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use crate::{
component::{ComponentHooks, ComponentId, StorageType},
observer::{ObserverDescriptor, ObserverTrigger},
prelude::*,
query::DebugCheckedUnwrap,
system::{IntoObserverSystem, ObserverSystem},
world::DeferredWorld,
};
use bevy_ptr::PtrMut;
/// Contains [`Observer`] information. This defines how a given observer behaves. It is the
/// "source of truth" for a given observer entity's behavior.
pub struct ObserverState {
pub(crate) descriptor: ObserverDescriptor,
pub(crate) runner: ObserverRunner,
pub(crate) last_trigger_id: u32,
pub(crate) despawned_watched_entities: u32,
}
impl Default for ObserverState {
fn default() -> Self {
Self {
runner: |_, _, _| {},
last_trigger_id: 0,
despawned_watched_entities: 0,
descriptor: Default::default(),
}
}
}
impl ObserverState {
/// Observe the given `event`. This will cause the [`Observer`] to run whenever an event with the given [`ComponentId`]
/// is triggered.
pub fn with_event(mut self, event: ComponentId) -> Self {
self.descriptor.events.push(event);
self
}
/// Observe the given event list. This will cause the [`Observer`] to run whenever an event with any of the given [`ComponentId`]s
/// is triggered.
pub fn with_events(mut self, events: impl IntoIterator<Item = ComponentId>) -> Self {
self.descriptor.events.extend(events);
self
}
/// Observe the given [`Entity`] list. This will cause the [`Observer`] to run whenever the [`Event`] is triggered
/// for any [`Entity`] target in the list.
pub fn with_entities(mut self, entities: impl IntoIterator<Item = Entity>) -> Self {
self.descriptor.entities.extend(entities);
self
}
/// Observe the given [`ComponentId`] list. This will cause the [`Observer`] to run whenever the [`Event`] is triggered
/// for any [`ComponentId`] target in the list.
pub fn with_components(mut self, components: impl IntoIterator<Item = ComponentId>) -> Self {
self.descriptor.components.extend(components);
self
}
}
impl Component for ObserverState {
const STORAGE_TYPE: StorageType = StorageType::SparseSet;
fn register_component_hooks(hooks: &mut ComponentHooks) {
hooks.on_add(|mut world, entity, _| {
world.commands().add(move |world: &mut World| {
world.register_observer(entity);
});
});
hooks.on_remove(|mut world, entity, _| {
let descriptor = std::mem::take(
&mut world
.entity_mut(entity)
.get_mut::<ObserverState>()
.unwrap()
.as_mut()
.descriptor,
);
world.commands().add(move |world: &mut World| {
world.unregister_observer(entity, descriptor);
});
});
}
}
/// Type for function that is run when an observer is triggered.
/// Typically refers to the default runner that runs the system stored in the associated [`ObserverSystemComponent`],
/// but can be overridden for custom behaviour.
pub type ObserverRunner = fn(DeferredWorld, ObserverTrigger, PtrMut);
/// An [`Observer`] system. Add this [`Component`] to an [`Entity`] to turn it into an "observer".
///
/// Observers listen for a "trigger" of a specific [`Event`]. Events are triggered by calling [`World::trigger`] or [`World::trigger_targets`].
///
/// Note that "buffered" events sent using [`EventReader`] and [`EventWriter`] are _not_ automatically triggered. They must be triggered at a specific
/// point in the schedule.
///
/// # Usage
///
/// The simplest usage
/// of the observer pattern looks like this:
///
/// ```
/// # use bevy_ecs::prelude::*;
/// # let mut world = World::default();
/// #[derive(Event)]
/// struct Speak {
/// message: String,
/// }
///
/// world.observe(|trigger: Trigger<Speak>| {
/// println!("{}", trigger.event().message);
/// });
///
/// // Observers currently require a flush() to be registered. In the context of schedules,
/// // this will generally be done for you.
/// world.flush();
///
/// world.trigger(Speak {
/// message: "Hello!".into(),
/// });
/// ```
///
/// Notice that we used [`World::observe`]. This is just a shorthand for spawning an [`Observer`] manually:
///
/// ```
/// # use bevy_ecs::prelude::*;
/// # let mut world = World::default();
/// # #[derive(Event)]
/// # struct Speak;
/// // These are functionally the same:
/// world.observe(|trigger: Trigger<Speak>| {});
/// world.spawn(Observer::new(|trigger: Trigger<Speak>| {}));
/// ```
///
/// Observers are systems. They can access arbitrary [`World`] data by adding [`SystemParam`]s:
///
/// ```
/// # use bevy_ecs::prelude::*;
/// # let mut world = World::default();
/// # #[derive(Event)]
/// # struct PrintNames;
/// # #[derive(Component, Debug)]
/// # struct Name;
/// world.observe(|trigger: Trigger<PrintNames>, names: Query<&Name>| {
/// for name in &names {
/// println!("{name:?}");
/// }
/// });
/// ```
///
/// Note that [`Trigger`] must always be the first parameter.
///
/// You can also add [`Commands`], which means you can spawn new entities, insert new components, etc:
///
/// ```
/// # use bevy_ecs::prelude::*;
/// # let mut world = World::default();
/// # #[derive(Event)]
/// # struct SpawnThing;
/// # #[derive(Component, Debug)]
/// # struct Thing;
/// world.observe(|trigger: Trigger<SpawnThing>, mut commands: Commands| {
/// commands.spawn(Thing);
/// });
/// ```
///
/// Observers can also trigger new events:
///
/// ```
/// # use bevy_ecs::prelude::*;
/// # let mut world = World::default();
/// # #[derive(Event)]
/// # struct A;
/// # #[derive(Event)]
/// # struct B;
/// world.observe(|trigger: Trigger<A>, mut commands: Commands| {
/// commands.trigger(B);
/// });
/// ```
///
/// When the commands are flushed (including these "nested triggers") they will be
/// recursively evaluated until there are no commands left, meaning nested triggers all
/// evaluate at the same time!
///
/// Events can be triggered for entities, which will be passed to the [`Observer`]:
///
/// ```
/// # use bevy_ecs::prelude::*;
/// # let mut world = World::default();
/// # let entity = world.spawn_empty().id();
/// #[derive(Event)]
/// struct Explode;
///
/// 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);
/// ```
///
/// You can trigger multiple entities at once:
///
/// ```
/// # use bevy_ecs::prelude::*;
/// # let mut world = World::default();
/// # let e1 = world.spawn_empty().id();
/// # let e2 = world.spawn_empty().id();
/// # #[derive(Event)]
/// # struct Explode;
/// world.trigger_targets(Explode, [e1, e2]);
/// ```
///
/// Observers can also watch _specific_ entities, which enables you to assign entity-specific logic:
///
/// ```
/// # use bevy_ecs::prelude::*;
/// # #[derive(Component, Debug)]
/// # struct Name(String);
/// # let mut world = World::default();
/// # let e1 = world.spawn_empty().id();
/// # let e2 = world.spawn_empty().id();
/// # #[derive(Event)]
/// # struct Explode;
/// world.entity_mut(e1).observe(|trigger: Trigger<Explode>, mut commands: Commands| {
/// println!("Boom!");
/// commands.entity(trigger.entity()).despawn();
/// });
///
/// world.entity_mut(e2).observe(|trigger: Trigger<Explode>, mut commands: Commands| {
/// println!("The explosion fizzles! This entity is immune!");
/// });
/// ```
///
/// If all entities watched by a given [`Observer`] are despawned, the [`Observer`] entity will also be despawned.
/// This protects against observer "garbage" building up over time.
///
/// The examples above calling [`EntityWorldMut::observe`] to add entity-specific observer logic are (once again)
/// just shorthand for spawning an [`Observer`] directly:
///
/// ```
/// # use bevy_ecs::prelude::*;
/// # let mut world = World::default();
/// # let entity = world.spawn_empty().id();
/// # #[derive(Event)]
/// # struct Explode;
/// let mut observer = Observer::new(|trigger: Trigger<Explode>| {});
/// observer.watch_entity(entity);
/// world.spawn(observer);
/// ```
///
/// Note that the [`Observer`] component is not added to the entity it is observing. Observers should always be their own entities!
///
/// You can call [`Observer::watch_entity`] more than once, which allows you to watch multiple entities with the same [`Observer`].
///
/// When first added, [`Observer`] will also create an [`ObserverState`] component, which registers the observer with the [`World`] and
/// serves as the "source of truth" of the observer.
///
/// [`SystemParam`]: crate::system::SystemParam
pub struct Observer<T: 'static, B: Bundle> {
system: BoxedObserverSystem<T, B>,
descriptor: ObserverDescriptor,
}
impl<E: Event, B: Bundle> Observer<E, B> {
/// Creates a new [`Observer`], which defaults to a "global" observer. This means it will run whenever the event `E` is triggered
/// for _any_ entity (or no entity).
pub fn new<M>(system: impl IntoObserverSystem<E, B, M>) -> Self {
Self {
system: Box::new(IntoObserverSystem::into_system(system)),
descriptor: Default::default(),
}
}
/// Observe the given `entity`. This will cause the [`Observer`] to run whenever the [`Event`] is triggered
/// for the `entity`.
pub fn with_entity(mut self, entity: Entity) -> Self {
self.descriptor.entities.push(entity);
self
}
/// Observe the given `entity`. This will cause the [`Observer`] to run whenever the [`Event`] is triggered
/// for the `entity`.
/// Note that if this is called _after_ an [`Observer`] is spawned, it will produce no effects.
pub fn watch_entity(&mut self, entity: Entity) {
self.descriptor.entities.push(entity);
}
/// Observe the given `component`. This will cause the [`Observer`] to run whenever the [`Event`] is triggered
/// with the given component target.
pub fn with_component(mut self, component: ComponentId) -> Self {
self.descriptor.components.push(component);
self
}
/// Observe the given `event`. This will cause the [`Observer`] to run whenever an event with the given [`ComponentId`]
/// is triggered.
/// # Safety
/// The type of the `event` [`ComponentId`] _must_ match the actual value
/// of the event passed into the observer system.
pub unsafe fn with_event(mut self, event: ComponentId) -> Self {
self.descriptor.events.push(event);
self
}
}
impl<E: Event, B: Bundle> Component for Observer<E, B> {
const STORAGE_TYPE: StorageType = StorageType::SparseSet;
fn register_component_hooks(hooks: &mut ComponentHooks) {
hooks.on_add(|mut world, entity, _| {
world.commands().add(move |world: &mut World| {
let event_type = world.init_component::<E>();
let mut components = Vec::new();
B::component_ids(&mut world.components, &mut world.storages, &mut |id| {
components.push(id);
});
let mut descriptor = ObserverDescriptor {
events: vec![event_type],
components,
..Default::default()
};
// Initialize System
let system: *mut dyn ObserverSystem<E, B> =
if let Some(mut observe) = world.get_mut::<Self>(entity) {
descriptor.merge(&observe.descriptor);
&mut *observe.system
} else {
return;
};
// SAFETY: World reference is exclusive and initialize does not touch system, so references do not alias
unsafe {
(*system).initialize(world);
}
{
let mut entity = world.entity_mut(entity);
if let crate::world::Entry::Vacant(entry) = entity.entry::<ObserverState>() {
entry.insert(ObserverState {
descriptor,
runner: observer_system_runner::<E, B>,
..Default::default()
});
}
}
});
});
}
}
/// Equivalent to [`BoxedSystem`](crate::system::BoxedSystem) for [`ObserverSystem`].
pub type BoxedObserverSystem<E = (), B = ()> = Box<dyn ObserverSystem<E, B>>;
fn observer_system_runner<E: Event, B: Bundle>(
mut world: DeferredWorld,
observer_trigger: ObserverTrigger,
ptr: PtrMut,
) {
let world = world.as_unsafe_world_cell();
// SAFETY: Observer was triggered so must still exist in world
let observer_cell = unsafe {
world
.get_entity(observer_trigger.observer)
.debug_checked_unwrap()
};
// SAFETY: Observer was triggered so must have an `ObserverState`
let mut state = unsafe {
observer_cell
.get_mut::<ObserverState>()
.debug_checked_unwrap()
};
// TODO: Move this check into the observer cache to avoid dynamic dispatch
// SAFETY: We only access world metadata
let last_trigger = unsafe { world.world_metadata() }.last_trigger_id();
if state.last_trigger_id == last_trigger {
return;
}
state.last_trigger_id = last_trigger;
// SAFETY: Caller ensures `ptr` is castable to `&mut T`
let trigger: Trigger<E, B> = Trigger::new(unsafe { ptr.deref_mut() }, observer_trigger);
// SAFETY: the static lifetime is encapsulated in Trigger / cannot leak out.
// Additionally, IntoObserverSystem is only implemented for functions starting
// with for<'a> Trigger<'a>, meaning users cannot specify Trigger<'static> manually,
// allowing the Trigger<'static> to be moved outside of the context of the system.
// This transmute is obviously not ideal, but it is safe. Ideally we can remove the
// static constraint from ObserverSystem, but so far we have not found a way.
let trigger: Trigger<'static, E, B> = unsafe { std::mem::transmute(trigger) };
// SAFETY: Observer was triggered so must have an `ObserverSystemComponent`
let system = unsafe {
&mut observer_cell
.get_mut::<Observer<E, B>>()
.debug_checked_unwrap()
.system
};
system.update_archetype_component_access(world);
// SAFETY:
// - `update_archetype_component_access` was just called
// - there are no outstanding references to world except a private component
// - system is an `ObserverSystem` so won't mutate world beyond the access of a `DeferredWorld`
// - system is the same type erased system from above
unsafe {
system.run_unsafe(trigger, world);
system.queue_deferred(world.into_deferred());
}
}