Scriptable Control Layer Over Rust Backend

Usage scenario

  • A system provides core functionalities, but no driving logic.

  • The driving logic must be dynamic and hot-loadable.

  • A script is used to drive the system and provide control intelligence.

Key concepts

  • Expose a Control API.

  • Leverage function overloading to simplify the API design.

  • Since Rhai is sand-boxed, it cannot mutate anything outside of its internal environment. To perform external actions via an API, the actual system must be wrapped in a RefCell (or RwLock/Mutex for sync) and shared to the Engine.

Using Rhai for games

Although this usage pattern appears a perfect fit for game logic, avoid writing the entire game in Rhai. Performance will not be acceptable.

Implement as much functionalities of the game engine in Rust as possible. Rhai integrates well with Rust so this is usually not a hinderance.

Lift as much out of Rhai as possible. Use Rhai only for the logic that must be dynamic or hot-loadable.


There are two broad ways for Rhai to control an external system, both of which involve wrapping the system in a shared, interior-mutated object.

This is one way which does not involve exposing the data structures of the external system, but only through exposing an abstract API primarily made up of functions.

Use this when the API is relatively simple and clean, and the number of functions is small enough.

For a complex API involving lots of functions, or an API that has a clear object structure, use the Singleton Command Object pattern instead.

Functional API

Assume that a system provides the following functional API:

struct EnergizerBunny;

impl EnergizerBunny {
    pub fn new () -> Self { ... }
    pub fn go (&mut self) { ... }
    pub fn stop (&mut self) { ... }
    pub fn is_going (&self) { ... }
    pub fn get_speed (&self) -> i64 { ... }
    pub fn set_speed (&mut self, speed: i64) { ... }

Wrap API in shared object

pub type SharedBunny = Rc<RefCell<EnergizerBunny>>;

or in multi-threaded environments with the sync feature, use one of the following:

pub type SharedBunny = Arc<RwLock<EnergizerBunny>>;

pub type SharedBunny = Arc<Mutex<EnergizerBunny>>;

Register control API

The trick to building a Control API is to clone the shared API object and move it into each function registration via a closure.

Therefore, it is not possible to use a plugin module to achieve this, and each function must be registered one after another.

// Notice 'move' is used to move the shared API object into the closure.
let b = bunny.clone();
engine.register_fn("bunny_power", move |on: bool| {
    if on {
        if b.borrow().is_going() {
            println!("Still going...");
        } else {
    } else {
        if b.borrow().is_going() {
        } else {
            println!("Already out of battery!");

let b = bunny.clone();
engine.register_fn("bunny_is_going", move || b.borrow().is_going());

let b = bunny.clone();
engine.register_fn("bunny_get_speed", move ||
    if b.borrow().is_going() { b.borrow().get_speed() } else { 0 }

let b = bunny.clone();
engine.register_fn("bunny_set_speed", move |speed: i64| -> Result<_, Box<EvalAltResult>>
    if speed <= 0 {
        return Err("Speed must be positive!".into());
    } else if speed > 100 {
        return Err("Bunny will be going too fast!".into());

    if b.borrow().is_going() {
    } else {
        return Err("Bunny is not yet going!".into());


Use the API

if !bunny_is_going() { bunny_power(true); }

if bunny_get_speed() > 50 { bunny_set_speed(50); }