Struct gecs::OneOf

pub struct OneOf { /* private fields */ }

A special parameter type for ECS query closures to match one of multiple components.

The OneOf<A, B, C, ...> pseudo-type argument to an ECS closure allows a query to match exactly one of the given component types. The component can be accessed as a &T (or &mut T for a &mut OneOf<...>), and is effectively duck-typed within the body of the closure – no traits or where clauses are needed to access elements of the component, so long as the same element or method is available on all potential results of the OneOf binding. If an archetype has more than one of the requested components in a OneOf, this will result in a compilation error. This query will only match archetypes that have one of the requested components.

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This is not a real struct and does not exist in any live code, it is a pseudo-type that only has meaning within an ECS query closure when parsed by the operation macro. It is presented here as a standalone struct for documentation purposes only.

Example

use gecs::prelude::*;

pub struct CompA(pub u32);
pub struct CompB(pub u32);
pub struct CompC(pub u32);

ecs_world! {
    ecs_archetype!(ArchFoo, 5, CompA, CompB);
    ecs_archetype!(ArchBar, 5, CompA, CompC);
}

fn main() {
    let mut world = World::default();

    let entity_a = world.archetype_mut::<ArchFoo>().push((CompA(1), CompB(10)));
    let entity_b = world.archetype_mut::<ArchBar>().push((CompA(1), CompC(10)));

    let mut sum_a = 0;
    let mut sum_b = 0;

    // All three of these queries match both ArchFoo and ArchBar:
    ecs_find!(world, entity_a, |v: &mut OneOf<CompB, CompC>| {
        v.0 += 1;
    });

    ecs_find!(world, entity_b, |v: &mut OneOf<CompB, CompC>| {
        v.0 += 1;
    });

    ecs_iter!(world, |u: &CompA, v: &OneOf<CompB, CompC>| {
        sum_a += u.0;
        sum_b += v.0;
    });

    assert_eq!(sum_a, 2);
    assert_eq!(sum_b, 22);
}