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use crate::entity::Entity;
use crate::system::{BoxedSystem, Command, IntoSystem};
use crate::world::World;
use crate::{self as bevy_ecs};
use bevy_ecs_macros::Component;
use thiserror::Error;
/// A small wrapper for [`BoxedSystem`] that also keeps track whether or not the system has been initialized.
#[derive(Component)]
struct RegisteredSystem {
initialized: bool,
system: BoxedSystem,
}
/// A system that has been removed from the registry.
/// It contains the system and whether or not it has been initialized.
///
/// This struct is returned by [`World::remove_system`].
pub struct RemovedSystem {
initialized: bool,
system: BoxedSystem,
}
impl RemovedSystem {
/// Is the system initialized?
/// A system is initialized the first time it's ran.
pub fn initialized(&self) -> bool {
self.initialized
}
/// The system removed from the storage.
pub fn system(self) -> BoxedSystem {
self.system
}
}
/// An identifier for a registered system.
///
/// These are opaque identifiers, keyed to a specific [`World`],
/// and are created via [`World::register_system`].
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub struct SystemId(Entity);
impl World {
/// Registers a system and returns a [`SystemId`] so it can later be called by [`World::run_system`].
///
/// It's possible to register the same systems more than once, they'll be stored separately.
///
/// This is different from adding systems to a [`Schedule`](crate::schedule::Schedule),
/// because the [`SystemId`] that is returned can be used anywhere in the [`World`] to run the associated system.
/// This allows for running systems in a pushed-based fashion.
/// Using a [`Schedule`](crate::schedule::Schedule) is still preferred for most cases
/// due to its better performance and abillity to run non-conflicting systems simultaneously.
pub fn register_system<M, S: IntoSystem<(), (), M> + 'static>(
&mut self,
system: S,
) -> SystemId {
self.register_boxed_system(Box::new(IntoSystem::into_system(system)))
}
/// Similar to [`Self::register_system`], but allows passing in a [`BoxedSystem`].
///
/// This is useful if the [`IntoSystem`] implementor has already been turned into a
/// [`System`](crate::system::System) trait object and put in a [`Box`].
pub fn register_boxed_system(&mut self, system: BoxedSystem) -> SystemId {
SystemId(
self.spawn(RegisteredSystem {
initialized: false,
system,
})
.id(),
)
}
/// Removes a registered system and returns the system, if it exists.
/// After removing a system, the [`SystemId`] becomes invalid and attempting to use it afterwards will result in errors.
/// Re-adding the removed system will register it on a new [`SystemId`].
///
/// If no system corresponds to the given [`SystemId`], this method returns an error.
/// Systems are also not allowed to remove themselves, this returns an error too.
pub fn remove_system(&mut self, id: SystemId) -> Result<RemovedSystem, RegisteredSystemError> {
match self.get_entity_mut(id.0) {
Some(mut entity) => {
let registered_system = entity
.take::<RegisteredSystem>()
.ok_or(RegisteredSystemError::SelfRemove(id))?;
entity.despawn();
Ok(RemovedSystem {
initialized: registered_system.initialized,
system: registered_system.system,
})
}
None => Err(RegisteredSystemError::SystemIdNotRegistered(id)),
}
}
/// Run stored systems by their [`SystemId`].
/// Before running a system, it must first be registered.
/// The method [`World::register_system`] stores a given system and returns a [`SystemId`].
/// This is different from [`RunSystemOnce::run_system_once`](crate::system::RunSystemOnce::run_system_once),
/// because it keeps local state between calls and change detection works correctly.
///
/// # Limitations
///
/// - Stored systems cannot be chained: they can neither have an [`In`](crate::system::In) nor return any values.
/// - Stored systems cannot be recursive, they cannot call themselves through [`Commands::run_system`](crate::system::Commands).
/// - Exclusive systems cannot be used.
///
/// # Examples
///
/// ```rust
/// # use bevy_ecs::prelude::*;
/// #[derive(Resource, Default)]
/// struct Counter(u8);
///
/// fn increment(mut counter: Local<Counter>) {
/// counter.0 += 1;
/// println!("{}", counter.0);
/// }
///
/// let mut world = World::default();
/// let counter_one = world.register_system(increment);
/// let counter_two = world.register_system(increment);
/// world.run_system(counter_one); // -> 1
/// world.run_system(counter_one); // -> 2
/// world.run_system(counter_two); // -> 1
/// ```
///
/// Change detection:
///
/// ```rust
/// # use bevy_ecs::prelude::*;
/// #[derive(Resource, Default)]
/// struct ChangeDetector;
///
/// let mut world = World::default();
/// world.init_resource::<ChangeDetector>();
/// let detector = world.register_system(|change_detector: ResMut<ChangeDetector>| {
/// if change_detector.is_changed() {
/// println!("Something happened!");
/// } else {
/// println!("Nothing happened.");
/// }
/// });
///
/// // Resources are changed when they are first added
/// let _ = world.run_system(detector); // -> Something happened!
/// let _ = world.run_system(detector); // -> Nothing happened.
/// world.resource_mut::<ChangeDetector>().set_changed();
/// let _ = world.run_system(detector); // -> Something happened!
/// ```
pub fn run_system(&mut self, id: SystemId) -> Result<(), RegisteredSystemError> {
// lookup
let mut entity = self
.get_entity_mut(id.0)
.ok_or(RegisteredSystemError::SystemIdNotRegistered(id))?;
// take ownership of system trait object
let RegisteredSystem {
mut initialized,
mut system,
} = entity
.take::<RegisteredSystem>()
.ok_or(RegisteredSystemError::Recursive(id))?;
// run the system
if !initialized {
system.initialize(self);
initialized = true;
}
system.run((), self);
system.apply_deferred(self);
// return ownership of system trait object (if entity still exists)
if let Some(mut entity) = self.get_entity_mut(id.0) {
entity.insert::<RegisteredSystem>(RegisteredSystem {
initialized,
system,
});
}
Ok(())
}
}
/// The [`Command`] type for [`World::run_system`].
///
/// This command runs systems in an exclusive and single threaded way.
/// Running slow systems can become a bottleneck.
#[derive(Debug, Clone)]
pub struct RunSystem {
system_id: SystemId,
}
impl RunSystem {
/// Creates a new [`Command`] struct, which can be added to [`Commands`](crate::system::Commands)
pub fn new(system_id: SystemId) -> Self {
Self { system_id }
}
}
impl Command for RunSystem {
#[inline]
fn apply(self, world: &mut World) {
let _ = world.run_system(self.system_id);
}
}
/// An operation with stored systems failed.
#[derive(Debug, Error)]
pub enum RegisteredSystemError {
/// A system was run by id, but no system with that id was found.
///
/// Did you forget to register it?
#[error("System {0:?} was not registered")]
SystemIdNotRegistered(SystemId),
/// A system tried to run itself recursively.
#[error("System {0:?} tried to run itself recursively")]
Recursive(SystemId),
/// A system tried to remove itself.
#[error("System {0:?} tried to remove itself")]
SelfRemove(SystemId),
}
mod tests {
use crate as bevy_ecs;
use crate::prelude::*;
#[derive(Resource, Default, PartialEq, Debug)]
struct Counter(u8);
#[test]
fn change_detection() {
#[derive(Resource, Default)]
struct ChangeDetector;
fn count_up_iff_changed(
mut counter: ResMut<Counter>,
change_detector: ResMut<ChangeDetector>,
) {
if change_detector.is_changed() {
counter.0 += 1;
}
}
let mut world = World::new();
world.init_resource::<ChangeDetector>();
world.init_resource::<Counter>();
assert_eq!(*world.resource::<Counter>(), Counter(0));
// Resources are changed when they are first added.
let id = world.register_system(count_up_iff_changed);
let _ = world.run_system(id);
assert_eq!(*world.resource::<Counter>(), Counter(1));
// Nothing changed
let _ = world.run_system(id);
assert_eq!(*world.resource::<Counter>(), Counter(1));
// Making a change
world.resource_mut::<ChangeDetector>().set_changed();
let _ = world.run_system(id);
assert_eq!(*world.resource::<Counter>(), Counter(2));
}
#[test]
fn local_variables() {
// The `Local` begins at the default value of 0
fn doubling(last_counter: Local<Counter>, mut counter: ResMut<Counter>) {
counter.0 += last_counter.0 .0;
last_counter.0 .0 = counter.0;
}
let mut world = World::new();
world.insert_resource(Counter(1));
assert_eq!(*world.resource::<Counter>(), Counter(1));
let id = world.register_system(doubling);
let _ = world.run_system(id);
assert_eq!(*world.resource::<Counter>(), Counter(1));
let _ = world.run_system(id);
assert_eq!(*world.resource::<Counter>(), Counter(2));
let _ = world.run_system(id);
assert_eq!(*world.resource::<Counter>(), Counter(4));
let _ = world.run_system(id);
assert_eq!(*world.resource::<Counter>(), Counter(8));
}
#[test]
fn nested_systems() {
use crate::system::SystemId;
#[derive(Component)]
struct Callback(SystemId);
fn nested(query: Query<&Callback>, mut commands: Commands) {
for callback in query.iter() {
commands.run_system(callback.0);
}
}
let mut world = World::new();
world.insert_resource(Counter(0));
let increment_two = world.register_system(|mut counter: ResMut<Counter>| {
counter.0 += 2;
});
let increment_three = world.register_system(|mut counter: ResMut<Counter>| {
counter.0 += 3;
});
let nested_id = world.register_system(nested);
world.spawn(Callback(increment_two));
world.spawn(Callback(increment_three));
let _ = world.run_system(nested_id);
assert_eq!(*world.resource::<Counter>(), Counter(5));
}
}