Class to handle SignalR subscriptions

hub.js

//+ Jonas Raoni Soares Silva
//@ http://raoni.org

const signalR = require('@aspnet/signalR');

export default class Hub {
  constructor (url, autoReloadTimeout = 1000) {
    Object.assign(this, {
      url,
      autoReloadTimeout,
      connection: null,
      isStarted: false,
      closeGracefully: false,
      eventHandlerMap: new Map()
    });
  }

  on (event, handler) {
    let eventHandler = this.eventHandlerMap.get(event);
    if (!eventHandler) {
      eventHandler = {
        subscriptionSet: new Set(),
        handler: (...args) => {
          try {
            for (const handler of eventHandler.subscriptionSet) {
              handler(...args);
            }
          } catch (e) {
            console.info(e);
          }
        }
      };
      this.eventHandlerMap.set(event, eventHandler);
      if (this.connection) {
        this.connection.on(event, eventHandler.handler);
      }
    }
    const subscriptionSet = eventHandler.subscriptionSet;
    subscriptionSet.add(handler);
    return this;
  }

  off (event, handler) {
    if (event === undefined) {
      if (this.connection) {
        for (const [event, eventHandler] of this.eventHandlerMap) {
          this.connection.off(event, eventHandler.handler);
        }
      }
      this.eventHandlerMap = new Map();
    } else {
      let eventHandler = this.eventHandlerMap.get(event);
      if (eventHandler) {
        let canRemoveEvent = !handler;
        if (handler !== undefined) {
          eventHandler.subscriptionSet.delete(handler);
          canRemoveEvent = !eventHandler.subscriptionSet.size;
        }
        if (canRemoveEvent) {
          if (this.connection) {
            this.connection.off(event, eventHandler.handler);
          }
          this.eventHandlerMap.delete(event);
        }
      }
    }
    return this;
  }

  start () {
    if (this.isStarted) { return; }
    this.isStarted = true;
    const connection = this.connection = new signalR.HubConnectionBuilder()
      .withUrl(this.url)
      .configureLogging(signalR.LogLevel.Debug)
      .build();

    for (const [event, eventHandler] of this.eventHandlerMap) {
      connection.on(event, eventHandler.handler);
    }

    connection.onclose((error) => {
      console.debug(`Connection to ${this.url} closed.`);
      if (!this.closeGracefully) {
        console.error(error);
        this.restart();
      }
      this.closeGracefully = false;
    });

    connection.start()
      .then(c => console.debug(`Connection to ${this.url} established.`))
      .catch(error => {
        console.debug(`Cannot connect to ${this.url}.`);
        console.error(error);
        setTimeout(() => this.restart(), this.autoReloadTimeout);
      });
  };

  stop () {
    this.closeGracefully = true;
    this.connection.stop();
    this.isStarted = false;
  }

  restart () {
    this.isStarted = false;
    this.start();
  }
}