JavaScript计算属性与监视(侦听)属性的使用

<div id="root">
    <input type="text" v-model="name">
</div>
<script src="./vue.js"></script>
<script>
    const vm = new Vue({
        el:'#root',
        data:{
            name:''
        },
        //所有的侦听器，都应该被定义到watch节点下
        watch:{
            // 侦听器本质上是一个函数，要监视哪个数据的变化，就把数据名作为方法名即可
            //newVal是“变化后的新值”，oldVal是“变化之前旧值”
            name(newVal,oldVal){ //监听name值的变化
                console.log("监听到了新值"+newVal, "监听到了旧值"+oldVal);
            }
        }
    })
</script>

<div id="root">
    <h2>今天天气很{{info}}</h2>
    <button @click="changeWeather">切换天气</button>
</div>
<script src="./vue.js"></script>
<script>
    const vm = new Vue({
        el:'#root',
        data:{
            isHot:true
        },
        computed:{
            info(){
                return this.isHot ? '炎热' : '凉爽'
            }
        },
        methods:{
            changeWeather(){
                this.isHot = !this.isHot
            }
        },
    })
    vm.$watch('info',{
        handler(newVal,oldVal){
            console.log('天气被修改了', newVal, oldVal);
        }
    })
</script>

<div id="root">
    <input type="text" v-model="name">
</div>
<script src="./vue.js"></script>
<script>
    const vm = new Vue({
        el:'#root',
        data:{
            name:'admin'
        },
        watch:{
            //定义对象格式的侦听器
            name:{
                handler(newVal,oldVal){
                    console.log(newVal, oldVal);
                },
                immediate:true
            }
        }
    })
</script>

<div id="root">
    <input type="text" v-model="info.name">
</div>
<script src="./vue.js"></script>
<script>
    const vm = new Vue({
        el:'#root',
        data:{
            info:{
                name:'admin'
            }
        },
        watch:{
            info: {
                handler(newVal){
                    console.log(newVal);
                },
                //开启深度监听
                deep:true
            }
        }
    })
</script>

watch:{
    "info.name"(newVal){
        console.log(newVal);
    }
}

<div id="root">
    <input type="text" v-model="firstname"><br>
    <input type="text" v-model="lastname"><br>
    全名：<span>{{fullname}}</span>
</div>
<script src="/vue/vue.js"></script>
<script>
    const vm = new Vue({
        el:"#root",
        data:{
            firstname:'张',
            lastname:'三',
            fullname:'张-三'
        },
        //watch能开启异步任务
        watch:{
            firstname(val){
                setTimeout(()=>{
                    this.fullname = val + '-' + this.lastname
                },1000)
            },
            lastname(val){
                this.fullname = this.firstname+'-'+val
            }
        }
    })
</script>