bevy/crates/bevy_ecs/src/system/function_system.rs

use crate::{
    archetype::{Archetype, ArchetypeComponentId, ArchetypeGeneration, ArchetypeId},
    component::ComponentId,
    query::{Access, FilteredAccessSet},
    system::{
        check_system_change_tick, ReadOnlySystemParamFetch, System, SystemId, SystemParam,
        SystemParamFetch, SystemParamState,
    },
    world::{World, WorldId},
};
use bevy_ecs_macros::all_tuples;
use std::{borrow::Cow, marker::PhantomData};

/// The metadata of a [`System`].
pub struct SystemMeta {
    pub(crate) id: SystemId,
    pub(crate) name: Cow<'static, str>,
    pub(crate) component_access_set: FilteredAccessSet<ComponentId>,
    pub(crate) archetype_component_access: Access<ArchetypeComponentId>,
    // NOTE: this must be kept private. making a SystemMeta non-send is irreversible to prevent
    // SystemParams from overriding each other
    is_send: bool,
    pub(crate) last_change_tick: u32,
}

impl SystemMeta {
    fn new<T>() -> Self {
        Self {
            name: std::any::type_name::<T>().into(),
            archetype_component_access: Access::default(),
            component_access_set: FilteredAccessSet::default(),
            is_send: true,
            id: SystemId::new(),
            last_change_tick: 0,
        }
    }

    /// Returns true if the system is [`Send`].
    #[inline]
    pub fn is_send(&self) -> bool {
        self.is_send
    }

    /// Sets the system to be not [`Send`].
    ///
    /// This is irreversible.
    #[inline]
    pub fn set_non_send(&mut self) {
        self.is_send = false;
    }
}

// TODO: Actually use this in FunctionSystem. We should probably only do this once Systems are constructed using a World reference
// (to avoid the need for unwrapping to retrieve SystemMeta)
/// Holds on to persistent state required to drive [`SystemParam`] for a [`System`].  
pub struct SystemState<Param: SystemParam> {
    meta: SystemMeta,
    param_state: <Param as SystemParam>::Fetch,
    world_id: WorldId,
    archetype_generation: ArchetypeGeneration,
}

impl<Param: SystemParam> SystemState<Param> {
    pub fn new(world: &mut World) -> Self {
        let config = <Param::Fetch as SystemParamState>::default_config();
        Self::with_config(world, config)
    }

    pub fn with_config(
        world: &mut World,
        config: <Param::Fetch as SystemParamState>::Config,
    ) -> Self {
        let mut meta = SystemMeta::new::<Param>();
        let param_state = <Param::Fetch as SystemParamState>::init(world, &mut meta, config);
        Self {
            meta,
            param_state,
            world_id: world.id(),
            archetype_generation: ArchetypeGeneration::initial(),
        }
    }

    #[inline]
    pub fn meta(&self) -> &SystemMeta {
        &self.meta
    }

    /// Retrieve the [`SystemParam`] values. This can only be called when all parameters are read-only.
    #[inline]
    pub fn get<'a>(&'a mut self, world: &'a World) -> <Param::Fetch as SystemParamFetch<'a>>::Item
    where
        Param::Fetch: ReadOnlySystemParamFetch,
    {
        self.validate_world_and_update_archetypes(world);
        // SAFE: Param is read-only and doesn't allow mutable access to World. It also matches the World this SystemState was created with.
        unsafe { self.get_unchecked_manual(world) }
    }

    /// Retrieve the mutable [`SystemParam`] values.
    #[inline]
    pub fn get_mut<'a>(
        &'a mut self,
        world: &'a mut World,
    ) -> <Param::Fetch as SystemParamFetch<'a>>::Item {
        self.validate_world_and_update_archetypes(world);
        // SAFE: World is uniquely borrowed and matches the World this SystemState was created with.
        unsafe { self.get_unchecked_manual(world) }
    }

    /// Applies all state queued up for [`SystemParam`] values. For example, this will apply commands queued up
    /// by a [`Commands`](`super::Commands`) parameter to the given [`World`].
    /// This function should be called manually after the values returned by [`SystemState::get`] and [`SystemState::get_mut`]  
    /// are finished being used.
    pub fn apply(&mut self, world: &mut World) {
        self.param_state.apply(world);
    }

    #[inline]
    pub fn matches_world(&self, world: &World) -> bool {
        self.world_id == world.id()
    }

    fn validate_world_and_update_archetypes(&mut self, world: &World) {
        assert!(self.matches_world(world), "Encountered a mismatched World. A SystemState cannot be used with Worlds other than the one it was created with.");
        let archetypes = world.archetypes();
        let new_generation = archetypes.generation();
        let old_generation = std::mem::replace(&mut self.archetype_generation, new_generation);
        let archetype_index_range = old_generation.value()..new_generation.value();

        for archetype_index in archetype_index_range {
            self.param_state.new_archetype(
                &archetypes[ArchetypeId::new(archetype_index)],
                &mut self.meta,
            );
        }
    }

    /// Retrieve the [`SystemParam`] values. This will not update archetypes automatically.
    ///
    /// # Safety
    /// This call might access any of the input parameters in a way that violates Rust's mutability rules. Make sure the data
    /// access is safe in the context of global [`World`] access. The passed-in [`World`] _must_ be the [`World`] the [`SystemState`] was
    /// created with.   
    #[inline]
    pub unsafe fn get_unchecked_manual<'a>(
        &'a mut self,
        world: &'a World,
    ) -> <Param::Fetch as SystemParamFetch<'a>>::Item {
        let change_tick = world.increment_change_tick();
        let param = <Param::Fetch as SystemParamFetch>::get_param(
            &mut self.param_state,
            &self.meta,
            world,
            change_tick,
        );
        self.meta.last_change_tick = change_tick;
        param
    }
}

/// Conversion trait to turn something into a [`System`].
///
/// Use this to get a system from a function. Also note that every system implements this trait as
/// well.
///
/// # Examples
///
/// ```
/// use bevy_ecs::system::IntoSystem;
/// use bevy_ecs::system::Res;
///
/// fn my_system_function(an_usize_resource: Res<usize>) {}
///
/// let system = my_system_function.system();
/// ```
pub trait IntoSystem<Params, SystemType: System> {
    /// Turns this value into its corresponding [`System`].
    fn system(self) -> SystemType;
}

// Systems implicitly implement IntoSystem
impl<Sys: System> IntoSystem<(), Sys> for Sys {
    fn system(self) -> Sys {
        self
    }
}

/// Wrapper type to mark a [`SystemParam`] as an input.
///
/// [`System`]s may take an optional input which they require to be passed to them when they
/// are being [`run`](System::run). For [`FunctionSystems`](FunctionSystem) the input may be marked
/// with this `In` type, but only the first param of a function may be tagged as an input. This also
/// means a system can only have one or zero input paramaters.
///
/// # Examples
///
/// Here is a simple example of a system that takes a [`usize`] returning the square of it.
///
/// ```
/// use bevy_ecs::prelude::*;
///
/// fn main() {
///     let mut square_system = square.system();
///
///     let mut world = World::default();
///     square_system.initialize(&mut world);
///     assert_eq!(square_system.run(12, &mut world), 144);
/// }
///
/// fn square(In(input): In<usize>) -> usize {
///     input * input
/// }
/// ```
pub struct In<In>(pub In);
pub struct InputMarker;

/// The [`System`] counter part of an ordinary function.
///
/// You get this by calling [`IntoSystem::system`]  on a function that only accepts
/// [`SystemParam`]s. The output of the system becomes the functions return type, while the input
/// becomes the functions [`In`] tagged parameter or `()` if no such paramater exists.
pub struct FunctionSystem<In, Out, Param, Marker, F>
where
    Param: SystemParam,
{
    func: F,
    param_state: Option<Param::Fetch>,
    system_meta: SystemMeta,
    config: Option<<Param::Fetch as SystemParamState>::Config>,
    // NOTE: PhantomData<fn()-> T> gives this safe Send/Sync impls
    #[allow(clippy::type_complexity)]
    marker: PhantomData<fn() -> (In, Out, Marker)>,
}

impl<In, Out, Param: SystemParam, Marker, F> FunctionSystem<In, Out, Param, Marker, F> {
    /// Gives mutable access to the systems config via a callback. This is useful to set up system
    /// [`Local`](crate::system::Local)s.
    ///
    /// # Examples
    ///
    /// ```
    /// # use bevy_ecs::prelude::*;
    /// # let world = &mut World::default();
    /// fn local_is_42(local: Local<usize>) {
    ///     assert_eq!(*local, 42);
    /// }
    /// let mut system = local_is_42.system().config(|config| config.0 = Some(42));
    /// system.initialize(world);
    /// system.run((), world);
    /// ```
    pub fn config(
        mut self,
        f: impl FnOnce(&mut <Param::Fetch as SystemParamState>::Config),
    ) -> Self {
        f(self.config.as_mut().unwrap());
        self
    }
}

impl<In, Out, Param, Marker, F> IntoSystem<Param, FunctionSystem<In, Out, Param, Marker, F>> for F
where
    In: 'static,
    Out: 'static,
    Param: SystemParam + 'static,
    Marker: 'static,
    F: SystemParamFunction<In, Out, Param, Marker> + Send + Sync + 'static,
{
    fn system(self) -> FunctionSystem<In, Out, Param, Marker, F> {
        FunctionSystem {
            func: self,
            param_state: None,
            config: Some(<Param::Fetch as SystemParamState>::default_config()),
            system_meta: SystemMeta::new::<F>(),
            marker: PhantomData,
        }
    }
}

impl<In, Out, Param, Marker, F> System for FunctionSystem<In, Out, Param, Marker, F>
where
    In: 'static,
    Out: 'static,
    Param: SystemParam + 'static,
    Marker: 'static,
    F: SystemParamFunction<In, Out, Param, Marker> + Send + Sync + 'static,
{
    type In = In;
    type Out = Out;

    #[inline]
    fn name(&self) -> Cow<'static, str> {
        self.system_meta.name.clone()
    }

    #[inline]
    fn id(&self) -> SystemId {
        self.system_meta.id
    }

    #[inline]
    fn new_archetype(&mut self, archetype: &Archetype) {
        let param_state = self.param_state.as_mut().unwrap();
        param_state.new_archetype(archetype, &mut self.system_meta);
    }

    #[inline]
    fn component_access(&self) -> &Access<ComponentId> {
        &self.system_meta.component_access_set.combined_access()
    }

    #[inline]
    fn archetype_component_access(&self) -> &Access<ArchetypeComponentId> {
        &self.system_meta.archetype_component_access
    }

    #[inline]
    fn is_send(&self) -> bool {
        self.system_meta.is_send
    }

    #[inline]
    unsafe fn run_unsafe(&mut self, input: Self::In, world: &World) -> Self::Out {
        let change_tick = world.increment_change_tick();
        let out = self.func.run(
            input,
            self.param_state.as_mut().unwrap(),
            &self.system_meta,
            world,
            change_tick,
        );
        self.system_meta.last_change_tick = change_tick;
        out
    }

    #[inline]
    fn apply_buffers(&mut self, world: &mut World) {
        let param_state = self.param_state.as_mut().unwrap();
        param_state.apply(world);
    }

    #[inline]
    fn initialize(&mut self, world: &mut World) {
        self.param_state = Some(<Param::Fetch as SystemParamState>::init(
            world,
            &mut self.system_meta,
            self.config.take().unwrap(),
        ));
    }

    #[inline]
    fn check_change_tick(&mut self, change_tick: u32) {
        check_system_change_tick(
            &mut self.system_meta.last_change_tick,
            change_tick,
            self.system_meta.name.as_ref(),
        );
    }
}

/// A trait implemented for all functions that can be used as [`System`]s.
pub trait SystemParamFunction<In, Out, Param: SystemParam, Marker>: Send + Sync + 'static {
    /// # Safety
    ///
    /// This call might access any of the input parameters in an unsafe way. Make sure the data
    /// access is safe in the context of the system scheduler.
    unsafe fn run(
        &mut self,
        input: In,
        state: &mut Param::Fetch,
        system_meta: &SystemMeta,
        world: &World,
        change_tick: u32,
    ) -> Out;
}

macro_rules! impl_system_function {
    ($($param: ident),*) => {
        #[allow(non_snake_case)]
        impl<Out, Func, $($param: SystemParam),*> SystemParamFunction<(), Out, ($($param,)*), ()> for Func
        where
            Func:
                FnMut($($param),*) -> Out +
                FnMut($(<<$param as SystemParam>::Fetch as SystemParamFetch>::Item),*) -> Out + Send + Sync + 'static, Out: 'static
        {
            #[inline]
            unsafe fn run(&mut self, _input: (), state: &mut <($($param,)*) as SystemParam>::Fetch, system_meta: &SystemMeta, world: &World, change_tick: u32) -> Out {
                let ($($param,)*) = <<($($param,)*) as SystemParam>::Fetch as SystemParamFetch>::get_param(state, system_meta, world, change_tick);
                self($($param),*)
            }
        }

        #[allow(non_snake_case)]
        impl<Input, Out, Func, $($param: SystemParam),*> SystemParamFunction<Input, Out, ($($param,)*), InputMarker> for Func
        where
            Func:
                FnMut(In<Input>, $($param),*) -> Out +
                FnMut(In<Input>, $(<<$param as SystemParam>::Fetch as SystemParamFetch>::Item),*) -> Out + Send + Sync + 'static, Out: 'static
        {
            #[inline]
            unsafe fn run(&mut self, input: Input, state: &mut <($($param,)*) as SystemParam>::Fetch, system_meta: &SystemMeta, world: &World, change_tick: u32) -> Out {
                let ($($param,)*) = <<($($param,)*) as SystemParam>::Fetch as SystemParamFetch>::get_param(state, system_meta, world, change_tick);
                self(In(input), $($param),*)
            }
        }
    };
}

all_tuples!(impl_system_function, 0, 16, F);