Vue中的watch观察模式
watch用于监测数据的变化，如果发生变化会及时返回修改前和修改后的数据
1）基础示例
<div id="app"><input type="text" v-model="val"><h1>{{change}}</h1></div><script>new Vue({el: '#app',data: {val: 'Hello World !',change: ''},watch: {val(news, olds){this.change = 'new: ' + news + ' old: ' + olds;}}})</script>
2）实例挂载
<div id="app"><input type="text" v-model="vals"><h1>{{vals}}</h1></div><script>new Vue({el: '#app', data: {vals: 'Hello World !'}}).$watch('vals', function(newVal, oldVal){console.log('newVal: ' + newVal);console.log('oldVal: ' + oldVal);})</script>
