Scriptable Event Handler with State


In many usage scenarios, a scripting engine is used to provide flexibility in event handling.

That means to execute certain actions in response to certain events that occur at run-time, and scripts are used to provide flexibility for coding those actions.

You’d be surprised how many applications fit under this pattern – they are all essentially event handling systems.


Because of the importance of this pattern, runnable examples are included.

See the Examples section for details.

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.

  • State may be provided by the system or the user, or both.

  • 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,
    • Scope containing system-provided default state.
  • User-provided state is initialized by a function called via Engine::call_fn_with_options.

  • 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.

Basic Infrastructure

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.

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

// 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 custom types

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

use rhai::plugin::*;

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

// Plugin module containing API to TestStruct
mod test_struct_api {
    pub fn new_state(value: i64) -> TestStruct {
        TestStruct { data: value }
    pub fn func1(obj: &mut TestStruct) -> bool {
    pub fn func2(obj: &mut TestStruct) -> i64 {
    pub fn process(data: i64) -> i64 {
    #[rhai_fn(get = "value", pure)]
    pub fn get_value(obj: &mut TestStruct) -> i64 {
    #[rhai_fn(set = "value")]
    pub fn set_value(obj: &mut TestStruct, 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. Optionally, call an initiation function to create new state variables; Engine::call_fn_with_options is used instead of Engine::call_fn so that variables created inside the function will not be removed from the custom Scope upon exit,
  5. Get the handler script and compile it,
  6. Store the compiled AST for future evaluations,
  7. Run the AST to initialize event handler state variables.
impl Handler {
    // Create a new 'Handler'.
    pub fn 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 any system-provided state into the custom 'Scope'.
        // Constants can be used to optimize the script.
        scope.push_constant("MY_CONSTANT", 42_i64);

        // Initialize state variables

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

        // The event handler is essentially these three items:
        Self { 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, via Engine::call_fn.

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) -> Dynamic {
        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,)).unwrap(),

            // 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,)).unwrap(),

            // 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.starts_with("update") =>
                              // Default implementation of 'update' event handler.
                              self.scope.set_value("obj_state", TestStruct::new(42));
                              // Turn function-not-found into a success.
                          _ => Err(err)

            // In a real application you'd be handling unknown events...
            _ => panic!("unknown event: {}", event_name)

Scripting Styles

Depending on needs and scripting style, there are three different ways to implement this pattern.

Main styleJS styleMap style
States storecustom Scopeobject map bound to thisobject map in custom Scope
Access state variablenormal variableproperty of thisproperty of state variable
Access global constants?yesyesyes
Add new state variable?init function onlyall functionsall functions
Add new global constants?yesnono
OOP-style functions on states?noyesyes
Detect system-provided initial states?noyesyes
Local variable may shadow state variable?yesnono
Benefitssimplefewer surprisesversatile
  • no new variables in functions (apart from init)
  • easy variable name collisions
  • all over the place
  • more complex implementation
  • all over the place
  • inconsistent syntax