Struct Local 

pub struct Local<'s, T: FromWorld + Send + 'static>(/* private fields */);

A SystemParam that provides a system-private value of T that persists across system calls.

The initial value is created by calling T’s FromWorld::from_world (or Default::default if T: Default).

A local may only be accessed by the system itself and is therefore not visible to other systems. If two or more systems specify the same local type each will have their own unique local. If multiple SystemParams within the same system each specify the same local type each will get their own distinct data storage.

The supplied lifetime parameter is the SystemParams 's lifetime.

Examples

fn counter(mut count: Local<u32>) -> u32 {
    *count += 1;
    *count
}
let mut counter_system = IntoSystem::into_system(counter);
counter_system.initialize(world);

// Counter is initialized to u32's default value of 0, and increases to 1 on first run.
assert_eq!(counter_system.run((), world).unwrap(), 1);
// Counter gets the same value and increases to 2 on its second call.
assert_eq!(counter_system.run((), world).unwrap(), 2);

A simple way to set a different default value for a local is by wrapping the value with an Option.

fn counter_from_10(mut count: Local<Option<u32>>) -> u32 {
    let count = count.get_or_insert(10);
    *count += 1;
    *count
}
let mut counter_system = IntoSystem::into_system(counter_from_10);
counter_system.initialize(world);

// Counter is initialized at 10, and increases to 11 on first run.
assert_eq!(counter_system.run((), world).unwrap(), 11);
// Counter is only increased by 1 on subsequent runs.
assert_eq!(counter_system.run((), world).unwrap(), 12);

A system can have multiple Local values with the same type, each with distinct values.

fn double_counter(mut count: Local<u32>, mut double_count: Local<u32>) -> (u32, u32) {
    *count += 1;
    *double_count += 2;
    (*count, *double_count)
}
let mut counter_system = IntoSystem::into_system(double_counter);
counter_system.initialize(world);

assert_eq!(counter_system.run((), world).unwrap(), (1, 2));
assert_eq!(counter_system.run((), world).unwrap(), (2, 4));

This example shows that two systems using the same type for their own Local get distinct locals.

fn write_to_local(mut local: Local<usize>) {
    *local = 42;
}
fn read_from_local(local: Local<usize>) -> usize {
    *local
}
let mut write_system = IntoSystem::into_system(write_to_local);
let mut read_system = IntoSystem::into_system(read_from_local);
write_system.initialize(world);
read_system.initialize(world);

assert_eq!(read_system.run((), world).unwrap(), 0);
write_system.run((), world);
// The read local is still 0 due to the locals not being shared.
assert_eq!(read_system.run((), world).unwrap(), 0);

You can use a Local to avoid reallocating memory every system call.

fn some_system(mut vec: Local<Vec<u32>>) {
    // Do your regular system logic, using the vec, as normal.

    // At end of function, clear the vec's contents so its empty for next system call.
    // If it's possible the capacity could get too large, you may want to check and resize that as well.
    vec.clear();
}

N.B. A Locals value cannot be read or written to outside of the containing system. To add configuration to a system, convert a capturing closure into the system instead:

struct Config(u32);
#[derive(Resource)]
struct MyU32Wrapper(u32);
fn reset_to_system(value: Config) -> impl FnMut(ResMut<MyU32Wrapper>) {
    move |mut val| val.0 = value.0
}

// .add_systems(reset_to_system(my_config))

Trait Implementations

impl<'s, T: Debug + FromWorld + Send + 'static> Debug for Local<'s, T>

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

Formats the value using the given formatter.

impl<'s, T: FromWorld + Send + 'static> Deref for Local<'s, T>

type Target = T

The resulting type after dereferencing.

fn deref(&self) -> &Self::Target

Dereferences the value.

impl<'s, T: FromWorld + Send + 'static> DerefMut for Local<'s, T>

fn deref_mut(&mut self) -> &mut Self::Target

Mutably dereferences the value.

impl<'_s, T: FromWorld + Send + 'static> ExclusiveSystemParam for Local<'_s, T>

type State = SyncCell<T>

Used to store data which persists across invocations of a system.

type Item<'s> = Local<'s, T>

The item type returned when constructing this system param. See SystemParam::Item.

fn init(world: &mut World, _system_meta: &mut SystemMeta) -> Self::State

Creates a new instance of this param’s State.

fn get_param<'s>( state: &'s mut Self::State, _system_meta: &SystemMeta, ) -> Self::Item<'s>

Creates a parameter to be passed into an ExclusiveSystemParamFunction.

impl<'s, 'a, T: FromWorld + Send + 'static> IntoIterator for &'a Local<'s, T>

where &'a T: IntoIterator,

type Item = <&'a T as IntoIterator>::Item

The type of the elements being iterated over.

type IntoIter = <&'a T as IntoIterator>::IntoIter

Which kind of iterator are we turning this into?

fn into_iter(self) -> Self::IntoIter

Creates an iterator from a value.

impl<'s, 'a, T: FromWorld + Send + 'static> IntoIterator for &'a mut Local<'s, T>

where &'a mut T: IntoIterator,

type Item = <&'a mut T as IntoIterator>::Item

The type of the elements being iterated over.

type IntoIter = <&'a mut T as IntoIterator>::IntoIter

Which kind of iterator are we turning this into?

fn into_iter(self) -> Self::IntoIter

Creates an iterator from a value.

impl<'a, T: FromWorld + Send + 'static> SystemParam for Local<'a, T>

type State = SyncCell<T>

Used to store data which persists across invocations of a system.

type Item<'w, 's> = Local<'s, T>

The item type returned when constructing this system param. The value of this associated type should be Self, instantiated with new lifetimes.

fn init_state(world: &mut World) -> Self::State

Creates a new instance of this param’s State.

fn init_access( _state: &Self::State, _system_meta: &mut SystemMeta, _component_access_set: &mut FilteredAccessSet, _world: &mut World, )

Registers any World access used by this SystemParam

unsafe fn get_param<'w, 's>( state: &'s mut Self::State, _system_meta: &SystemMeta, _world: UnsafeWorldCell<'w>, _change_tick: Tick, ) -> Self::Item<'w, 's>

Creates a parameter to be passed into a SystemParamFunction.

fn apply(state: &mut Self::State, system_meta: &SystemMeta, world: &mut World)

Applies any deferred mutations stored in this SystemParam’s state. This is used to apply Commands during ApplyDeferred.

fn queue( state: &mut Self::State, system_meta: &SystemMeta, world: DeferredWorld<'_>, )

Queues any deferred mutations to be applied at the next ApplyDeferred.

unsafe fn validate_param( state: &mut Self::State, system_meta: &SystemMeta, world: UnsafeWorldCell<'_>, ) -> Result<(), SystemParamValidationError>

Validates that the param can be acquired by the get_param.

impl<'s, T: FromWorld + Send + 'static> SystemParamBuilder<Local<'s, T>> for LocalBuilder<T>

fn build(self, _world: &mut World) -> <Local<'s, T> as SystemParam>::State

Registers any World access used by this SystemParam and creates a new instance of this param’s State.

fn build_state(self, world: &mut World) -> SystemState<P>

Create a SystemState from a SystemParamBuilder. To create a system, call SystemState::build_system on the result.

impl<'s, T: FromWorld + Send + 'static> ReadOnlySystemParam for Local<'s, T>