https://bevyengine.org/learn/quick-start/getting-started/ecs/
All application logic in Bevy uses the Entity-Component-System paradigm, commonly abbreviated as ECS. ECS is a software pattern that involves breaking a program into entities, components, and systems. An entity is a unique "thing" that is assigned a group of components, which are then processed using systems.
For example, one entity may have a Position and Velocity component, while another entity may have a Position and UI component. Systems are the logic that runs on a specific set of component types. You might have a system that runs on all entities with a Position and Velocity component. ECS encourages clean, decoupled design by forcing you to break down your application data and logic into its core components. It also helps speed up code by optimizing memory access patterns and simplifying parallelism.
Try writing a System yourself
fn hello_world(){
println!("helloWorld");
}
fn main() {
App::new()
.add_systems(Update, hello_world)
.run();
}
Then run it
cargo run