nodejs/stdlib/

events.rs

//! Node `events` module: `EventEmitter`. The emitter is an object tagged
//! `@@native = "EventEmitter"` with hidden `@@on`/`@@once` maps (event name →
//! listener array). `emit` collects listeners, releases the host borrow, then
//! invokes each so callbacks can re-enter the host.

use super::arg_str;
use crate::host::{call_method, invoke, with_host, JsObj};
use fusevm::Value;
use indexmap::IndexMap;

/// Construct a fresh `EventEmitter`.
pub fn new_emitter() -> Value {
    with_host(|h| {
        let on = h.new_object(IndexMap::new());
        let once = h.new_object(IndexMap::new());
        let mut m = IndexMap::new();
        m.insert("@@native".into(), h.new_str("EventEmitter"));
        m.insert("@@on".into(), on);
        m.insert("@@once".into(), once);
        h.new_object(m)
    })
}

/// The EventEmitter method names, exposed so `EventEmitter.prototype` can be
/// enumerated / copied (express does `mixin(app, EventEmitter.prototype)` to make
/// its `app` *function* an emitter).
pub const METHODS: &[&str] = &[
    "on",
    "addListener",
    "prependListener",
    "once",
    "prependOnceListener",
    "emit",
    "removeListener",
    "off",
    "removeAllListeners",
    "listenerCount",
    "listeners",
    "eventNames",
    "setMaxListeners",
    "getMaxListeners",
];

pub fn instance_call(recv: &Value, method: &str, args: Vec<Value>) -> Result<Value, String> {
    match method {
        "listeners" => Ok(with_host(|h| {
            h.new_array(listeners(recv, &arg_str(&args, 0)))
        })),
        // A no-op accessor pair kept for API completeness; the emitter has no cap.
        "setMaxListeners" => Ok(recv.clone()),
        "getMaxListeners" => Ok(Value::Float(10.0)),
        "on" | "addListener" | "prependListener" => {
            add(
                recv,
                "@@on",
                &arg_str(&args, 0),
                args.get(1).cloned().unwrap_or(Value::Undef),
            );
            Ok(recv.clone())
        }
        "once" | "prependOnceListener" => {
            add(
                recv,
                "@@once",
                &arg_str(&args, 0),
                args.get(1).cloned().unwrap_or(Value::Undef),
            );
            Ok(recv.clone())
        }
        "emit" => emit(
            recv,
            &arg_str(&args, 0),
            &args.get(1..).map(|s| s.to_vec()).unwrap_or_default(),
        ),
        "removeListener" | "off" => {
            remove(recv, &arg_str(&args, 0), args.get(1).cloned());
            Ok(recv.clone())
        }
        "removeAllListeners" => {
            let name = if args.is_empty() {
                None
            } else {
                Some(arg_str(&args, 0))
            };
            remove_all(recv, name.as_deref());
            Ok(recv.clone())
        }
        "listenerCount" => Ok(Value::Float(
            listeners(recv, &arg_str(&args, 0)).len() as f64
        )),
        "eventNames" => Ok(with_host(|h| {
            let mut keys: Vec<String> = Vec::new();
            for map in ["@@on", "@@once"] {
                if let Some(JsObj::Object(p)) = named_map(h, recv, map).and_then(|v| h.get(&v)) {
                    keys.extend(p.keys().cloned());
                }
            }
            let names: Vec<Value> = keys.into_iter().map(|k| h.new_str(k)).collect();
            h.new_array(names)
        })),
        _ => Err(crate::host::type_error(&format!(
            "emitter.{method} is not a function"
        ))),
    }
}

/// Read a hidden emitter field (`@@on`/`@@once`). Works for a plain emitter
/// object AND for a function/class receiver (express's `app` is a function whose
/// emitter maps live in the fn-prop side table).
fn named_map(h: &crate::host::JsHost, recv: &Value, which: &str) -> Option<Value> {
    match h.get(recv) {
        Some(JsObj::Object(p)) => p.get(which).cloned(),
        Some(JsObj::Func(_)) | Some(JsObj::Class(_)) => h.fn_prop(recv, which),
        _ => None,
    }
}

/// Store a hidden emitter field, routing to props or the fn-prop table.
fn set_named_map(h: &mut crate::host::JsHost, recv: &Value, which: &str, val: Value) {
    match h.get(recv) {
        Some(JsObj::Func(_)) | Some(JsObj::Class(_)) => h.set_fn_prop(recv, which, val),
        _ => {
            if let Some(JsObj::Object(p)) = h.get_mut(recv) {
                p.insert(which.to_string(), val);
            }
        }
    }
}

fn add(recv: &Value, which: &str, name: &str, f: Value) {
    with_host(|h| {
        // Lazily create the listener map (a mixed-in function emitter has none).
        let map = match named_map(h, recv, which) {
            Some(m) => m,
            None => {
                let m = h.new_object(IndexMap::new());
                set_named_map(h, recv, which, m.clone());
                m
            }
        };
        // Ensure `map[name]` is an array, then push.
        let arr = match h.get(&map) {
            Some(JsObj::Object(p)) => p.get(name).cloned(),
            _ => None,
        };
        let arr = match arr {
            Some(a) if matches!(h.get(&a), Some(JsObj::Array(_))) => a,
            _ => {
                let a = h.new_array(Vec::new());
                if let Some(JsObj::Object(p)) = h.get_mut(&map) {
                    p.insert(name.to_string(), a.clone());
                }
                a
            }
        };
        if let Some(JsObj::Array(items)) = h.get_mut(&arr) {
            items.push(f);
        }
    });
}

fn listeners(recv: &Value, name: &str) -> Vec<Value> {
    with_host(|h| {
        let mut out = Vec::new();
        for which in ["@@on", "@@once"] {
            if let Some(map) = named_map(h, recv, which) {
                if let Some(JsObj::Object(p)) = h.get(&map) {
                    if let Some(a) = p.get(name) {
                        if let Some(JsObj::Array(items)) = h.get(a) {
                            out.extend(items.iter().cloned());
                        }
                    }
                }
            }
        }
        out
    })
}

fn emit(recv: &Value, name: &str, args: &[Value]) -> Result<Value, String> {
    let to_call = listeners(recv, name);
    // Once-listeners fire a single time: clear them before invoking.
    remove_all_of(recv, "@@once", Some(name));
    let had = !to_call.is_empty();
    for f in to_call {
        invoke(&f, args.to_vec(), Some(recv.clone()))?;
    }
    // Settle any `events.once(emitter, name)` promise waiters for this event.
    resolve_waiters(recv, name, args);
    Ok(Value::Bool(had))
}

// ── `events.once` promise waiters ───────────────────────────────────────────
//
// `once(emitter, name)` returns a real Promise. We cannot register a Rust
// closure as a JS listener (listeners must be callable Values), so instead a
// pending promise is parked under the emitter's hidden `@@waiters` map keyed by
// event name; `emit` (above) settles them. On `error`, waiters of every other
// event reject with the error, mirroring Node's `once` semantics.

/// Park `promise` to be resolved when `name` next fires on `recv`.
fn add_waiter(recv: &Value, name: &str, promise: Value) {
    with_host(|h| {
        let map = match waiter_map(h, recv) {
            Some(m) => m,
            None => {
                let m = h.new_object(IndexMap::new());
                if let Some(JsObj::Object(p)) = h.get_mut(recv) {
                    p.insert("@@waiters".into(), m.clone());
                }
                m
            }
        };
        let existing = match h.get(&map) {
            Some(JsObj::Object(p)) => p.get(name).cloned(),
            _ => None,
        };
        let arr = match existing {
            Some(a) if matches!(h.get(&a), Some(JsObj::Array(_))) => a,
            _ => {
                let a = h.new_array(Vec::new());
                if let Some(JsObj::Object(p)) = h.get_mut(&map) {
                    p.insert(name.to_string(), a.clone());
                }
                a
            }
        };
        if let Some(JsObj::Array(items)) = h.get_mut(&arr) {
            items.push(promise);
        }
    });
}

fn waiter_map(h: &crate::host::JsHost, recv: &Value) -> Option<Value> {
    match h.get(recv) {
        Some(JsObj::Object(p)) => p.get("@@waiters").cloned(),
        _ => None,
    }
}

/// Remove and return the promises parked on `name`.
fn take_waiters(recv: &Value, name: &str) -> Vec<Value> {
    with_host(|h| {
        let Some(map) = waiter_map(h, recv) else {
            return Vec::new();
        };
        let arr = match h.get_mut(&map) {
            Some(JsObj::Object(p)) => p.shift_remove(name),
            _ => None,
        };
        let Some(arr) = arr else { return Vec::new() };
        match h.get(&arr) {
            Some(JsObj::Array(items)) => items.clone(),
            _ => Vec::new(),
        }
    })
}

/// Remove and return every parked promise except those on `keep`.
fn take_waiters_except(recv: &Value, keep: &str) -> Vec<Value> {
    with_host(|h| {
        let Some(map) = waiter_map(h, recv) else {
            return Vec::new();
        };
        let keys: Vec<String> = match h.get(&map) {
            Some(JsObj::Object(p)) => p.keys().filter(|k| k.as_str() != keep).cloned().collect(),
            _ => Vec::new(),
        };
        let mut out = Vec::new();
        for k in keys {
            let arr = match h.get_mut(&map) {
                Some(JsObj::Object(p)) => p.shift_remove(&k),
                _ => None,
            };
            if let Some(arr) = arr {
                if let Some(JsObj::Array(items)) = h.get(&arr) {
                    out.extend(items.iter().cloned());
                }
            }
        }
        out
    })
}

fn resolve_waiters(recv: &Value, name: &str, args: &[Value]) {
    let waiting = take_waiters(recv, name);
    if !waiting.is_empty() {
        let arr = with_host(|h| h.new_array(args.to_vec()));
        for p in &waiting {
            if let Some(id) = with_host(|h| h.promise_id(p)) {
                crate::host::resolve_promise_val(id, arr.clone());
            }
        }
    }
    if name == "error" {
        let err = args.first().cloned().unwrap_or(Value::Undef);
        for p in take_waiters_except(recv, "error") {
            if let Some(id) = with_host(|h| h.promise_id(&p)) {
                crate::host::reject_promise_val(id, err.clone());
            }
        }
    }
}

// ── static module functions (`require('events').once`, `.listenerCount`, …) ──

/// Static functions on the `events` module namespace. `EventEmitter` (the
/// self-ref ctor) and `EventEmitterAsyncResource` are handled by the parent;
/// `on` (async iterator) is deferred (see module docs / final report).
pub const STATIC_METHODS: &[&str] = &[
    "once",
    "listenerCount",
    "getEventListeners",
    "getMaxListeners",
    "setMaxListeners",
    "addAbortListener",
    "init",
];

/// Dispatch a static `events.<method>(...)`. Returns `None` for names this
/// module does not own (e.g. `EventEmitter`) so the parent's specific arm wins.
pub fn static_call(method: &str, args: &[Value]) -> Option<Result<Value, String>> {
    let emitter = args.first().cloned().unwrap_or(Value::Undef);
    Some(match method {
        "once" => Ok(once_static(emitter, &arg_str(args, 1))),
        "listenerCount" => Ok(Value::Float(
            listeners(&emitter, &arg_str(args, 1)).len() as f64
        )),
        "getEventListeners" => Ok(with_host(|h| {
            h.new_array(listeners(&emitter, &arg_str(args, 1)))
        })),
        // No per-emitter cap is tracked; report Node's default and accept sets.
        "getMaxListeners" => Ok(Value::Float(10.0)),
        "setMaxListeners" => Ok(Value::Undef),
        "addAbortListener" => Ok(add_abort_listener(args)),
        "init" => Ok(init_emitter(emitter)),
        _ => return None,
    })
}

/// `events.once(emitter, name)` → a Promise resolving with the event args (or
/// rejecting with the error if `error` fires first).
fn once_static(emitter: Value, name: &str) -> Value {
    let p = with_host(|h| h.new_promise());
    add_waiter(&emitter, name, p.clone());
    p
}

/// `EventEmitter.init(emitter)` — ensure the hidden emitter maps exist on
/// `emitter` (used when mixing the emitter surface into a plain object).
fn init_emitter(emitter: Value) -> Value {
    with_host(|h| {
        let has = matches!(h.get(&emitter), Some(JsObj::Object(p)) if p.contains_key("@@on"));
        if !has {
            let on = h.new_object(IndexMap::new());
            let once = h.new_object(IndexMap::new());
            let native = h.new_str("EventEmitter");
            if let Some(JsObj::Object(p)) = h.get_mut(&emitter) {
                p.entry("@@native".to_string()).or_insert(native);
                p.insert("@@on".to_string(), on);
                p.insert("@@once".to_string(), once);
            }
        }
    });
    emitter
}

/// `events.addAbortListener(signal, listener)` — best-effort: register a
/// one-time `abort` listener if `signal` is emitter-like. `AbortSignal` is not
/// modeled natively, so this is a no-op for plain signals. Returns a disposable
/// placeholder object.
fn add_abort_listener(args: &[Value]) -> Value {
    let signal = args.first().cloned().unwrap_or(Value::Undef);
    let listener = args.get(1).cloned().unwrap_or(Value::Undef);
    let name = with_host(|h| h.new_str("abort"));
    let _ = call_method(&signal, "once", vec![name, listener]);
    with_host(|h| h.new_object(IndexMap::new()))
}

fn remove(recv: &Value, name: &str, f: Option<Value>) {
    let Some(f) = f else { return };
    with_host(|h| {
        for which in ["@@on", "@@once"] {
            if let Some(map) = named_map(h, recv, which) {
                let arr = match h.get(&map) {
                    Some(JsObj::Object(p)) => p.get(name).cloned(),
                    _ => None,
                };
                if let Some(a) = arr {
                    let now_empty = if let Some(JsObj::Array(items)) = h.get_mut(&a) {
                        if let Some(pos) = items.iter().position(|x| x == &f) {
                            items.remove(pos);
                        }
                        items.is_empty()
                    } else {
                        false
                    };
                    // Node drops an event key once its last listener is removed,
                    // so `eventNames()` no longer lists it.
                    if now_empty {
                        if let Some(JsObj::Object(p)) = h.get_mut(&map) {
                            p.shift_remove(name);
                        }
                    }
                }
            }
        }
    });
}

fn remove_all(recv: &Value, name: Option<&str>) {
    remove_all_of(recv, "@@on", name);
    remove_all_of(recv, "@@once", name);
}

fn remove_all_of(recv: &Value, which: &str, name: Option<&str>) {
    with_host(|h| {
        if let Some(map) = named_map(h, recv, which) {
            if let Some(JsObj::Object(p)) = h.get_mut(&map) {
                match name {
                    Some(n) => {
                        p.shift_remove(n);
                    }
                    None => p.clear(),
                }
            }
        }
    });
}