ljm0/modules/util/EventEmitterForwarder.ts

/**
 * Implements utility to forward events from one eventEmitter to another.
 * @param src {object} instance of EventEmitter or another class that implements
 * addListener method which will register listener to EventEmitter instance.
 * @param dest {object} instance of EventEmitter or another class that
 * implements emit method which will emit an event.
 */
export default class EventEmitterForwarder {
    src: any;
    dest: any;
    listeners: Map<any, any>;

    /**
     * @constructor
     */
    constructor(src: any, dest: any) {
        if (!src || !dest || typeof src.addListener !== 'function' || typeof dest.emit !== 'function') {
            throw new Error('Invalid arguments passed to EventEmitterForwarder');
        }
        this.src = src;
        this.dest = dest;
        this.listeners = new Map();
    }

    /**
     * Adds event to be forwarded from src to dest.
     * @param srcEvent {string} the event that EventEmitterForwarder is listening
     * for.
     * @param dstEvent {string} the event that will be fired from dest.
     * @param arguments all other passed arguments are going to be fired with
     * dstEvent.
     */
    forward(...args) {
        const srcEvent = args[0];

        // This will be the "this" value for emit function.
        args[0] = this.dest;

        // Using bind.apply to pass the arguments as Array-like object ("arguments")
        const newListener = Function.prototype.bind.apply(this.dest.emit, args);

        this.src.addListener(srcEvent, newListener);
        this.listeners.set(srcEvent, newListener);
    }

    /**
     * Clears the listeners for the supplied events.
     *
     * @param args all the events which listeners to be cleaned.
     */
    removeListeners(...args) {
        args.forEach(a => {
            const l = this.listeners.get(a);

            if (l) {
                this.src.removeListener(a, l);
                this.listeners.delete(a);
            }
        });
    }
}