text: export default class Dep {
      constructor() {
          this.subs = {}
      }
  
      addSub(key, watcher) {
          const subs = this.subs
          subs[key]
              ? subs[key].push(watcher)
              : (subs[key] = [watcher])
      }
  
      notify(key) {
          this.subs[key] && this.subs[key].forEach(watcher => watcher.update())
      }
  }