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. Systems are logic that runs on a specific set of component types. 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.

Bevy ECS #

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 datatypes for all of these concepts:

  • Components: Rust structs that implement the Component trait

    #[derive(Component)]
    struct Position { x: f32, y: f32 }
    
  • Systems: normal Rust functions

    fn print_position_system(query: Query<&Position>) {
        for position in &query {
            println!("position: {} {}", position.x, position.y);
        }
    }
    
  • Entities: a simple type containing a unique integer

    struct Entity(u64);
    

Now let's see how this works in practice!

Your First System #

Paste the following function into your main.rs file:

fn hello_world() {
    println!("hello world!");
}

This will be our first system. The only remaining step is to add it to our App!

use bevy::prelude::*;

fn main() {
    App::new()
        .add_systems(Update, hello_world)
        .run();
}

The add_systems function adds the system to your App's Update Schedule, but we'll cover that more later.

Now run your app again using cargo run. You should see hello world! printed once in your terminal.

Your First Components #

Greeting the whole world is great, but what if we want to greet specific people? In ECS, you would generally model people as entities with a set of components that define them. Let's start simple with a Person component.

Add this struct to your main.rs file:

#[derive(Component)]
struct Person;

But what if we want our people to have a name? In a more traditional design, we might just tack on a name: String field to Person. But other entities might have names too! For example, dogs should probably also have a name. It often makes sense to break datatypes up in to small pieces to encourage code reuse. So let's make Name its own component:

#[derive(Component)]
struct Name(String);

We can then add people to our World using a "startup system". Startup systems are just like normal systems, but they run exactly once, before all other systems, right when our app starts. Let's use Commands to spawn some entities into our World:

fn add_people(mut commands: Commands) {
    commands.spawn((Person, Name("Elaina Proctor".to_string())));
    commands.spawn((Person, Name("Renzo Hume".to_string())));
    commands.spawn((Person, Name("Zayna Nieves".to_string())));
}

Now register the startup system like this:

fn main() {
    App::new()
        .add_systems(Startup, add_people)
        .add_systems(Update, hello_world)
        .run();
}

Your First Query #

We could run this now and the add_people system would run first, followed by hello_world. But our new people don't have anything to do yet! Let's make a system that properly greets the new citizens of our World:

fn greet_people(query: Query<&Name, With<Person>>) {
    for name in &query {
        println!("hello {}!", name.0);
    }
}

The parameters we pass into a "system function" define what data the system runs on. In this case, greet_people will run on all entities with the Person and Name component.

You can interpret the [Query] above as: "iterate over every Name component for entities that also have a Person component".

Now we just register the system in our App. Note that you can pass more than one system into an add_systems call by using a tuple! [Query]: https://docs.rs/bevy/latest/bevy/ecs/system/struct.Query.html

fn main() {
    App::new()
        .add_systems(Startup, add_people)
        .add_systems(Update, (hello_world, greet_people))
        .run();
}

Running our app will result in the following output:

Quick Note: "hello world!" might show up in a different order than it does below. This is because systems run in parallel by default whenever possible.

hello world!
hello Elaina Proctor!
hello Renzo Hume!
hello Zayna Nieves!

Marvelous!

Your First mutable Query #

If we want to change the names of some people (perhaps they got married!), for example, we can do this using a mutable query:

fn update_people(mut query: Query<&mut Name, With<Person>>) {
    for mut name in &mut query {
        if name.0 == "Elaina Proctor" {
            name.0 = "Elaina Hume".to_string();
            break; // We don’t need to change any other names
        }
    }
}

We need to make query mutable, and use a mutable reference (&mut) to the components we want to change.

Don’t forget to add the system to the Update schedule:

fn main() {
    App::new()
        .add_systems(Startup, add_people)
        .add_systems(Update, (hello_world, (update_people, greet_people).chain()))
        .run();
}

Note that we have used .chain() on the two systems. This is because we want them two to run in exactly the order they're listed in the code: with update_people occurring before greet_people. If they weren’t, the name might change after we greet the people.

But we don’t add the hello_world system to the chain, because it doesn’t matter when it runs. This way, Bevy can run hello_world in parallel while the other systems are running.