Rust Bevy States

Rust is a powerful systems programming language that is known for its speed and reliability. Bevy is a game engine built in Rust that provides an easy way for game developers to create games using Rust. One of the key features of the Bevy engine is its state management system.

What are states?

In game development, states can be thought of as different modes or stages that a game can be in. For example, a game could have a "Main Menu" state, a "Playing" state, and a "Game Over" state. Each state has its own set of rules and behaviors that define how the game works.

Bevy's state management system

Bevy's state management system allows developers to easily create and switch between different states in their games. This system is built on top of Rust's State trait, which is used to define the behavior of each state.

Defining states

To define a state in Bevy, you first need to create a struct that implements the State trait. This struct should contain all the data and behavior needed for that state.

use bevy::prelude::*;

struct MainMenuState;

impl State for MainMenuState {
    fn on_enter(&mut self, _world: &mut World) {
        // code to run when entering the state
    }

    fn on_exit(&mut self, _world: &mut World) {
        // code to run when exiting the state
    }
}

In this example, we're defining a MainMenuState struct that implements the State trait. We've also defined two methods, on_enter and on_exit, which will be called when the state is entered and exited, respectively.

Switching between states

Once you've defined your states, you can use Bevy's StateSwitcher resource to switch between them. The StateSwitcher resource is responsible for keeping track of the current state and switching between states when necessary.

use bevy::prelude::*;

fn main() {
    App::build()
        .add_resource(StateSwitcher::<AppState>::new(AppState::MainMenu))
        // other setup
        .run();
}

In this example, we're creating an App and adding a StateSwitcher resource to it. We've also set the initial state to MainMenu.

To switch between states, you can call the switch method on the StateSwitcher resource.

use bevy::prelude::*;

fn playing_system(
    mut state: ResMut<StateSwitcher<AppState>>,
) {
    // switch to the Game Over state
    state.switch(AppState::GameOver);
}

In this example, we're defining a system that switches the state to GameOver when the game is over.

Conclusion

Bevy's state management system makes it easy for game developers to create and switch between different states in their games. The State trait provides a flexible way to define the behavior of each state, and the StateSwitcher resource makes it easy to switch between states. Whether you're a beginner or an experienced game developer, Bevy and Rust are powerful tools that can help you create the games of your dreams.