code: function PubSub(context) {
      if (_typeof(context) !== 'object') {
        throw new Error('Invalid Context passed to initialize PubSub (jsPDF-module)');
      }
  
      var topics = {};
  
      this.subscribe = function (topic, callback, once) {
        once = once || false;
  
        if (typeof topic !== 'string' || typeof callback !== 'function' || typeof once !== 'boolean') {
          throw new Error('Invalid arguments passed to PubSub.subscribe (jsPDF-module)');
        }
  
        if (!topics.hasOwnProperty(topic)) {
          topics[topic] = {};
        }
  
        var token = Math.random().toString(35);
        topics[topic][token] = [callback, !!once];
        return token;
      };
  
      this.unsubscribe = function (token) {
        for (var topic in topics) {
          if (topics[topic][token]) {
            delete topics[topic][token];
  
            if (Object.keys(topics[topic]).length === 0) {
              delete topics[topic];
            }
  
            return true;
          }
        }
  
        return false;
      };
  
      this.publish = function (topic) {
        if (topics.hasOwnProperty(topic)) {
          var args = Array.prototype.slice.call(arguments, 1),
              tokens = [];
  
          for (var token in topics[topic]) {
            var sub = topics[topic][token];
  
            try {
              sub[0].apply(context, args);
            } catch (ex) {
              if (global.console) {
                console.error('jsPDF PubSub Error', ex.message, ex);
              }
            }
  
            if (sub[1]) tokens.push(token);
          }
  
          if (tokens.length) tokens.forEach(this.unsubscribe);
        }
      };
  
      this.getTopics = function () {
        return topics;
      };
    }
docs: jsPDF's Internal PubSub Implementation.
  Backward compatible rewritten on 2014 by
  Diego Casorran, https://github.com/diegocr
  
  @class
  @name PubSub
  @ignore