侦听属性

侦听属性，可以帮助我们侦听data某个数据的变化，从而做相应的自定义操作。

侦听属性是一个对象，它的键是要监听的对象或者变量，值一般是函数，当侦听的data数据发生变化时，会自定执行的对应函数，这个函数在被调用时，vue会传入两个形参，第一个是变化前的数据值，第二个是变化后的数据值。

案例1：

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script src="js/vue.min.js"></script>
    <script>
      window.onload = function(){
         var vm = new Vue({
            el:"#app",
            data:{
                num:20
            },
            watch:{
                num:function(newval,oldval){
                    //num发生变化的时候，要执行的代码
                    console.log("num已经发生了变化！");
                }
            }
        })
      }
    </script>
</head>
<body>
    <div id="app">
        <p>{{ num }}</p>
        <button @click="num++">按钮</button>
    </div>
</body>
</html>

案例2：

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script src="js/vue.js"></script>
    <style>
    input[type=password]{
        outline-color: red;
    }
    input.is_pass{
        outline-color: blue;
    }
    input.is_fail{
        outline-color: red;
    }
    </style>
</head>
<body>
    <div id="app">
        <button @click="num--">-</button>
        <input type="text" size="1" v-model="num">
        <button @click="num++">+</button>
        <p>
            <input type="password" :class="pwd_tips" v-model="password">
        </p>
    </div>
    <script>
        const vm = new Vue({
            el:"#app",
            data(){
                return {
                    num: 0,
                    password: "",
                    pwd_tips: "is_fail",
                }
            },
            // 侦听属性，往往开发中用于验证用户的操作和输入内容，当然也可以在用户输入合法结果时，自动完成对应的操作
            // 例如：http提交数据，提示。
            watch:{
                num(){
                    if(this.num < 0){
                        this.num = 0;
                    }
                },
                password(){
                    if(this.password.length>6 && this.password.length<16){
                        this.pwd_tips = "is_pass"
                    }else{
                        this.pwd_tips = "is_fail";
                    }
                }
            },
            methods:{
 
            }
        })
    </script>
</body>
</html>