Type-safe event hooks and boxed values (aka "getter-setters") in Typescript

hooks.ts

/// This interface defines an event listener
interface IListener<Event> {
    (event: Event): void
}

/// This interface represents an event hook
interface IHook<Event> {
    /// Attach a handler to this hookable
    (handler: IListener<Event>): void

    /// Send an event to all listeners
    (event: Event): void
    
    or<Other>(other: IHook<Other>) : IHook<Event|Other>
}

/// This interface represents a hookable boxed value
interface IBox<T> extends IHook<T> {
    /// Retrieve the value
    () : T
}

/// Create an event hook for a specific type of message
function hook<Event>(getter? : { (): Event }): IHook<Event> {
    var _handlers: Array<Function> = []
    var _busy = false
    var _getter = getter; // || function () { return null; };
    
    function invoke(message)
    function invoke(event: Event)
    function invoke() {
        if (arguments.length === 0) {
            return _getter();
        } else if (arguments[0] instanceof Function) {
            _handlers.push(arguments[0])
        } else if (!_busy) {
            _busy = true
            for (var i = 0; i < _handlers.length; i++) {
                _handlers[i].apply(this, arguments)
            }
            _busy = false
        }
    }
    
    invoke["or"] = function <Other> (other: IHook<Other>) : IHook<Event|Other> {
        return <IHook<Event>> function (event: Event|Other) {
            invoke(<Event> event)
            other(<Other> event)
        }
    }
    
    return <IHook<Event>> <any> invoke
}

/// Create a hookable boxed value (optionally with an initial value)
function box<T>(value?: T): IBox<T> {
    var _value: T = value
    var _hook = hook(() => _value)
    
    _hook((value: T) => _value = value)
    
    return <IBox<T>> _hook;
}

// EXAMPLE:

class User {
    first_name = box("Foo")
    last_name = box("Bar")
}

class Model {
    onLogin = hook<{user: User}>()
}

var model = new Model()

model.onLogin(e => console.log("Logged in: " + e.user.first_name()))

var user = new User()

user.first_name.or(user.last_name)((value) => console.log("updated: ", value))

user.first_name("Rasmus")
user.last_name("Schultz")

model.onLogin({user})