Scriptable Event Handler with State

Usage Scenario

  • A system sends events that must be handled.

  • Flexibility in event handling must be provided, through user-side scripting.

  • State must be kept between invocations of event handlers.

  • Default implementations of event handlers can be provided.

Key Concepts

  • An event handler object is declared that holds the following items:

    • Engine with registered functions serving as an API,
    • AST of the user script,
    • a Scope containing state.
  • Upon an event, the appropriate event handler function in the script is called via Engine::call_fn.

  • Optionally, trap the EvalAltResult::ErrorFunctionNotFound error to provide a default implementation.


Declare Handler Object

In most cases, it would be simpler to store an Engine instance together with the handler object because it only requires registering all API functions only once.

In rare cases where handlers are created and destroyed in a tight loop, a new Engine instance can be created for each event. See One Engine Instance Per Call for more details.

fn main() {
use rhai::{Engine, Scope, AST, EvalAltResult};

// Event handler
struct Handler {
    // Scripting engine
    pub engine: Engine,
    // Use a custom 'Scope' to keep stored state
    pub scope: Scope<'static>,
    // Program script
    pub ast: AST

Register API for Any Custom Type

Custom types are often used to hold state. The easiest way to register an entire API is via a plugin module.

fn main() {
use rhai::plugin::*;

// A custom type to a hold state value.
#[derive(Debug, Clone, Eq, PartialEq, Hash, Default)]
pub struct SomeType {
    data: i64;

mod SomeTypeAPI {
    pub func1(obj: &mut SomeType) -> bool { ... }
    pub func2(obj: &mut SomeType) -> bool { ... }
    pub process(data: i64) -> i64 { ... }
    #[rhai_fn(get = "value")]
    pub get_value(obj: &mut SomeType) -> i64 { }
    #[rhai_fn(set = "value")]
    pub set_value(obj: &mut SomeType, value: i64) { = value; }

Initialize Handler Object

Steps to initialize the event handler:

  1. Register an API with the Engine,
  2. Create a custom Scope to serve as the stored state,
  3. Add default state variables into the custom Scope,
  4. Get the handler script and compile it,
  5. Store the compiled AST for future evaluations,
  6. Run the AST to initialize event handler state variables.

fn main() {
impl Handler {
    pub new(path: impl Into<PathBuf>) -> Self {
        let mut engine = Engine::new();

        // Register custom types and API's

        // Create a custom 'Scope' to hold state
        let mut scope = Scope::new();

        // Add initialized state into the custom 'Scope'
        scope.push("state1", false);
        scope.push("state2", SomeType::new(42));

        // Compile the handler script.
        // In a real application you'd be handling errors...
        let ast = engine.compile_file(path).unwrap();

        // Evaluate the script to initialize it and other state variables.
        // In a real application you'd again be handling errors...
        engine.consume_ast_with_scope(&mut scope, &ast).unwrap();

        // The event handler is essentially these three items:
        Handler { engine, scope, ast }

Hook up events

There is usually an interface or trait that gets called when an event comes from the system.

Mapping an event from the system into a scripted handler is straight-forward:

fn main() {
impl Handler {
    // Say there are three events: 'start', 'end', 'update'.
    // In a real application you'd be handling errors...
    pub fn on_event(&mut self, event_name: &str, event_data: i64) -> Result<(), Error> {
        let engine = &self.engine;
        let scope = &mut self.scope;
        let ast = &self.ast;

        match event_name {
            // The 'start' event maps to function 'start'.
            // In a real application you'd be handling errors...
            "start" => engine.call_fn(scope, ast, "start", (event_data,))?,

            // The 'end' event maps to function 'end'.
            // In a real application you'd be handling errors...
            "end" => engine.call_fn(scope, ast, "end", (event_data,))?,

            // The 'update' event maps to function 'update'.
            // This event provides a default implementation when the scripted function
            // is not found.
            "update" =>
                engine.call_fn(scope, ast, "update", (event_data,))
                      .or_else(|err| match *err {
                         EvalAltResult::ErrorFunctionNotFound(fn_name, _) if fn_name == "update" => {
                            // Default implementation of 'update' event handler
                            self.scope.set_value("state2", SomeType::new(42));
                            // Turn function-not-found into a success
                         _ => Err(err.into())

Sample Handler Script

Because the stored state is kept in a custom Scope, it is possible for all functions defined in the handler script to access and modify these state variables.

The API registered with the Engine can be also used throughout the script.

fn main() {
fn start(data) {
    if state1 {
        throw "Already started!";
    if state2.func1() || state2.func2() {
        throw "Conditions not yet ready to start!";
    state1 = true;
    state2.value = data;

fn end(data) {
    if !state1 {
        throw "Not yet started!";
    if state2.func1() || state2.func2() {
        throw "Conditions not yet ready to start!";
    state1 = false;
    state2.value = data;

fn update(data) {
    state2.value += process(data);