Vue3监听属性

我们可以通过watch来响应数据的变化。

<!--
 * @Date: 2023-04-02 19:41:53
 * @LastEditors: Mei
 * @LastEditTime: 2023-04-02 19:50:34
 * @FilePath: \vscode\Vue3_listen.html
 * @Description: 
 * 
 * Copyright (c) 2023 by ${git_name_email}, All Rights Reserved. 
-->
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script src="vue_doc/vue.global3.js"></script>
</head>
<body>
    <div id="app">
        <p style = "font-size: 25px;">计数器：{{counter}}</p>
        <button @click="counter++" style="font-size: 25px;">点我</button>
    </div>

    <script>
        const app={
            data(){
                return{
                    counter:1
                }
            }
        }
        vm=Vue.createApp(app).mount('#app')
        vm.$watch('counter',function(a,b){
            alert('数字变化，后面的编程前面的：'+b+'编程'+a+'!!!!!!')
        })
    </script>
</body>
</html>

Vue3学习笔记（8.0）_vue.js

 以下实例进行千米和米之间的换算：

<!--
 * @Date: 2023-04-02 19:41:53
 * @LastEditors: Mei
 * @LastEditTime: 2023-04-03 09:31:26
 * @FilePath: \vscode\Vue3_listen.html
 * @Description: 
 * 
 * Copyright (c) 2023 by ${git_name_email}, All Rights Reserved. 
-->
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script src="vue_doc/vue.global3.js"></script>
</head>
<body>
    <div id="app">
        <!-- <p style = "font-size: 25px;">计数器：{{counter}}</p>
        <button @click="counter++" style="font-size: 25px;">点我</button> -->
        千米：<input type="text" v-model="kilometers" @focus="currentlyActiveField='kilometers'">
        米：<input type="text" v-model="meters" @focus="currentlyActiveField='meters'">

    </div>
    <p id="info"></p>

    <script>
        const app={
            data(){
                return{
                    // counter:1
                    kilometers:0,
                    meters:0
                }
            },
            watch:{
                meters:function(newValue,oldValue){
                    if(this.currentlyActiveField==='meters'){
                        this.kilometers=newValue/1000;
                        this.meters=newValue;
                    }
                },
                kilometers:function(newValue,oldValue){
                    if(this.currentlyActiveField==='kilometers'){
                        this.kilometers=newValue;
                        this.meters=newValue*1000;
                    }
                }
            }
        }

        vm=Vue.createApp(app).mount('#app')
        vm.$watch('kilometers',function(a,b){
            document.getElementById("info").innerHTML='数字变化，修改前的值是：'+b+'修改后的值是'+a+'!!!!!!'
        })
    </script>
</body>
</html>

Vue3学习笔记（8.0）_html5_02

 异步加载中使用watch

异步数据的加载Vue通过watch选项提供了一个更通用的方法，来响应数据的变化。

以下实例我们使用axios库

<script>
  const watchExampleVM = Vue.createApp({
    data() {
      return {
        question: '',
        answer: '每个问题结尾需要输入 ? 号。'
      }
    },
    watch: {
      // 每当问题改变时，此功能将运行，以 ? 号结尾，兼容中英文 ?
      question(newQuestion, oldQuestion) {
        if (newQuestion.indexOf('?') > -1 || newQuestion.indexOf('？') > -1) {
          this.getAnswer()
        }
      }
    },
    methods: {
      getAnswer() {
        this.answer = '加载中...'
        axios
          .get('/try/ajax/json_vuetest.php')
          .then(response => {
            this.answer = response.data.answer
          })
          .catch(error => {
            this.answer = '错误! 无法访问 API。 ' + error
          })
      }
    }
  }).mount('#watch-example')
</script>