERVER),
             ON_REMOVE => Some(ArchetypeFlags::ON_REMOVE_OBSERVER),
+            ON_MUTATE => Some(ArchetypeFlags::ON_MUTATE_OBSERVER),
             _ => None,
         }
     }
@@ -378,6 +382,14 @@ impl Observers {
         {
             flags.insert(ArchetypeFlags::ON_REMOVE_OBSERVER);
         }
+
+        if self
+            .on_mutate
+            .component_observers
+            .contains_key(&component_id)
+        {
+            flags.insert(ArchetypeFlags::ON_MUTATE_OBSERVER);
+        }
     }
 }
 
@@ -565,6 +577,7 @@ impl World {
     }
 }
 
+// TODO GRACE: write tests here
 #[cfg(test)]
 mod tests {
     use alloc::vec;

crates/bevy_ecs/src/system/system_param.rs

@@ -1,9 +1,9 @@
 pub use crate::change_detection::{NonSendMut, Res, ResMut};
 use crate::{
-    archetype::{Archetype, Archetypes},
+    archetype::{self, Archetype, Archetypes},
     bundle::Bundles,
     change_detection::{Ticks, TicksMut},
-    component::{ComponentId, ComponentTicks, Components, Tick},
+    component::{self, ComponentId, ComponentTicks, Components, Tick},
     entity::Entities,
     query::{
         Access, FilteredAccess, FilteredAccessSet, QueryData, QueryFilter, QuerySingleError,
@@ -12,8 +12,7 @@ use crate::{
     storage::ResourceData,
     system::{Query, Single, SystemMeta},
     world::{
-        unsafe_world_cell::UnsafeWorldCell, DeferredWorld, FilteredResources, FilteredResourcesMut,
-        FromWorld, World,
+        unsafe_world_cell::UnsafeWorldCell, DeferredWorld, FilteredResources, FilteredResourcesMut, FromWorld, OnMutate, World, ON_MUTATE
     },
 };
 use bevy_ecs_macros::impl_param_set;
@@ -29,7 +28,7 @@ use core::{
     ops::{Deref, DerefMut},
 };
 
-use super::Populated;
+use super::{IntoSystem, Populated};
 
 /// A parameter that can be used in a [`System`](super::System).
 ///
@@ -320,6 +319,64 @@ unsafe impl<D: QueryData + 'static, F: QueryFilter + 'static> SystemParam for Qu
         // world data that the query needs.
         unsafe { Query::new(world, state, system_meta.last_run, change_tick) }
     }
+
+    fn apply(state: &mut Self::State, system_meta: &SystemMeta, world: &mut World) {
+        let mut vec = Vec::new();
+        for archetype_id in state.matched_archetypes() {
+            let Some(archetype) = world.archetypes().get(archetype_id) else { continue; };
+            if !archetype.has_mutate_observer() { continue; }
+            // println!("{:?}", archetype_id);
+            let (writes, cant_write) = system_meta.component_access_set.combined_access().component_writes();
+            let writes = if cant_write { vec![] } else { writes.collect() };
+            // println!("{:?}", writes);
+
+            for archetype_entity in archetype.entities() {
+                let entity = archetype_entity.id();
+                let entity_ref = world.entity(entity);
+                let writes_here = writes.iter().cloned().filter(|c| 
+                    entity_ref
+                        .get_change_ticks_by_id(*c)
+                        .map(|ticks| {
+                            // println!("{:?}", ticks);
+                            ticks.changed.get() >= system_meta.last_run.get()
+                        }) // FIXME GRACE: get this_run
+                        .unwrap_or(false)
+                    ).collect::<Vec<_>>(); 
+                vec.push((entity, writes_here));
+            }
+        }
+
+        let mut deferred_world = DeferredWorld::from(world);
+        for (e, modified_components) in vec {
+            unsafe {
+                deferred_world.trigger_observers(ON_MUTATE, e, modified_components.iter().cloned());
+            }
+        }
+    }
+
+    // fn apply(state: &mut Self::State, system_meta: &SystemMeta, world: &mut World) {
+
+
+
+    //     for archetype_id in state.matched_archetypes() {
+    //         let Some(archetype) = world.archetypes().get(archetype_id) else { continue; };
+    //         if !archetype.has_mutate_observer() { continue; }
+    //         let (writes, are_writes) = system_meta.archetype_component_access.component_writes();
+    //         if are_writes {
+    //             for comp in writes {
+
+    //             }
+    //         }
+    //         for archetype_entity in archetype.entities() {
+
+    //             let entity = archetype_entity.id();
+    //             for component in archetype.components() {
+
+    //             }
+    //         }
+    //     }
+    //     // world.commands().trigger_targets(OnMutate, targets);
+    // }
 }
 
 pub(crate) fn init_query_param<D: QueryData + 'static, F: QueryFilter + 'static>(

crates/bevy_ecs/src/world/component_constants.rs

@@ -13,6 +13,8 @@ pub const ON_INSERT: ComponentId = ComponentId::new(1);
 pub const ON_REPLACE: ComponentId = ComponentId::new(2);
 /// [`ComponentId`] for [`OnRemove`]
 pub const ON_REMOVE: ComponentId = ComponentId::new(3);
+/// [`ComponentId`] for [`OnMutate`]
+pub const ON_MUTATE: ComponentId = ComponentId::new(4);
 
 /// Trigger emitted when a component is added to an entity. See [`crate::component::ComponentHooks::on_add`]
 /// for more information.
@@ -41,3 +43,9 @@ pub struct OnReplace;
 #[cfg_attr(feature = "bevy_reflect", derive(Reflect))]
 #[cfg_attr(feature = "bevy_reflect", reflect(Debug))]
 pub struct OnRemove;
+
+/// Trigger emitted when a component is mutated.
+#[derive(Event, Debug)]
+#[cfg_attr(feature = "bevy_reflect", derive(Reflect))]
+#[cfg_attr(feature = "bevy_reflect", reflect(Debug))]
+pub struct OnMutate;

crates/bevy_ecs/src/world/mod.rs

@@ -182,6 +182,7 @@ impl World {
         assert_eq!(ON_INSERT, self.register_component::<OnInsert>());
         assert_eq!(ON_REPLACE, self.register_component::<OnReplace>());
         assert_eq!(ON_REMOVE, self.register_component::<OnRemove>());
+        assert_eq!(ON_MUTATE, self.register_component::<OnMutate>());
     }
     /// Creates a new empty [`World`].
     ///

examples/playground.rs

@@ -0,0 +1,55 @@
+
+#![allow(missing_docs)]
+
+use bevy::prelude::*;
+
+// TODO GRACE: remove this file
+
+fn main() {
+    App::new()
+        .add_plugins(DefaultPlugins)
+        .add_systems(Startup, setup)
+        .add_systems(Update, on_space_press)
+        .add_observer(on_a_mutated)
+        .run();
+}
+
+#[derive(Component)]
+struct A(i32);
+
+fn setup(mut commands: Commands) {
+    commands.spawn(A(0));
+}
+
+fn on_space_press(input: Res<ButtonInput<KeyCode>>, mut query: Query<&mut A>) {
+    if input.just_pressed(KeyCode::Space) {
+        for mut a in query.iter_mut() {
+            println!("asdfjkaskldfjlksadfljsdfk");
+            a.0 = 100;
+        }
+    }
+}
+
+fn on_a_mutated(trigger: Trigger<OnMutate, A>) {
+    println!("A mutation happened!");
+}
+
+
+// fn main() {
+//     App::new()
+//         .add_plugins(DefaultPlugins)
+//         .add_systems(Startup, setup)
+//         .add_observer(on_a_added)
+//         .run();
+// }
+
+// #[derive(Component)]
+// struct A(i32);
+
+// fn setup(mut commands: Commands) {
+//     commands.spawn_empty().insert(A(0)).insert(A(1));
+// }
+
+// fn on_a_added(trigger: Trigger<OnAdd, A>) {
+//     println!("Abafjgklasdfgjp;kl")
+// }