const lifeCycleInstance = Symbol('LifeCycleInstance');
const singletonEnforcer = Symbol('SingletonEnforcer');

const VU_DRAWN_EVT = 'visual-unit.drawn';
const CANVAS_INIT_EVT = 'canvas.init';

const EVENT_LIST = [VU_DRAWN_EVT, CANVAS_INIT_EVT];

const resolver = resolveFn => notifier => resolveFn(notifier);

class LifeCycleManager {

  constructor(enforcer) {
    if (enforcer !== singletonEnforcer) {
      throw new Error('Can not construct LifeCycle');
    }
    this._eventList = EVENT_LIST;
    this._promises = new Map();
    this._notifiers = {};
    this._init();
  }


  _init() {
    this._notifiers = this._eventList.reduce((acc, name) => {
      acc[name] = resolver;
      return acc;
    }, {});
    this._initPromises();
  }

  retrieve(promiseName) {
    return this._promises.get(promiseName);
  }

  _makeNotifierPromise(eventName) {
    return new Promise((resolve) => {
      this._notifiers[eventName] = this._notifiers[eventName](resolve);
    });
  }

  _initPromises() {
    this._eventList.forEach((eventName) => {
      this._promises.set(eventName, this._makeNotifierPromise(eventName));
    });
  }

  notify(notifier) {
    let formalName = notifier.formalName || notifier.client.constructor.formalName();
    this._notifiers[`${formalName}.${notifier.action}`](notifier);
  }

  static get instance() {
    if (!this[lifeCycleInstance]) {
      this[lifeCycleInstance] = new LifeCycleManager(singletonEnforcer);
    }
    return this[lifeCycleInstance];
  }
}

// ----------- Picasso Library --------------- //

class VisualUnit {

  constructor() {
    this.lifeCycleManager = LifeCycleManager.instance;
  }

  static formalName() {
    return 'visual-unit';
  }

  draw() {
    console.log('drawing visual-unit');
  }
}


class Canvas {
  constructor() {
    console.log('init canvas');
    this.lifeCycleManager = LifeCycleManager.instance;
    this.lifeCycleManager.notify({
      client: this,
      action: 'init'
    });
  }

  static formalName() {
    return 'canvas';
  }

  hooks(hookList) {
    const promiseBag = [];
    hookList.forEach(hookName => {
      promiseBag.push(this.lifeCycleManager.retrieve(hookName));
    });
    return promiseBag;
  }

  mount() {
    // create multiple visual units
    let units = [];
    for (let i = 0; i < 10; i++) {
      let visualUnit = new VisualUnit();
      visualUnit.draw();
      units.push(visualUnit);
    }

    this.lifeCycleManager.notify({
      client: units,
      action: 'drawn',
      formalName: 'visual-unit'
    });
  }
}


// -------------- User Code ------------ //

let canvas = new Canvas();

const promises = canvas.hooks(['visual-unit.drawn', 'canvas.init']);

Promise.all(promises).then(d => {
  console.log(d);
})

canvas.mount();