20Vue - 计算属性（观察 Watchers)

<div id="watch-example">
  <p>
    Ask a yes/no question:
    <input v-model="question">
  </p>
  <p>{{ answer }}</p>
</div>

<!-- Since there is already a rich ecosystem of ajax libraries    -->
<!-- and collections of general-purpose utility methods, Vue core -->
<!-- is able to remain small by not reinventing them. This also   -->
<!-- gives you the freedom to just use what you're familiar with. -->
<script src="https://unpkg.com/axios@0.12.0/dist/axios.min.js"></script>
<script src="https://unpkg.com/lodash@4.13.1/lodash.min.js"></script>
<script>
var watchExampleVM = new Vue({
  el: '#watch-example',
  data: {
    question: '',
    answer: 'I cannot give you an answer until you ask a question!'
  },
  watch: {
    // 如果 question 发生改变，这个函数就会运行
    question: function (newQuestion) {
      this.answer = 'Waiting for you to stop typing...'
      this.getAnswer()
    }
  },
  methods: {
    // _.debounce 是一个通过 lodash 限制操作频率的函数。
    // 在这个例子中，我们希望限制访问yesno.wtf/api的频率
    // ajax请求直到用户输入完毕才会发出
    // 学习更多关于 _.debounce function (and its cousin
    // _.throttle), 参考: https://lodash.com/docs#debounce
    getAnswer: _.debounce(
      function () {
        var vm = this
        if (this.question.indexOf('?') === -1) {
          vm.answer = 'Questions usually contain a question mark. ;-)'
          return
        }
        vm.answer = 'Thinking...'
        axios.get('https://yesno.wtf/api')
          .then(function (response) {
            vm.answer = _.capitalize(response.data.answer)
          })
          .catch(function (error) {
            vm.answer = 'Error! Could not reach the API. ' + error
          })
      },
      // 这是我们为用户停止输入等待的毫秒数
      500
    )
  }
})
</script>