SOURCE

function Watcher (vm, exp, cb) {
  this.vm = vm
  this.exp = exp
  this.cb = cb
  this.value = this.get()
}

Watcher.prototype = {
  update: function () {
    var value = this.vm.data[this.exp]
    var oldValue = this.value
    
    if (value !== oldValue) {
      this.value = value
      this.cb.call(this.vm, value, oldValue)
    }
  },
  get: function () {
    Dep.target = this

    var value = this.vm.data[this.exp]
    
    Dep.target = null
    
    return value
  }
}