Nature of Code in Bevy: Random Walker

Today, I'll start reading and exploring the Nature of Code book and the Bevy Game Engine! I've been interested in the book for a couple of years and I also completed some of the first chapters in both Processing (the Java framework used in the book) and Haskell. In my early days at University, I was also a semi-regular follower of The Coding Train YouTube channel, which the author of the book Daniel Shiffman runs. As for Bevy, I've looked at it now and again since it came into my awareness a year ago. I think it is a refreshing new engine and I love the principles they adhere to.

With the motivation explained, let's move on to the first example from the introduction chapter: a random walker, which simply is a dot on the screen that moves around randomly.

Random Walker

Let us first create a new project and import the most recent version of bevy along with the rand crate, which we'll also need.

# Cargo.toml
[dependencies]
bevy = { version = "0.6" }
rand = "0.8.5"

Next, we create a new bevy App with the DefaultPlugins and run it. And voila, an empty gray window shows up. The DefaultPlugins add a game loop, a window system, audio and all sort of things you expect from a game engine.

use bevy::prelude::*;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .run();
}

Then we need a black dot that can function as our random walker. At first, I looked at the bevy_prototype_lyon crate, but I opted for a simpler option: using a sprite.

Let's add the black dot with our first system. We'll specify the system to run once at startup and we'll make it spawn an orthographic camera and a sprite with a black circle texture. The AssetServer will look for files in an /assets folder by default, so that's where we'll put the image file ./assets/black-circle.png.

use bevy::prelude::*;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_startup_system(startup_system) // <- Also added this line.
        .run();
}

fn startup_system(mut commands: Commands, asset_server: Res<AssetServer>) {
    commands.spawn_bundle(OrthographicCameraBundle::new_2d());
    commands
        .spawn_bundle(SpriteBundle {
            texture: asset_server.load("black-circle.png"),
            ..Default::default()
        });
}

Okay, so far so good. But a little boring. Let's add some random movement. For this, we'll create our first component, a Walker struct, and derive Component for it. It doesn't need any fields, because it only serves as a marker for the entity.

#[derive(Component)]
struct Walker;

We'll use the rand crate and create a walk system, which generates two random numbers and moves the black circle by the random amount. Take note of the query type Query<&mut Transform, With<Walker>>, which specifies that we want all Transform components on entities that also has a Walker component.

use bevy::prelude::*;
use rand::prelude::*;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_startup_system(startup_system)
        .add_system(walk)
        .run();
}

// fn startup_system(...){...}

fn walk(mut query: Query<&mut Transform, With<Walker>>) {
    let speed = 5.;
    let mut rng = rand::thread_rng();
    let mut walker_transform = query.single_mut();
    walker_transform.translation.x += rng.gen_range(-speed..speed);
    walker_transform.translation.y += rng.gen_range(-speed..speed);
}

If you run this, it will crash immediately, because we use single_mut() on the query, which will crash if there is anything other than a single entity that matches the given query. At present, our little game has two entities with a Transform component. The camera and the sprite. But no entities have the Walker component yet. Let's address that.

...
commands.spawn_bundle(SpriteBundle {
    texture: asset_server.load("black-circle.png"),
    ..Default::default()
}).insert(Walker); // <- Add a Walker component to the SpriteBundle.
...

And.. It works!

Final thoughts

I tried to trace the path of the walker by not drawing the background at every frame, similar to what as Daniel Shiffman does in his example, but I couldn't figure out how to do it in Bevy. I'll skip it for now, as this is just a warmup exercise to get our feet wet. Perhaps I'll figure out how to do it once I get deeper into Bevy.

You can see the full example on GitHub.

I guess that's it for now. Stay tuned for more Nature of Code in Bevy posts ;)

– Kasper