watcher.js

// See https://github.com/vuejs/vue/blob/be9ac624c81bb698ed75628fe0cbeaba4a2fc991/src/core/observer/watcher.js
// for full implementation

class Watcher {
  constructor(getter, cb) {
    this.getter = getter // function that returns a value based on reactive properties
    this.cb = cb // function that is run on value updates, and is passed value and old value
    this.value = this.get()
    this.cb(this.value, null)
  }

  get() {
    pushTarget(this) // from dep.js
    const value = this.getter()
    popTarget() // from dep.js

    return value
  }

  addDep(dep) {
    dep.addSub(this)
  }

  update() {
    const value = this.get()
    const oldValue = this.value
    this.value = value

    this.cb(value, oldValue)
  }
}