observer.js

// See https://github.com/vuejs/vue/blob/61187596b9af48f1cb7b1848ad3eccc02ac2509d/src/core/observer/index.js
// for full implementation

/* Walk through each property and convert them into
* getter/setters. This method should only be called when
* value type is Object.
*/
function walk(obj) {
  const keys = Object.keys(obj)
  for (let i = 0; i < keys.length; i++) {
    defineReactive(obj, keys[i], obj[keys[i]])
  }
}

function defineReactive(obj, key, val) {
  if (val !== null && typeof val === 'object') {
    walk(val) // Add reactivity to all children of val
  }

  const dep = new Dep()
  Object.defineProperty(obj, key, {
    enumerable: true,
    configurable: true,
    get: function reactiveGetter() {
      dep.depend()

      return val
    },
    set: function reactiveSetter(newVal) {
      val = newVal
      dep.notify()
    }
  })
}