Bevy ECS is an Entity Component System custom-built for the [Bevy][bevy] game engine. It aims to be simple to use, ergonomic, fast, massively parallel, opinionated, and featureful. It was created specifically for Bevy's needs, but it can easily be used as a standalone crate in other projects.
## ECS
All app logic in Bevy uses the Entity Component System paradigm, which is often shortened to ECS. ECS is a software pattern that involves breaking your program up into Entities, Components, and Systems. Entities are unique "things" that are assigned groups of Components, which are then processed using Systems.
For example, one entity might have a `Position` and `Velocity` component, whereas another entity might have a `Position` and `UI` component. You might have a movement system that runs on all entities with a Position and Velocity component.
The ECS pattern encourages clean, decoupled designs by forcing you to break up your app data and logic into its core components. It also helps make your code faster by optimizing memory access patterns and making parallelism easier.
## Concepts
Bevy ECS is Bevy's implementation of the ECS pattern. Unlike other Rust ECS implementations, which often require complex lifetimes, traits, builder patterns, or macros, Bevy ECS uses normal Rust data types for all of these concepts:
### Components
Components are normal Rust structs. They are data stored in a `World` and specific instances of Components correlate to Entities.
```rust
struct Position { x: f32, y: f32 }
```
### Worlds
Entities, Components, and Resources are stored in a `World`. Worlds, much like Rust std collections like HashSet and Vec, expose operations to insert, read, write, and remove the data they store.
```rust
let world = World::default();
```
### Entities
Entities are unique identifiers that correlate to zero or more Components.
```rust
let entity = world.spawn()
.insert(Position { x: 0.0, y: 0.0 })
.insert(Velocity { x: 1.0, y: 0.0 })
.id();
let entity_ref = world.entity(entity);
let position = entity_ref.get::<Position>().unwrap();
let velocity = entity_ref.get::<Velocity>().unwrap();
```
### Systems
Systems are normal Rust functions. Thanks to the Rust type system, Bevy ECS can use function parameter types to determine what data needs to be sent to the system. It also uses this "data access" information to determine what Systems can run in parallel with each other.
println!("Entity {:?} is at position: x {}, y {}", entity, position.x, position.y);
}
}
```
### Resources
Apps often require unique resources, such as asset collections, renderers, audio servers, time, etc. Bevy ECS makes this pattern a first class citizen. `Resource` is a special kind of component that does not belong to any entity. Instead, it is identified uniquely by its type:
```rust
#[derive(Default)]
struct Time {
seconds: f32,
}
world.insert_resource(Time::default());
let time = world.get_resource::<Time>().unwrap();
// You can also access resources from Systems
fn print_time(time: Res<Time>) {
println!("{}", time.seconds);
}
```
The [`resources.rs`](examples/resources.rs) example illustrates how to read and write a Counter resource from Systems.
### Schedules
Schedules consist of zero or more Stages, which run a set of Systems according to some execution strategy. Bevy ECS provides a few built in Stage implementations (ex: parallel, serial), but you can also implement your own! Schedules run Stages one-by-one in an order defined by the user.
The built in "parallel stage" considers dependencies between systems and (by default) run as many of them in parallel as possible. This maximizes performance, while keeping the system execution safe. You can also define explicit dependencies between systems.
## Using Bevy ECS
Bevy ECS should feel very natural for those familiar with Rust syntax:
```rust
use bevy_ecs::prelude::*;
struct Velocity {
x: f32,
y: f32,
}
struct Position {
x: f32,
y: f32,
}
// This system moves each entity with a Position and Velocity component
* **Tables**: Fast and cache friendly iteration, but slower adding and removing of components. This is the default storage type.
* **Sparse Sets**: Fast adding and removing of components, but slower iteration.
Component storage types are configurable, and they default to table storage if the storage is not manually defined. The [`component_storage.rs`](examples/component_storage.rs) example shows how to configure the storage type for a component.
// Bundles play well with Rust's struct update syntax
world.spawn().insert_bundle(PlayerBundle {
position: Position { x: 1.0, y: 1.0 },
..Default::default()
});
```
### Events
Events offer a communication channel between one or more systems. Events can be sent using the system parameter `EventWriter` and received with `EventReader`.
```rust
struct MyEvent {
message: String,
}
fn writer(mut writer: EventWriter<MyEvent>) {
writer.send(MyEvent {
message: "hello!".to_string(),
});
}
fn reader(mut reader: EventReader<MyEvent>) {
for event in reader.iter() {
}
}
```
A minimal set up using events can be seen in [`events.rs`](examples/events.rs).
