Scriptable Event Handler with State
JS Style


A runnable example of this implementation is included.

See the Examples section for details.

Keep State in Object Map

This style allows defining new user state variables everywhere by packaging them all inside an object map, which is then exposed via the this pointer.

Because this scripting style resembles JavaScript, it is so named.

State variables can be freely created by all functions (not just the init function).

The event handler type needs to hold this object map instead of a custom Scope.

use rhai::{Engine, Scope, Dynamic, AST};

// Event handler
struct Handler {
    // Scripting engine
    pub engine: Engine,
    // The custom 'Scope' can be used to hold global constants
    pub scope: Scope<'static>,
    // Use an object map (as a 'Dynamic') to keep stored state
    pub states: Dynamic,
    // Program script
    pub ast: AST

Bind Object Map to this Pointer

Initialization can simply be done via binding the object map containing global states to the this pointer.

impl Handler {
    // Create a new 'Handler'.
    pub fn new(path: impl Into<PathBuf>) -> Self {
        let mut engine = Engine::new();

            // Code omitted

        // Use an object map to hold state
        let mut states = Map::new();

        // Default states can be added
        states.insert("bool_state".into(), Dynamic::FALSE);

        // Convert the object map into 'Dynamic'
        let mut states: Dynamic = states.into();

        // Use 'call_fn_raw' instead of 'call_fn' to bind the 'this' pointer
        // In a real application you'd again be handling errors...
        engine.call_fn_raw(&mut scope, &ast, false, true, "init", Some(&mut states), []).unwrap();
        //                                          ^^^^          ^^^^^^^^^^^^^^^^^
        //                                      rewind scope      bind 'this' pointer

            // Code omitted

        Self { engine, scope, states, ast }

Bind this Pointer During Events Handling

Events handling should also use Engine::call_fn_raw to bind the object map containing global states to the this pointer.

pub fn on_event(&mut self, event_name: &str, event_data: i64) -> Dynamic {
    let engine = &self.engine;
    let scope = &mut self.scope;
    let states = &mut self.states;
    let ast = &self.ast;

    match event_name {
        // In a real application you'd be handling errors...
        "start" => engine.call_fn_raw(scope, ast, false, true, "start",
                                      Some(states), [event_data.into()]).unwrap(),
                                   // ^^^^^^^^^^^^ bind 'this' pointer

Handler Scripting Style

No shadowing

Notice that this can never be shadowed because it is not a valid variable name.

Because the stored state is kept in an object map, which in turn is bound to this, it is necessary for functions to always access or modify these state variables via the this pointer.

As it is impossible to declare a local variable named this, there is no risk of accidentally shadowing a state variable.

Because an object map is used to hold state values, it is even possible to add user-defined functions, leveraging the OOP support for object maps.

Sample script

/// Initialize user-provided state.
/// State is stored inside an object map bound to 'this'.
fn init() {
    // Can detect system-provided default states!
    // Add 'bool_state' as new state variable if one does not exist
    if !("bool_state" in this) {
        this.bool_state = false;
    // Add 'obj_state' as new state variable (overwrites any existing)
    this.obj_state = new_state(0);

    // Can also add OOP-style functions!
    this.log = |x| print(`State = ${this.obj_state.value}, data = ${x}`);

/// 'start' event handler
fn start(data) {
    // Access state variables via 'this'
    if this.bool_state {
        throw "Already started!";

    // New state variables can be created anywhere
    this.start_mode = data;

    if this.obj_state.func1() || this.obj_state.func2() {
        throw "Conditions not yet ready to start!";
    this.bool_state = true;
    this.obj_state.value = data;

    // Constant 'MY_CONSTANT' in custom scope is also visible!
    print(`MY_CONSTANT = ${MY_CONSTANT}`);

/// 'end' event handler
fn end(data) {
    if !this.bool_state || !("start_mode" in this) {
        throw "Not yet started!";
    if !this.obj_state.func1() && !this.obj_state.func2() {
        throw "Conditions not yet ready to end!";
    this.bool_state = false;
    this.obj_state.value = data;

/// 'update' event handler
fn update(data) {
    this.obj_state.value += process(data);

    // Call user-defined function OOP-style!