Trait System

pub trait System:
    Send
    + Sync
    + 'static {
    type In: SystemInput;
    type Out;

    // Required methods
    fn name(&self) -> Cow<'static, str>;
    fn component_access(&self) -> &Access<ComponentId>;
    fn archetype_component_access(&self) -> &Access<ArchetypeComponentId>;
    fn is_send(&self) -> bool;
    fn is_exclusive(&self) -> bool;
    fn has_deferred(&self) -> bool;
    unsafe fn run_unsafe(
        &mut self,
        input: SystemIn<'_, Self>,
        world: UnsafeWorldCell<'_>,
    ) -> Self::Out;
    fn apply_deferred(&mut self, world: &mut World);
    fn queue_deferred(&mut self, world: DeferredWorld<'_>);
    unsafe fn validate_param_unsafe(
        &mut self,
        world: UnsafeWorldCell<'_>,
    ) -> bool;
    fn initialize(&mut self, _world: &mut World);
    fn update_archetype_component_access(&mut self, world: UnsafeWorldCell<'_>);
    fn check_change_tick(&mut self, change_tick: Tick);
    fn get_last_run(&self) -> Tick;
    fn set_last_run(&mut self, last_run: Tick);

    // Provided methods
    fn type_id(&self) -> TypeId { ... }
    fn run(&mut self, input: SystemIn<'_, Self>, world: &mut World) -> Self::Out { ... }
    fn validate_param(&mut self, world: &World) -> bool { ... }
    fn default_system_sets(&self) -> Vec<InternedSystemSet> { ... }
}

An ECS system that can be added to a Schedule

Systems are functions with all arguments implementing SystemParam.

Systems are added to an application using App::add_systems(Update, my_system) or similar methods, and will generally run once per pass of the main loop.

Systems are executed in parallel, in opportunistic order; data access is managed automatically. It’s possible to specify explicit execution order between specific systems, see IntoSystemConfigs.

Required Associated Types

type In: SystemInput

The system’s input.

type Out

The system’s output.

Required Methods

fn name(&self) -> Cow<'static, str>

Returns the system’s name.

fn component_access(&self) -> &Access<ComponentId>

Returns the system’s component Access.

fn archetype_component_access(&self) -> &Access<ArchetypeComponentId>

Returns the system’s archetype component Access.

fn is_send(&self) -> bool

Returns true if the system is Send.

fn is_exclusive(&self) -> bool

Returns true if the system must be run exclusively.

fn has_deferred(&self) -> bool

Returns true if system has deferred buffers.

unsafe fn run_unsafe( &mut self, input: SystemIn<'_, Self>, world: UnsafeWorldCell<'_>, ) -> Self::Out

Runs the system with the given input in the world. Unlike System::run, this function can be called in parallel with other systems and may break Rust’s aliasing rules if used incorrectly, making it unsafe to call.

Unlike System::run, this will not apply deferred parameters, which must be independently applied by calling System::apply_deferred at later point in time.

Safety

• The caller must ensure that world has permission to access any world data registered in archetype_component_access. There must be no conflicting simultaneous accesses while the system is running.
• The method System::update_archetype_component_access must be called at some point before this one, with the same exact World. If System::update_archetype_component_access panics (or otherwise does not return for any reason), this method must not be called.

fn apply_deferred(&mut self, world: &mut World)

Applies any Deferred system parameters (or other system buffers) of this system to the world.

This is where Commands get applied.

fn queue_deferred(&mut self, world: DeferredWorld<'_>)

Enqueues any Deferred system parameters (or other system buffers) of this system into the world’s command buffer.

unsafe fn validate_param_unsafe(&mut self, world: UnsafeWorldCell<'_>) -> bool

Validates that all parameters can be acquired and that system can run without panic. Built-in executors use this to prevent invalid systems from running.

However calling and respecting System::validate_param_unsafe or it’s safe variant is not a strict requirement, both System::run and System::run_unsafe should provide their own safety mechanism to prevent undefined behavior.

This method has to be called directly before System::run_unsafe with no other (relevant) world mutations in between. Otherwise, while it won’t lead to any undefined behavior, the validity of the param may change.

Safety

• The caller must ensure that world has permission to access any world data registered in archetype_component_access. There must be no conflicting simultaneous accesses while the system is running.
• The method System::update_archetype_component_access must be called at some point before this one, with the same exact World. If System::update_archetype_component_access panics (or otherwise does not return for any reason), this method must not be called.

fn initialize(&mut self, _world: &mut World)

Initialize the system.

fn update_archetype_component_access(&mut self, world: UnsafeWorldCell<'_>)

Update the system’s archetype component Access.

Note for implementors

world may only be used to access metadata. This can be done in safe code via functions such as UnsafeWorldCell::archetypes.

fn check_change_tick(&mut self, change_tick: Tick)

Checks any Ticks stored on this system and wraps their value if they get too old.

This method must be called periodically to ensure that change detection behaves correctly. When using bevy’s default configuration, this will be called for you as needed.

fn get_last_run(&self) -> Tick

Gets the tick indicating the last time this system ran.

fn set_last_run(&mut self, last_run: Tick)

Overwrites the tick indicating the last time this system ran.

Warning

This is a complex and error-prone operation, that can have unexpected consequences on any system relying on this code. However, it can be an essential escape hatch when, for example, you are trying to synchronize representations using change detection and need to avoid infinite recursion.

Provided Methods

fn type_id(&self) -> TypeId

Returns the TypeId of the underlying system type.

fn run(&mut self, input: SystemIn<'_, Self>, world: &mut World) -> Self::Out

Runs the system with the given input in the world.

For read-only systems, see run_readonly, which can be called using &World.

Unlike System::run_unsafe, this will apply deferred parameters immediately.

fn validate_param(&mut self, world: &World) -> bool

Safe version of System::validate_param_unsafe. that runs on exclusive, single-threaded world pointer.

fn default_system_sets(&self) -> Vec<InternedSystemSet>

Returns the system’s default system sets.

Each system will create a default system set that contains the system.

Trait Implementations

impl<In, Out> Debug for dyn System<In = In, Out = Out>

where In: SystemInput + 'static, Out: 'static,

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

Implementors

impl<A, B> System for PipeSystem<A, B>

where A: System, B: System, for<'a> B::In: SystemInput<Inner<'a> = A::Out>,

type In = <A as System>::In

type Out = <B as System>::Out

impl<A, B, Func> System for CombinatorSystem<Func, A, B>

where Func: Combine<A, B> + 'static, A: System, B: System,

type In = <Func as Combine<A, B>>::In

type Out = <Func as Combine<A, B>>::Out

impl<Func, S> System for AdapterSystem<Func, S>

where Func: Adapt<S>, S: System,

type In = <Func as Adapt<S>>::In

type Out = <Func as Adapt<S>>::Out

impl<Marker, F> System for ExclusiveFunctionSystem<Marker, F>

where Marker: 'static, F: ExclusiveSystemParamFunction<Marker>,

type In = <F as ExclusiveSystemParamFunction<Marker>>::In

type Out = <F as ExclusiveSystemParamFunction<Marker>>::Out

impl<Marker, F> System for FunctionSystem<Marker, F>

where Marker: 'static, F: SystemParamFunction<Marker>,

type In = <F as SystemParamFunction<Marker>>::In

type Out = <F as SystemParamFunction<Marker>>::Out