In this lesson we will build a simple reactivity system using the very same techniques you’ll find in the Vue source code. This will give you a better understanding of Vue.js and it’s design patterns, as well as get you familiar with watchers and the Dep class.

The Reactivity System

Vue’s reactivity system can look like magic when you see it working for the first time.

Take this simple app:

    <div id="app">
      <div>Price: ${{ price }}</div>
      <div>Total: ${{ price * quantity }}</div>
      <div>Taxe