为什么要用nextTick
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>example</title>
</head>
<body>
<div id="app">
<div v-if="isShow">
<input type="text" ref="userName" />
</div>
<button @click="showInput">点击显示输入框</button>
</div>
</body>
</html>
<script src="https://cdn.jsdelivr.net/npm/vue/dist/vue.js"></script>
<script>
var app = new Vue({
el: '#app',
data: {
isShow: false
},
methods:{
showInput(){
this.isShow = true
this.$refs.userName.focus()
}
}
})
</script>
运行结果是报错，找不到节点。也就是说，当你执行到isShow=true时，此时dom节点尚未更新，只能等待dom更新后，你才能执行下面的focus。
用MessageChannel实现
<!DOCTYPE html>
<html lang="en-zh">
<head>
<meta charset="utf-8" />
<style type="text/css">
</style>
</head>
<body>
<div id="app">
<div v-if="isShow">
<input type="text" ref="userName" />
</div>
<button @click="showInput">点击显示输入框</button>
</div>
</body>
</html>
<script src="https://cdn.jsdelivr.net/npm/vue/dist/vue.js"></script>
<script>
var app = new Vue({
el: '#app',
data: {
isShow: false
},
methods: {
showInput(){
this.isShow = true;
this.myNextTick(() => {
this.$refs.userName.focus();
})
},
myNextTick(fanc){
var that = this;
const ch = new MessageChannel();
const port1 = ch.port1;
const port2 = ch.port2;
port2.onmessage = (() => {
fanc();
})
port1.postMessage(1);
}
}
})
</script>
用MutationObserver实现
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>example</title>
</head>
<body>
<div id="app">
<div v-if="isShow">
<input type="text" ref="userName" />
</div>
<button @click="showInput">点击显示输入框</button>
</div>
</body>
</html>
<script src="https://cdn.jsdelivr.net/npm/vue/dist/vue.js"></script>
<script>
var app = new Vue({
el: '#app',
data: {
isShow: false
},
methods:{
showInput(){
this.isShow = true
this.mynextTick(()=>{
this.$refs.userName.focus()
})
},
mynextTick(func){
var textNode = document.createTextNode(0)//新建文本节点
var that = this
var callback = function(mutationsList, observer) {
func.call(that);
// 或
// fanc();
}
var observer = new MutationObserver(callback);
observer.observe(textNode,{characterData:true })
textNode.data = 1//修改文本信息，触发dom更新
}
}
})
</script>
为何弃用MessageChannel使用MutationObserver
vue2中在2018年12月20号用MutationObser替换了MessageChannel（有很多博客说是相反的，但查git上的时间线是这样的），把2个都使用下对比便知结果：
因为MutationObserver是宏任务，MessageChannel是微任务，比它先执行，选用它肯定考虑了这部分因素。
<!DOCTYPE html>
<html lang="en-zh">
<head>
<meta charset="utf-8" />
<style type="text/css">
</style>
</head>
<body>
<div id="app">
<div v-if="isShow">
<input type="text" ref="userName" />
</div>
<button @click="showInput">点击显示输入框</button>
</div>
</body>
</html>
<script src="https://cdn.jsdelivr.net/npm/vue/dist/vue.js"></script>
<script>
var app = new Vue({
el: '#app',
data: {
isShow: false
},
methods: {
showInput(){
this.isShow = true;
this.myNextTick1(() => {
this.$refs.userName.focus();
})
this.myNextTick2(() => {
this.$refs.userName.focus();
})
},
myNextTick1(fanc){
var that = this;
const ch = new MessageChannel();
const port1 = ch.port1;
const port2 = ch.port2;
port2.onmessage = (() => {
console.log('1')
fanc();
})
port1.postMessage(1);
},
myNextTick2 (fanc) {
var that = this;
var textNode = document.createTextNode('0');
var callback = function(mutationList, observer){
console.log('2')
fanc();
}
var observer = new MutationObserver(callback);
observer.observe(textNode, {characterData: true});
textNode.data = 1;
}
}
})
</script>
打印出来：
2
1
并不是按顺序执行，而是先宏任务MutationObserver，再微任务MessageChannel。