bevy_ecs/schedule/executor/
single_threaded.rs

use core::panic::AssertUnwindSafe;
use fixedbitset::FixedBitSet;

#[cfg(feature = "trace")]
use tracing::info_span;

#[cfg(feature = "std")]
use std::eprintln;

use crate::{
    error::{default_error_handler, BevyError, ErrorContext},
    schedule::{is_apply_deferred, BoxedCondition, ExecutorKind, SystemExecutor, SystemSchedule},
    world::World,
};

use super::__rust_begin_short_backtrace;

/// Runs the schedule using a single thread.
///
/// Useful if you're dealing with a single-threaded environment, saving your threads for
/// other things, or just trying minimize overhead.
#[derive(Default)]
pub struct SingleThreadedExecutor {
    /// System sets whose conditions have been evaluated.
    evaluated_sets: FixedBitSet,
    /// Systems that have run or been skipped.
    completed_systems: FixedBitSet,
    /// Systems that have run but have not had their buffers applied.
    unapplied_systems: FixedBitSet,
    /// Setting when true applies deferred system buffers after all systems have run
    apply_final_deferred: bool,
}

impl SystemExecutor for SingleThreadedExecutor {
    fn kind(&self) -> ExecutorKind {
        ExecutorKind::SingleThreaded
    }

    fn init(&mut self, schedule: &SystemSchedule) {
        // pre-allocate space
        let sys_count = schedule.system_ids.len();
        let set_count = schedule.set_ids.len();
        self.evaluated_sets = FixedBitSet::with_capacity(set_count);
        self.completed_systems = FixedBitSet::with_capacity(sys_count);
        self.unapplied_systems = FixedBitSet::with_capacity(sys_count);
    }

    fn run(
        &mut self,
        schedule: &mut SystemSchedule,
        world: &mut World,
        _skip_systems: Option<&FixedBitSet>,
        error_handler: fn(BevyError, ErrorContext),
    ) {
        // If stepping is enabled, make sure we skip those systems that should
        // not be run.
        #[cfg(feature = "bevy_debug_stepping")]
        if let Some(skipped_systems) = _skip_systems {
            // mark skipped systems as completed
            self.completed_systems |= skipped_systems;
        }

        for system_index in 0..schedule.systems.len() {
            #[cfg(feature = "trace")]
            let name = schedule.systems[system_index].name();
            #[cfg(feature = "trace")]
            let should_run_span = info_span!("check_conditions", name = &*name).entered();

            let mut should_run = !self.completed_systems.contains(system_index);
            for set_idx in schedule.sets_with_conditions_of_systems[system_index].ones() {
                if self.evaluated_sets.contains(set_idx) {
                    continue;
                }

                // evaluate system set's conditions
                let set_conditions_met =
                    evaluate_and_fold_conditions(&mut schedule.set_conditions[set_idx], world);

                if !set_conditions_met {
                    self.completed_systems
                        .union_with(&schedule.systems_in_sets_with_conditions[set_idx]);
                }

                should_run &= set_conditions_met;
                self.evaluated_sets.insert(set_idx);
            }

            // evaluate system's conditions
            let system_conditions_met =
                evaluate_and_fold_conditions(&mut schedule.system_conditions[system_index], world);

            should_run &= system_conditions_met;

            let system = &mut schedule.systems[system_index];
            if should_run {
                let valid_params = match system.validate_param(world) {
                    Ok(()) => true,
                    Err(e) => {
                        if !e.skipped {
                            error_handler(
                                e.into(),
                                ErrorContext::System {
                                    name: system.name(),
                                    last_run: system.get_last_run(),
                                },
                            );
                        }
                        false
                    }
                };

                should_run &= valid_params;
            }

            #[cfg(feature = "trace")]
            should_run_span.exit();

            // system has either been skipped or will run
            self.completed_systems.insert(system_index);

            if !should_run {
                continue;
            }

            if is_apply_deferred(system) {
                self.apply_deferred(schedule, world);
                continue;
            }

            let f = AssertUnwindSafe(|| {
                if system.is_exclusive() {
                    if let Err(err) = __rust_begin_short_backtrace::run(system, world) {
                        error_handler(
                            err,
                            ErrorContext::System {
                                name: system.name(),
                                last_run: system.get_last_run(),
                            },
                        );
                    }
                } else {
                    // Use run_unsafe to avoid immediately applying deferred buffers
                    let world = world.as_unsafe_world_cell();
                    system.update_archetype_component_access(world);
                    // SAFETY: We have exclusive, single-threaded access to the world and
                    // update_archetype_component_access is being called immediately before this.
                    unsafe {
                        if let Err(err) = __rust_begin_short_backtrace::run_unsafe(system, world) {
                            error_handler(
                                err,
                                ErrorContext::System {
                                    name: system.name(),
                                    last_run: system.get_last_run(),
                                },
                            );
                        }
                    };
                }
            });

            #[cfg(feature = "std")]
            #[expect(clippy::print_stderr, reason = "Allowed behind `std` feature gate.")]
            {
                if let Err(payload) = std::panic::catch_unwind(f) {
                    eprintln!("Encountered a panic in system `{}`!", &*system.name());
                    std::panic::resume_unwind(payload);
                }
            }

            #[cfg(not(feature = "std"))]
            {
                (f)();
            }

            self.unapplied_systems.insert(system_index);
        }

        if self.apply_final_deferred {
            self.apply_deferred(schedule, world);
        }
        self.evaluated_sets.clear();
        self.completed_systems.clear();
    }

    fn set_apply_final_deferred(&mut self, apply_final_deferred: bool) {
        self.apply_final_deferred = apply_final_deferred;
    }
}

impl SingleThreadedExecutor {
    /// Creates a new single-threaded executor for use in a [`Schedule`].
    ///
    /// [`Schedule`]: crate::schedule::Schedule
    pub const fn new() -> Self {
        Self {
            evaluated_sets: FixedBitSet::new(),
            completed_systems: FixedBitSet::new(),
            unapplied_systems: FixedBitSet::new(),
            apply_final_deferred: true,
        }
    }

    fn apply_deferred(&mut self, schedule: &mut SystemSchedule, world: &mut World) {
        for system_index in self.unapplied_systems.ones() {
            let system = &mut schedule.systems[system_index];
            system.apply_deferred(world);
        }

        self.unapplied_systems.clear();
    }
}

fn evaluate_and_fold_conditions(conditions: &mut [BoxedCondition], world: &mut World) -> bool {
    let error_handler: fn(BevyError, ErrorContext) = default_error_handler();

    #[expect(
        clippy::unnecessary_fold,
        reason = "Short-circuiting here would prevent conditions from mutating their own state as needed."
    )]
    conditions
        .iter_mut()
        .map(|condition| {
            match condition.validate_param(world) {
                Ok(()) => (),
                Err(e) => {
                    if !e.skipped {
                        error_handler(
                            e.into(),
                            ErrorContext::System {
                                name: condition.name(),
                                last_run: condition.get_last_run(),
                            },
                        );
                    }
                    return false;
                }
            }
            __rust_begin_short_backtrace::readonly_run(&mut **condition, world)
        })
        .fold(true, |acc, res| acc && res)
}
bevy_ecs/schedule/executor/single_threaded.rs

bevy_ecs/schedule/executor/
single_threaded.rs
