es6 javascript class with pub sub pattern, losing context

Question

i'm kinda new to es6 features, i'm trying to implement some basic class, like this

class App {
  constructor() {
    this.init();
  }

  init() {
    console.log("init!");
    this.hello();
  }

  hello(){
    console.log("Hello!");
  }
}

let app = new App;

everything works fine. this is my output:

init
Hello!

then i tried to implement the same thing with publish / subscribe pattern (using PubSubJs plugin):

class App {
  constructor() {
    PubSub.subscribe('say hello', this.init);
  }

  init() {
    console.log("init!");
    this.hello();
  }

  hello(){
    console.log("Hello!");
  }
}

let app = new App;

PubSub.publish('say hello');

This will call correctly the init method, but then will lose the class context when trying to call this.hello()

infact i get this output:

init //correct
Uncaught TypeError: this.hello is not a function //<- why this?

DEMO HERE

any idea? am i missing something? thanks