multiple-listeners.js

export type Channel = {
    on(event : string, listener : Function): *
};

export type Subscriber = {
    listener(config : { window : HTMLIFrameElement, domain : string }): Channel
};

const subscribe = (channel : Channel) => {
    let listeners = {};
    let stopChannel = {};

    const hasListeners = (eventName : string) : boolean => {
        return listeners[eventName] && listeners[eventName].length > 0 ? true : false;
    };

    const addListener = (eventName : string, listener : Function) => {
        listeners[eventName].push(listener);
    };

    const initializeChannel = (eventName : string, listener : Function) => {
        listeners[eventName] = [
            listener
        ];

        stopChannel[eventName] = channel.on(eventName, (...args) => {
            listeners[eventName].forEach(l => l(...args));
        });
    };

    return {
        on(eventName : string, listener : Function) : Function {
            if (hasListeners(eventName)) {
                addListener(eventName, listener);
            } else {
                initializeChannel(eventName, listener);
            }
    
            return function unsubscribe() {
                const index = listeners[eventName].indexOf(listener);
                listeners[eventName].splice(index, 1);
                
                if (!hasListeners(eventName)) {
                    stopChannel[eventName]();
                }
            };
        }
    };
};

export function createChannel(subscriber : Subscriber, iframe : HTMLIFrameElement) : Channel {
    const channel = postRobot.listener({ window: iframe.contentWindow, domain: __HOST__ });

    return subscribe(channel);
}