event-emitters/dist.es2018/QueueingEventEmitter.js

event emitters

import { EventEmitter } from './EventEmitter';
/**
 * Acts the same as EventEmitter but queues messages when there are no subscribers, delivering them
 * as soon as the first subscriber subscribes.
 */
export class QueueingEventEmitter extends EventEmitter {
    constructor() {
        super(...arguments);
        this.messageQueue = [];
    }
    emit(newMessage) {
        if (!this.listeners.length) {
            this.messageQueue.push(newMessage);
        }
        else {
            super.emit(newMessage);
        }
    }
    /**
     * The queue will still be drained if the subscriber throws error, in the case that the
     * there are multiple messages with multiple exceptions thrown, only the last one will
     * be re-thrown.
     */
    subscribe(listener) {
        super.subscribe(listener);
        if (this.messageQueue.length) {
            let error;
            this.messageQueue.forEach((queuedMessage) => {
                try {
                    super.emit(queuedMessage);
                }
                catch (e) {
                    error = e;
                }
            });
            this.messageQueue.splice(0);
            if (error) {
                throw error;
            }
        }
        return listener;
    }
}
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