Iterators

Iteration is one of the most important features of an ECS.

World

let world = World::new();

for (i, j) in &mut world.iter::<(&mut Pos, &Vel)>() {
    i.0 += j.0;
}

THe "extra" &mut is unfortunate but necessary.

Views

Iteration on views is achieved using IntoIter::iter.

let world = World::new();

world.run(|mut vm_pos: ViewMut<Pos>, v_vel: View<Vel>| {
    for i in vm_pos.iter() {
        dbg!(i);
    }
    
    for (i, j) in (&mut vm_pos, &v_vel).iter() {
        i.0 += j.0;
    }
});

You can use views in any order. However, using the same combination of views in different positions may yield components in a different order.
You shouldn't expect specific ordering from Shipyard's iterators in general.

With Id

You can ask an iterator to tell you which entity owns each component by using WithId::with_id:

let world = World::new();

world.run(|v_pos: View<Pos>| {
    for (id, i) in v_pos.iter().with_id() {
        println!("{:?} belongs to entity {:?}", i, id);
    }
});

Not

It's possible to filter entities that don't have a certain component using Not by adding ! in front of the view reference.

let world = World::new();

world.run(|v_pos: View<Pos>, v_vel: View<Vel>| {
    for (i, _) in (&v_pos, !&v_vel).iter() {
        dbg!(i);
    }
});