bevy_mod_index

A Rust crate that allows efficient querying for components by their values in the game engine Bevy.

Use Cases

It is quite common to want to write code in a system that only operates on components that have a certain value, e.g.:

fn move_living_players(mut players: Query<&mut Transform, &Player>) {
  for (mut transform, player) in &players {
    if player.is_alive() {
      move_player(transform);
    }
  }
}

With an index, we can change the code to:

fn move_living_players(
  mut transforms: Query<&mut Transform>, 
  player_alive_idx: Index<PlayerAlive>
) {
  for entity in &player_alive_idx.get(true) {
      transforms.get(entity).unwrap().move_player(transform);
  }
}

There are a few cases where a change like this may be beneficial:

• If is_alive is expensive to calculate, indexes can we can save work by caching the results and only recomputing when the data actually changes.
  • If the component data that the result is calculated from doesn't change often, we can use the cached values across frames.
  • If components tend to change only in the beginning of a frame, and the results are needed multiple times later on, we can use the cached values across different systems, (or even the same system if it had been calculated multiple times).
• If we don't care too much about performance, indexes can provide a nicer API to work with.

Indexes add a non-zero amount of overhead, though, so introducing them can make your systems slower. Make sure to profile your systems before and after introducing indexes if you care about performance.

Getting Started

First, import the prelude.

use bevy_mod_index::prelude::*;

Next, implement the IndexInfo trait. If your component only needs one index, you can implement this trait directly on the component. If you need more than one, you can use a simple unit struct for each index beyond the first. You can also use unit structs to give more descriptive names, even if you only need one index.

You must specify:

• the type of component to be indexed,
• the type of value that you want to be able to use for lookups,
• a function for calculating that value for a component,
• how to store the relationship between an entity and the value calculated from its appropriate component, and
• when the index should refresh itself with the latest data.

struct NearOrigin {}
impl IndexInfo for NearOrigin {
  type Component = Transform;
  type Value = bool;
  type Storage = HashmapStorage<Self>;
  type RefreshPolicy = ConservativeRefrehPolicy;

  fn value(t: &Transform) -> bool {
    t.translation.length() < 5.0
  }
}

Finally, include the Index system param in your systems and use it to query for entities!

fn count_players_and_enemies_near_spawn(
  players: Query<(), With<(&Player, &Transform)>>,
  enemies: Query<(), With<(&Enemy, &Transform)>>,
  index: Index<NearOrigin>,
) {
  let (mut player_count, mut enemy_count) = (0, 0);
  
  let entities_near_spawn: HashSet<Entity> = index.lookup(true);
  for entity in entities_near_spawn.into_iter() {
    if let Ok(()) = players.get(entity) {
      player_count += 1;
    }
    if let Ok(()) = enemies.get(entity) {
      enemy_count += 1;
    }
  }
  
  println!("There are {} players and {} enemies near spawn!", player_count, enemy_count)
}

Storage Implementations

HashmapStorage uses a Resource to cache a mapping between Entitys and the values computed from their components. It uses a custom SystemParam in order to update itself when the index is used. This is a good default choice, especially when the number of Entitys returned by a lookup is expected to be just a small percentage of those in the entire query.

NoStorage, as the name implies, does not store any index data. Instead, it loops over all data each time it is queried, computing the value function for each component, exactly like the first move_living_players example above. This option allows you to use the index API without incurring as much overhead as HashmapStorage (though still more than directly looping over all components yourself).

Refresh Policies

Refresh policies describe how often the index should check for component changes and update its internal storage.

ConservativeRefreshPolicy is a good default that will never be out of date, but if your systems that use your Index always run every frame (and therefore will never miss a despawn/component removal), SimpleRefreshPolicy can be used instead to reduce some overhead.

Other more specialized refresh policies can be found in the docs, and you can also define your own by implementing the IndexRefreshPolicy trait.

Compatability

API Stability

Consider the API to be extremely unstable as I experiment with what names and patterns feel most natural and expressive, and also work on supporting new features.

Performance

I have not put a lot of effort into optimizing the performance indexes yet. However, I have done some initial tests under to get a sense of approximately how much overhead they add.

With 1 million entities, while none of the components change frame-to-frame, using the component itself as the index value, operation on ~300 entities takes:

• 2-4x as long as a naive iteration when using NoStorage.
• 3-5x as long as a naive iteration when using HashmapStorage.

With the same setup, except that 5% of the entities are updated every frame, performance for HashmapStorage drops to 30-40x as long as naive iteration.

I am currently in the process of adding more concrete benchmarks, and I do have some plans for changes that will affect performance.

Get in contact

If you have suggestions for improvements to the API, or ideas about improving performance, I'd love to hear them. File an issue, or even better, reach out in the bevy_mod_index #crate-help thread on Bevy's discord.

Troubleshooting

• Query<(bevy_ecs::entity::Entity, &bevy_mod_index::index::test::Number, bevy_ecs::query::fetch::ChangeTrackers<bevy_mod_index::index::test::Number>), ()> in system bevy_mod_index::index::test::adder_some::{{closure}} accesses component(s) bevy_mod_index::index::test::Number in a way that conflicts with a previous system parameter. Consider using ``Without<T>`` to create disjoint Queries or merging conflicting Queries into a ``ParamSet``.
  • Indexes use a read-only query of their components to update the index before it is used. If you have a query that mutably access these components in the same system as an Index, you can combine them into a ParamSet.

Future work

• Docs
• Cleanup removed components and despawned entities without needing to run every frame.
• Option to update the index when components change instead of when the index is used.
  • Naively, requires engine support for custom DerefMut hooks, but this would likely add overhead even when indexes aren't used. Other solutions may be possible.
    • Perhaps the Component derive will one day accept an attribute that enables/disables change detection by specifying &mut T or Mut<T> as the reference type, and we could add a third option for IndexedMut<T> that would automatically look up all indexes for the component in some resource and add the entity to a list to be re-indexed.
      • See https://github.com/bevyengine/bevy/pull/7499 for a draft implementation.
• More storage options besides HashMap.
  • Sorted container to allow for querying "nearby" values.
    • 1D data should be simple enough, but would also like to support kd-trees for positions.
• Indexes over more than one Component.
• Indexes for subsets of a Component
  • Replacing Components with arbitrary queries may cover both of these cases.
• Derive for simple cases of IndexInfo where the component itself is used as the value.