FAQ for common scenarios

[dakom]

Just having a bit of trouble understanding the API... the tutorial and cargo docs are great but I feel I need to do a bit of head-scratching to figure out some things like this:

• how to get all entities that have a certain component?
• how to get all entities that do not have a certain component?
• how to get a specific component from an entity?
• can I have multiple dispatchers (e.g. to run at different frequencies)?
• if I'm not using parallel, should I use dispatchers anyway if I have multiple systems?
• can entities have multiple instances of the same component (like two Transforms) ?

Maybe there can be a FAQ style page in the docs that asks and answers these (and more)...

[dakom]

Gonna try and answer some of my own questions:

how to get all entities that have a certain component?
how to get all entities that do not have a certain component?

Both of these are the same: call join() with the entities and component storage.

Inside of a system, use SystemData to get each of those (this is detailed in the docs).
Outside of a system, use world directly:

 let parents = world.read_storage::<Parent>();

 //all the root nodes, i.e. entities that don't have parent components
 for (entity, _) in (&world.entities(), !&parents).join() {
 }

 //all the children, i.e. entities that *have* parent components
 for (entity, parent) in (&world.entities(), &parents).join() { 
 }

how to get a specific component from an entity?

entities are used as keys in a component storage lookup. example:

world.read_storage::<Parent>().get(entity)

Don't know the answers to the other questions yet...

[WaDelma]

Nothing stops you from having multiple dispatchers.

You can only have one of each type of component for single entity.

[dakom]

thanks! btw by "not using parallel" I mean the Cargo feature :)

[arlyon]

Here's an interesting one: how do I add a components to all entities that don't have a given component? This, for example doesn't work due to the borrow checker:

for (entity, entity_position, ()) in (&entities, &positions, !&transforms).join() {
    let mut t = Transform::default();
    t.set_translation_xyz(0.0, 0.0, 0.0);
    transforms.insert(entity, t).unwrap();
}

error[E0502]: cannot borrow `transforms` as mutable because it is also borrowed as immutable
   --> src/systems.rs:169:13
    |
166 |         for (entity, entity_position, ()) in (&entities, &positions, !&transforms).join() {
    |                                              --------------------------------------------
    |                                              |                        |
    |                                              |                        immutable borrow occurs here
    |                                              immutable borrow later used here
...
169 |             transforms.insert(entity, t).unwrap();
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ mutable borrow occurs here

[willglynn]

@arlyon I would reach for LazyUpdate::insert() here.

[drmorr0]

Is there a way to get all components attached to a given entity?