Subscription.js

class Subscription {
  constructor(store, parentSub, onStateChange) {
    this.store = store //global store from context
    this.parentSub = parentSub //parentSub from context
    this.onStateChange = onStateChange //its own listener
    this.subscribed = false //a flag for testing whether itself has been subscribed
    this.listeners = [] //nested subscription listeners subscribe here
  }
  //notify the listeners subscribed to itself
  notifyNestedSubs() {
    this.listeners.forEach(l=>l());
  }
  trySubscribe() {
    //if not yet subscribed
    if (!this.subscribed) {
      if (this.parentSub !== null) {
        //if has parentSub(from context), subscribe to parentSub
        this.parentSub.addNestedSub(this.onStateChange)
      }
      else {
        //if root component, subscribe directly to store
        this.store.subscribe(this.onStateChange)
      }
      //mark it as subscribed
      this.subscribed = true;
    }
  }
  //for adding nested subscription listener to itself
  addNestedSub(listener) {
    //FIRST, ensure itself has been subscribed, that's how the order is maintained
    this.trySubscribe()
    //THEN, subscribe the nested listener to its own listener collection
    this.listeners.push(listener)
  }
}