Unpacking the Entity Component System Pattern in Bevy

Understanding the ECS Pattern

The ECS pattern is a design paradigm that separates game logic into three interconnected components:

• Entities: Unique objects in the game world, such as characters, enemies, or obstacles.
• Components: Data structures attached to entities, defining their properties and behavior.
• Systems: Functions that process entities and their components, controlling the application’s behavior.

Bevy’s ECS Implementation

Bevy’s API allows users to write regular functions in Rust, which are then automatically dispatched to the correct data. This is made possible by Bevy’s use of traits, which define a set of methods that can be implemented for different types.

Defining Systems

In Bevy, systems are defined using the <code:system< code=””> trait, which provides a run method for executing the system’s logic. To create a system, you need to implement the <code<system< code=””> trait for a specific type.</code<system<></code:system<>

trait System {
    fn run(&mut self);
}

struct MySystem;

impl System for MySystem {
    fn run(&mut self) {
        // System logic goes here
    }
}

Storing Generic Systems

Bevy stores generic systems in a list using trait objects, which are essentially fat pointers that point to both the system’s data and its virtual table. This allows Bevy to store and execute systems with different type parameters.

let mut systems: Vec<box> = Vec::new();
systems.push(Box::new(MySystem));
</box

Fetching Parameters

To fetch parameters for a system, Bevy uses the SystemParamFetch trait, which provides a way to extract values from a World. The World is a central repository of entities and their components, allowing systems to access the data they need.

trait SystemParamFetch {
    fn fetch(&mut self) -> &World;
}

struct MySystemFetch;

impl SystemParamFetch for MySystemFetch {
    fn fetch(&mut self) -> &World {
        // Fetch logic goes here
    }
}

Same Pattern, Different Framework: Extractors in Axum

Axum, a web framework, also uses the ECS pattern to define handler functions for specific routes. Axum’s implementation is similar to Bevy’s, with a few key differences. Axum uses extractors to extract values from requests, which are then passed to the handler functions.

struct MyExtractor;

impl Extractor for MyExtractor {
    fn extract(&self, request: &Request) -> Result {
        // Extraction logic goes here
    }
}

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