Scriptable Event Handler with State
Main Style


A runnable example of this implementation is included.

See the Examples section for details.

Initialize Handler Instance with Engine::call_fn_with_options

Use Engine::call_fn_with_options instead of Engine::call_fn in order to retain new variables defined inside the custom Scope when running the init function.

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

            // Code omitted

        // Run the 'init' function to initialize the state, retaining variables.
        let options = CallFnOptions::new()
                        .eval_ast(false)        // do not re-evaluate the AST
                        .rewind_scope(false);   // do not rewind scope

        // In a real application you'd again be handling errors...
        engine.call_fn_with_options(options, &mut scope, &ast, "init", ()).unwrap();

            // Code omitted

        Self { engine, scope, ast }

Handler Scripting Style

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.

Sample script

/// Initialize user-provided state (shadows system-provided state, if any).
/// Because 'CallFnOptions::rewind_scope' is 'false', new variables introduced
/// will remain inside the custom 'Scope'.
fn init() {
    // Add 'bool_state' and 'obj_state' as new state variables
    let bool_state = false;
    let obj_state = new_state(0);

    // Constants can also be added!
    const EXTRA_CONSTANT = "hello, world!";

/// Without 'OOP' support, the can only be a function.
fn log(value, data) {
    print(`State = ${value}, data = ${data}`);

/// 'start' event handler
fn start(data) {
    if bool_state {
        throw "Already started!";
    if obj_state.func1() || obj_state.func2() {
        throw "Conditions not yet ready to start!";
    bool_state = true;
    obj_state.value = data;

    // Constants 'MY_CONSTANT' and 'EXTRA_CONSTANT'
    // in custom scope are also visible!
    print(`MY_CONSTANT = ${MY_CONSTANT}`);

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

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

    // Without OOP support, can only call function
    log(obj_state.value, data);

Disadvantages of This Style

This style is simple to implement and intuitive to use, but it is not very flexible.

New user state variables are introduced by evaluating a special initialization script function (e.g. init) that defines them, and Engine::call_fn_with_scope is used to keep them inside the custom Scope (together with setting CallFnOptions::rewind_scope to false).

However, this has the disadvantage that no other function can introduce new state variables, otherwise they’d simply shadow existing variables in the custom Scope. Thus, functions are called during events via Engine::call_fn which does not retain any variables.

When there are a large number of state variables, this style also makes it easy for local variables defined in user functions to accidentally shadow a state variable with a variable that just happens to be the same name.

// 'start' event handler
fn start(data) {
    let bool_state = false; // <- oops! bad variable name!
        :                   // there is now no way to access the
        :                   // state variable 'bool_state'...

    if bool_state {         // <- 'bool_state' is not the right thing
        ...                 //    unless this is what you actually want