bevy_scriptum 📜

bevy_scriptum is a a plugin for Bevy that allows you to write some of your game logic in a scripting language. Currently Rhai and Lua are supported, but more languages may be added in the future.

API docs are available in docs.rs

bevy_scriptum's main advantages include:

  • low-boilerplate
  • easy to use
  • asynchronicity with a promise-based API
  • flexibility
  • hot-reloading

Scripts are separate files that can be hot-reloaded at runtime. This allows you to quickly iterate on your game logic without having to recompile your game.

All you need to do is register callbacks on your Bevy app like this:

use bevy::prelude::*;
use bevy_scriptum::prelude::*;
use bevy_scriptum::runtimes::lua::prelude::*;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_scripting::<LuaRuntime>(|runtime| {
             runtime.add_function(String::from("hello_bevy"), || {
               println!("hello bevy, called from script");
             });
        })
        .run();
}

And you can call them in your scripts like this:

hello_bevy()

Every callback function that you expose to the scripting language is also a Bevy system, so you can easily query and mutate ECS components and resources just like you would in a regular Bevy system:

use bevy::prelude::*;
use bevy_scriptum::prelude::*;
use bevy_scriptum::runtimes::lua::prelude::*;

#[derive(Component)]
struct Player;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_scripting::<LuaRuntime>(|runtime| {
            runtime.add_function(
                String::from("print_player_names"),
                |players: Query<&Name, With<Player>>| {
                    for player in &players {
                        println!("player name: {}", player);
                    }
                },
            );
        })
        .run();
}

You can also pass arguments to your callback functions, just like you would in a regular Bevy system - using In structs with tuples:

use bevy::prelude::*;
use bevy_scriptum::prelude::*;
use bevy_scriptum::runtimes::lua::prelude::*;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_scripting::<LuaRuntime>(|runtime| {
            runtime.add_function(
                String::from("fun_with_string_param"),
                |In((x,)): In<(String,)>| {
                    println!("called with string: '{}'", x);
                },
            );
        })
        .run();
}

which you can then call in your script like this:

fun_with_string_param("Hello world!")

Usage

Add the following to your Cargo.toml:

[dependencies]
bevy_scriptum = { version = "0.6", features = ["lua"] }

or execute cargo add bevy_scriptum --features lua from your project directory.

You can now start exposing functions to the scripting language. For example, you can expose a function that prints a message to the console:

use bevy::prelude::*;
use bevy_scriptum::prelude::*;
use bevy_scriptum::runtimes::lua::prelude::*;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_scripting::<LuaRuntime>(|runtime| {
           runtime.add_function(
               String::from("my_print"),
               |In((x,)): In<(String,)>| {
                   println!("my_print: '{}'", x);
               },
           );
        })
        .run();
}

Then you can create a script file in assets directory called script.lua that calls this function:

my_print("Hello world!")

And spawn an entity with attached Script component with a handle to a script source file:

use bevy::prelude::*;
use bevy_scriptum::prelude::*;
use bevy_scriptum::runtimes::lua::prelude::*;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_scripting::<LuaRuntime>(|runtime| {
           runtime.add_function(
               String::from("my_print"),
               |In((x,)): In<(String,)>| {
                   println!("my_print: '{}'", x);
               },
           );
        })
        .add_systems(Startup,|mut commands: Commands, asset_server: Res<AssetServer>| {
            commands.spawn(Script::<LuaScript>::new(asset_server.load("script.lua")));
        })
        .run();
}

You should then see my_print: 'Hello world!' printed in your console.

Provided examples

You can also try running provided examples by cloning this repository and running cargo run --example <example_name>_<language_name>. For example:

cargo run --example hello_world_lua

The examples live in examples directory and their corresponding scripts live in assets/examples directory within the repository.

Promises - getting return values from scripts

Every function called from script returns a promise that you can call :and_then with a callback function on. This callback function will be called when the promise is resolved, and will be passed the return value of the function called from script. For example:

get_player_name():and_then(function(name)
    print(name)
end)

which will print out John when used with following exposed function:

use bevy::prelude::*;
use bevy_scriptum::prelude::*;
use bevy_scriptum::runtimes::lua::prelude::*;

fn main() {
    App::new()
       .add_plugins(DefaultPlugins)
       .add_scripting::<LuaRuntime>(|runtime| {
               runtime.add_function(String::from("get_player_name"), || String::from("John"));
       });
}

Access entity from script

A variable called entity is automatically available to all scripts - it represents bevy entity that the Script component is attached to. It exposes index property that returns bevy entity index. It is useful for accessing entity's components from scripts. It can be used in the following way:

print("Current entity index: " .. entity.index)

entity variable is currently not available within promise callbacks.

Contributing

Contributions are welcome! Feel free to open an issue or submit a pull request.

License

bevy_scriptum is licensed under either of the following, at your option: Apache License, Version 2.0, (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0) or MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)