vue计算属性无法监听到数组内部变化的解决方案

var vm = new Vue({
 el: '#demo',
 data: {
 firstName: 'Foo',
 lastName: 'Bar'
 },
 computed: {
 fullName: function () {
  return this.firstName + ' ' + this.lastName
 }
 }
})

<div id="demo">{{ fullName }}</div>

var vm = new Vue({
 el: '#demo',
 data: {
  nameArray:['Foo' ,'Bar']
 },
 computed: {
 fullName: function () {
  return this.nameArray[0] + ' ' + this.nameArray[1]
 }
 }
})

methods: {
 change () {
  this.nameArray[0] = '123'
 }

this.nameArray.splice(0, 1, '123')

this.$set(this.nameArray, 0, '123')