add Flags<T> as a query to get flags of component (#1172)

add `Flags` as a query to get flags of component

Cargo.toml

@@ -198,6 +198,10 @@ path = "examples/diagnostics/custom_diagnostic.rs"
 name = "log_diagnostics"
 path = "examples/diagnostics/log_diagnostics.rs"
 
+[[example]]
+name = "change_detection"
+path = "examples/ecs/change_detection.rs"
+
 [[example]]
 name = "event"
 path = "examples/ecs/event.rs"

crates/bevy_ecs/src/core/mod.rs

@@ -51,7 +51,7 @@ pub use entities::{Entity, EntityReserver, Location, NoSuchEntity};
 pub use entity_builder::{BuiltEntity, EntityBuilder};
 pub use entity_map::*;
 pub use filter::{Added, Changed, EntityFilter, Mutated, Or, QueryFilter, With, Without};
-pub use query::{Batch, BatchedIter, Mut, QueryIter, ReadOnlyFetch, WorldQuery};
+pub use query::{Batch, BatchedIter, Flags, Mut, QueryIter, ReadOnlyFetch, WorldQuery};
 pub use world::{ArchetypesGeneration, Component, ComponentError, SpawnBatchIter, World};
 pub use world_builder::*;
 

crates/bevy_ecs/src/core/query.rs

@@ -130,6 +130,41 @@ impl<T: WorldQuery> WorldQuery for Option<T> {
     type Fetch = TryFetch<T::Fetch>;
 }
 
+/// Flags on component `T` that happened since the start of the frame.
+#[derive(Debug, Clone)]
+pub struct Flags<T: Component> {
+    _marker: std::marker::PhantomData<T>,
+    with: bool,
+    added: bool,
+    mutated: bool,
+}
+
+impl<T: Component> Flags<T> {
+    /// Does the entity have this component
+    pub fn with(&self) -> bool {
+        self.with
+    }
+
+    /// Has this component been added since the start of the frame.
+    pub fn added(&self) -> bool {
+        self.added
+    }
+
+    /// Has this component been mutated since the start of the frame.
+    pub fn mutated(&self) -> bool {
+        self.mutated
+    }
+
+    /// Has this component been either mutated or added since the start of the frame.
+    pub fn changed(&self) -> bool {
+        self.added || self.mutated
+    }
+}
+
+impl<T: Component> WorldQuery for Flags<T> {
+    type Fetch = FlagsFetch<T>;
+}
+
 /// Unique borrow of an entity's component
 pub struct Mut<'a, T: Component> {
     pub(crate) value: &'a mut T,
@@ -237,6 +272,51 @@ impl<'a, T: Fetch<'a>> Fetch<'a> for TryFetch<T> {
     }
 }
 
+#[doc(hidden)]
+pub struct FlagsFetch<T>(Option<NonNull<ComponentFlags>>, PhantomData<T>);
+unsafe impl<T> ReadOnlyFetch for FlagsFetch<T> {}
+
+impl<'a, T: Component> Fetch<'a> for FlagsFetch<T> {
+    type Item = Flags<T>;
+
+    const DANGLING: Self = Self(None, PhantomData::<T>);
+
+    #[inline]
+    fn access() -> QueryAccess {
+        QueryAccess::read::<T>()
+    }
+
+    unsafe fn get(archetype: &'a Archetype, offset: usize) -> Option<Self> {
+        Some(Self(
+            archetype
+                .get_type_state(std::any::TypeId::of::<T>())
+                .map(|type_state| {
+                    NonNull::new_unchecked(type_state.component_flags().as_ptr().add(offset))
+                }),
+            PhantomData::<T>,
+        ))
+    }
+
+    unsafe fn fetch(&self, n: usize) -> Self::Item {
+        if let Some(flags) = self.0.as_ref() {
+            let flags = *flags.as_ptr().add(n);
+            Self::Item {
+                _marker: PhantomData::<T>,
+                with: true,
+                added: flags.contains(ComponentFlags::ADDED),
+                mutated: flags.contains(ComponentFlags::MUTATED),
+            }
+        } else {
+            Self::Item {
+                _marker: PhantomData::<T>,
+                with: false,
+                added: false,
+                mutated: false,
+            }
+        }
+    }
+}
+
 struct ChunkInfo<Q: WorldQuery, F: QueryFilter> {
     fetch: Q::Fetch,
     filter: F::EntityFilter,
@@ -466,7 +546,7 @@ smaller_tuples_too!(tuple_impl, O, N, M, L, K, J, I, H, G, F, E, D, C, B, A);
 
 #[cfg(test)]
 mod tests {
-    use crate::core::{Added, Changed, Component, Entity, Mutated, Or, QueryFilter, World};
+    use crate::core::{Added, Changed, Component, Entity, Flags, Mutated, Or, QueryFilter, World};
     use std::{vec, vec::Vec};
 
     use super::Mut;
@@ -640,6 +720,38 @@ mod tests {
         assert_eq!(get_changed(&world), vec![e1]);
     }
 
+    #[test]
+    fn flags_query() {
+        let mut world = World::default();
+        let e1 = world.spawn((A(0), B(0)));
+        world.spawn((B(0),));
+
+        fn get_flags(world: &World) -> Vec<Flags<A>> {
+            world.query::<Flags<A>>().collect::<Vec<Flags<A>>>()
+        }
+        let flags = get_flags(&world);
+        assert!(flags[0].with());
+        assert!(flags[0].added());
+        assert!(!flags[0].mutated());
+        assert!(flags[0].changed());
+        assert!(!flags[1].with());
+        assert!(!flags[1].added());
+        assert!(!flags[1].mutated());
+        assert!(!flags[1].changed());
+        world.clear_trackers();
+        let flags = get_flags(&world);
+        assert!(flags[0].with());
+        assert!(!flags[0].added());
+        assert!(!flags[0].mutated());
+        assert!(!flags[0].changed());
+        *world.get_mut(e1).unwrap() = A(1);
+        let flags = get_flags(&world);
+        assert!(flags[0].with());
+        assert!(!flags[0].added());
+        assert!(flags[0].mutated());
+        assert!(flags[0].changed());
+    }
+
     #[test]
     fn exact_size_query() {
         let mut world = World::default();

crates/bevy_ecs/src/lib.rs

@@ -16,7 +16,7 @@ pub mod prelude {
         resource::{ChangedRes, FromResources, Local, Res, ResMut, Resource, Resources},
         schedule::{Schedule, State, StateStage, SystemStage},
         system::{Commands, IntoSystem, Query, System},
-        Added, Bundle, Changed, Component, Entity, In, IntoChainSystem, Mut, Mutated, Or, QuerySet,
-        Ref, RefMut, With, Without, World,
+        Added, Bundle, Changed, Component, Entity, Flags, In, IntoChainSystem, Mut, Mutated, Or,
+        QuerySet, Ref, RefMut, With, Without, World,
     };
 }

examples/README.md

@@ -114,6 +114,7 @@ Example | File | Description
 
 Example | File | Description
 --- | --- | ---
+`change_detection` | [`ecs/change_detection.rs`](./ecs/change_detection.rs) | Change detection on components
 `ecs_guide` | [`ecs/ecs_guide.rs`](./ecs/ecs_guide.rs) | Full guide to Bevy's ECS
 `event` | [`ecs/event.rs`](./ecs/event.rs) | Illustrates event creation, activation, and reception
 `hierarchy` | [`ecs/hierarchy.rs`](./ecs/hierarchy.rs) | Creates a hierarchy of parents and children entities

examples/ecs/change_detection.rs

@@ -0,0 +1,53 @@
+use bevy::prelude::*;
+use rand::Rng;
+
+// This example illustrates how to react to component change
+fn main() {
+    App::build()
+        .add_plugins(DefaultPlugins)
+        .add_startup_system(setup.system())
+        .add_system(change_component.system())
+        .add_system(change_detection.system())
+        .add_system(flags_monitoring.system())
+        .run();
+}
+
+#[derive(Debug)]
+struct MyComponent(f64);
+
+fn setup(commands: &mut Commands) {
+    commands.spawn((MyComponent(0.),));
+    commands.spawn((Transform::default(),));
+}
+
+fn change_component(time: Res<Time>, mut query: Query<(Entity, &mut MyComponent)>) {
+    for (entity, mut component) in query.iter_mut() {
+        if rand::thread_rng().gen_bool(0.1) {
+            info!("changing component {:?}", entity);
+            component.0 = time.seconds_since_startup();
+        }
+    }
+}
+
+// There are query filters for `Changed<T>`, `Added<T>` and `Mutated<T>`
+// Only entities matching the filters will be in the query
+fn change_detection(query: Query<(Entity, &MyComponent), Changed<MyComponent>>) {
+    for (entity, component) in query.iter() {
+        info!("{:?} changed: {:?}", entity, component,);
+    }
+}
+
+// By looking at flags, the query is not filtered but the information is available
+fn flags_monitoring(query: Query<(Entity, Option<&MyComponent>, Flags<MyComponent>)>) {
+    for (entity, component, flags) in query.iter() {
+        info!(
+            "{:?}: {:?} -> with: {:?} - added: {:?} - mutated: {:?} - changed: {:?}",
+            entity,
+            component,
+            flags.with(),
+            flags.added(),
+            flags.mutated(),
+            flags.changed()
+        );
+    }
+}