pub trait Join {
type Type;
type Value;
type Mask: BitSetLike;
// Required methods
unsafe fn open(self) -> (Self::Mask, Self::Value);
unsafe fn get(value: &mut Self::Value, index: Index) -> Self::Type;
// Provided methods
fn join(self) -> JoinIter<Self> ⓘ
where Self: Sized { ... }
fn maybe(self) -> MaybeJoin<Self>
where Self: Sized { ... }
fn is_unconstrained() -> bool { ... }
}Expand description
The purpose of the Join trait is to provide a way
to access multiple storages at the same time with
the merged bit set.
Joining component storages means that you’ll only get values where for a given entity every storage has an associated component.
§Example
let mut world = World::default();
world.register_component::<Pos>();
world.register_component::<Vel>();
{
let pos = world.component::<Pos>();
let vel = world.component::<Vel>();
// There are no entities yet, so no pair will be returned.
let joined: Vec<_> = (&pos, &vel).join().collect();
assert_eq!(joined, vec![]);
}
world.create_entity().with(Pos).build();
{
let pos = world.component::<Pos>();
let vel = world.component::<Vel>();
// Although there is an entity, it only has `Pos`.
let joined: Vec<_> = (&pos, &vel).join().collect();
assert_eq!(joined, vec![]);
}
let ent = world.create_entity().with(Pos).with(Vel).build();
{
let pos = world.component::<Pos>();
let vel = world.component::<Vel>();
// Now there is one entity that has both a `Vel` and a `Pos`.
let joined: Vec<_> = (&pos, &vel).join().collect();
assert_eq!(joined, vec![(&Pos, &Vel)]);
// If we want to get the entity the components are associated to,
// we need to join over `Entities`:
let entities = world.entities();
// note: `Entities` is the fetched resource; we get back
// `Read<Entities>`.
// `Read<Entities>` can also be referred to by `Entities` which
// is a shorthand type definition to the former type.
let joined: Vec<_> = (&entities, &pos, &vel).join().collect();
assert_eq!(joined, vec![(ent, &Pos, &Vel)]);
}§Iterating over a single storage
Join can also be used to iterate over a single
storage, just by writing (&storage).join().
Required Associated Types§
Sourcetype Mask: BitSetLike
type Mask: BitSetLike
Type of joined bit mask.
Required Methods§
Sourceunsafe fn open(self) -> (Self::Mask, Self::Value)
unsafe fn open(self) -> (Self::Mask, Self::Value)
Open this join by returning the mask and the storages.
§Safety
This is unsafe because implementations of this trait can permit
the Value to be mutated independently of the Mask.
If the Mask does not correctly report the status of the Value
then illegal memory access can occur.
Provided Methods§
Sourcefn join(self) -> JoinIter<Self> ⓘwhere
Self: Sized,
fn join(self) -> JoinIter<Self> ⓘwhere
Self: Sized,
Create a joined iterator over the contents.
Sourcefn maybe(self) -> MaybeJoin<Self>where
Self: Sized,
fn maybe(self) -> MaybeJoin<Self>where
Self: Sized,
Returns a Join-able structure that yields all indices, returning
None for all missing elements and Some(T) for found elements.
WARNING: Do not have a join of only MaybeJoins. Otherwise the join
will iterate over every single index of the bitset. If you want a
join with all MaybeJoins, add an Entities to the join as well
to bound the join to all entities that are alive.
struct ExampleSystem;
impl<'a> System<'a> for ExampleSystem {
type SystemData = (WriteStorage<'a, Pos>, ReadStorage<'a, Vel>);
fn run(&mut self, (mut positions, velocities): Self::SystemData) {
for (mut position, maybe_velocity) in (&mut positions, velocities.maybe()).join() {
if let Some(velocity) = maybe_velocity {
position.x += velocity.x;
position.y += velocity.y;
}
}
}
}
#[tokio::main]
async fn main() {
let mut world = World::default();
let mut dispatcher = Dispatcher::setup_builder(&mut world)
.with(ExampleSystem, "example_system", &[])
.unwrap()
.build();
let e1 = world
.create_entity()
.with(Pos { x: 0, y: 0 })
.with(Vel { x: 5, y: 2 })
.build();
let e2 = world.create_entity().with(Pos { x: 0, y: 0 }).build();
dispatcher.dispatch(&mut world).await;
let positions = world.component::<Pos>();
assert_eq!(positions.get(e1), Some(&Pos { x: 5, y: 2 }));
assert_eq!(positions.get(e2), Some(&Pos { x: 0, y: 0 }));
}Sourcefn is_unconstrained() -> bool
fn is_unconstrained() -> bool
If this Join typically returns all indices in the mask, then iterating
over only it or combined with other joins that are also dangerous
will cause the JoinIter/ParJoin to go through all indices which
is usually not what is wanted and will kill performance.
Dyn Compatibility§
This trait is not dyn compatible.
In older versions of Rust, dyn compatibility was called "object safety", so this trait is not object safe.