Trait bevy::ecs::system::SystemParam

pub trait SystemParam: Sized {
    type Fetch: for<'s, 'w> SystemParamFetch<'w, 's>;
}
Expand description

A parameter that can be used in a System.

Derive

This trait can be derived with the super::SystemParam macro. This macro only works if each field on the derived struct implements SystemParam. Note: There are additional requirements on the field types. See the Generic SystemParams section for details and workarounds of the probable cause if this derive causes an error to be emitted.

The struct for which SystemParam is derived must (currently) have exactly two lifetime parameters. The first is the lifetime of the world, and the second the lifetime of the parameter’s state.

Attributes

#[system_param(ignore)]: Can be added to any field in the struct. Fields decorated with this attribute will be created with the default value upon realisation. This is most useful for PhantomData fields, to ensure that the required lifetimes are used, as shown in the example.

Example

use std::marker::PhantomData;
use bevy_ecs::system::SystemParam;

#[derive(SystemParam)]
struct MyParam<'w, 's> {
    foo: Res<'w, SomeResource>,
    #[system_param(ignore)]
    marker: PhantomData<&'s ()>,
}

fn my_system(param: MyParam) {
    // Access the resource through `param.foo`
}

Generic SystemParams

When using the derive macro, you may see an error in the form of:

expected ... [ParamType]
found associated type `<<[ParamType] as SystemParam>::Fetch as SystemParamFetch<'_, '_>>::Item`

where [ParamType] is the type of one of your fields. To solve this error, you can wrap the field of type [ParamType] with StaticSystemParam (i.e. StaticSystemParam<[ParamType]>).

Details

The derive macro requires that the SystemParam implementation of each field F’s Fetch’s Item is itself F (ignoring lifetimes for simplicity). This assumption is due to type inference reasons, so that the derived SystemParam can be used as an argument to a function system. If the compiler cannot validate this property for [ParamType], it will error in the form shown above.

This will most commonly occur when working with SystemParams generically, as the requirement has not been proven to the compiler.

Required Associated Types§

Implementations on Foreign Types§

Implementors§