详解vue2父组件传递props异步数据到子组件的问题
测试环境:vue v2.3.3, vue v2.3.1
案例一
父组件parent.vue
// asyncData为异步获取的数据,想传递给子组件使用 <template> <div> 父组件 <child :child-data="asyncData"></child> </div> </template> <script> import child from './child' export default { data: () => ({ asyncData: '' }), components: { child }, created () { }, mounted () { // setTimeout模拟异步数据 setTimeout(() => { this.asyncData = 'async data' console.log('parent finish') }, 2000) } } </script>
子组件child.vue
<template> <div> 子组件{{childData}} </div> </template> <script> export default { props: ['childData'], data: () => ({ }), created () { console.log(this.childData) // 空值 }, methods: { } } </script>
上面按照这里的解析,子组件的html中的{{childData}}的值会随着父组件的值而改变,但是created里面的却不会发生改变(生命周期问题)
案例二
parent.vue
<template> <div> 父组件 <child :child-object="asyncObject"></child> </div> </template> <script> import child from './child' export default { data: () => ({ asyncObject: '' }), components: { child }, created () { }, mounted () { // setTimeout模拟异步数据 setTimeout(() => { this.asyncObject = {'items': [1, 2, 3]} console.log('parent finish') }, 2000) } } </script>
child.vue
<template> <div> 子组件<!--这里很常见的一个问题,就是{{childObject}}可以获取且没有报错,但是{{childObject.items[0]}}不行,往往有个疑问为什么前面获取到值,后面获取不到呢?--> <p>{{childObject.items[0]}}</p> </div> </template> <script> export default { props: ['childObject'], data: () => ({ }), created () { console.log(this.childObject) // 空值 }, methods: { } } </script>
created里面的却不会发生改变, 子组件的html中的{{{childObject.items[0]}}的值虽然会随着父组件的值而改变,但是过程中会报错
// 首先传过来的是空,然后在异步刷新值,也开始时候childObject.items[0]等同于''.item[0]这样的操作,所以就会报下面的错 vue.esm.js?8910:434 [Vue warn]: Error in render function: "TypeError: Cannot read property '0' of undefined"
针对二的解决方法:
使用v-if可以解决报错问题,和created为空问题
// parent.vue <template> <div> 父组件 <child :child-object="asyncObject" v-if="flag"></child> </div> </template> <script> import child from './child' export default { data: () => ({ asyncObject: '', flag: false }), components: { child }, created () { }, mounted () { // setTimeout模拟异步数据 setTimeout(() => { this.asyncObject = {'items': [1, 2, 3]} this.flag = true console.log('parent finish') }, 2000) } } </script>
child.vue
<template> <div> 子组件 <!--不报错--> <p>{{childObject.items[0]}}</p> </div> </template> <script> export default { props: ['childObject'], data: () => ({ }), created () { console.log(this.childObject)// Object {items: [1,2,3]} }, methods: { } } </script>
子组件使用watch来监听父组件改变的prop,使用methods来代替created
parent.vue
<template> <div> 父组件 <child :child-object="asyncObject"></child> </div> </template> <script> import child from './child' export default { data: () => ({ asyncObject: '' }), components: { child }, created () { }, mounted () { // setTimeout模拟异步数据 setTimeout(() => { this.asyncObject = {'items': [1, 2, 3]} console.log('parent finish') }, 2000) } } </script>
child.vue
<template> <div> 子组件<!--1--> <p>{{test}}</p> </div> </template> <script> export default { props: ['childObject'], data: () => ({ test: '' }), watch: { 'childObject.items': function (n, o) { this.test = n[0] this.updata() } }, methods: { updata () { // 既然created只会执行一次,但是又想监听改变的值做其他事情的话,只能搬到这里咯 console.log(this.test)// 1 } } } </script>
子组件watch computed data 相结合,有点麻烦
parent.vue
<template> <div> 父组件 <child :child-object="asyncObject"></child> </div> </template> <script> import child from './child' export default { data: () => ({ asyncObject: undefined }), components: { child }, created () { }, mounted () { // setTimeout模拟异步数据 setTimeout(() => { this.asyncObject = {'items': [1, 2, 3]} console.log('parent finish') }, 2000) } } </script>
child.vue
<template> <div> 子组件<!--这里很常见的一个问题,就是{{childObject}}可以获取且没有报错,但是{{childObject.items[0]}}不行,往往有个疑问为什么前面获取到值,后面获取不到呢?--> <p>{{test}}</p> </div> </template> <script> export default { props: ['childObject'], data: () => ({ test: '' }), watch: { 'childObject.items': function (n, o) { this._test = n[0] } }, computed: { _test: { set (value) { this.update() this.test = value }, get () { return this.test } } }, methods: { update () { console.log(this.childObject) // {items: [1,2,3]} } } } </script>
使用emit,on,bus相结合
parent.vue
<template> <div> 父组件 <child></child> </div> </template> <script> import child from './child' export default { data: () => ({ }), components: { child }, mounted () { // setTimeout模拟异步数据 setTimeout(() => { // 触发子组件,并且传递数据过去 this.$bus.emit('triggerChild', {'items': [1, 2, 3]}) console.log('parent finish') }, 2000) } } </script>
child.vue
<template> <div> 子组件 <p>{{test}}</p> </div> </template> <script> export default { props: ['childObject'], data: () => ({ test: '' }), created () { // 绑定 this.$bus.on('triggerChild', (parmas) => { this.test = parmas.items[0] // 1 this.updata() }) }, methods: { updata () { console.log(this.test) // 1 } } } </script>
这里使用了bus这个库,parent.vue和child.vue必须公用一个事件总线(也就是要引入同一个js,这个js定义了一个类似let bus = new Vue()的东西供这两个组件连接),才能相互触发
使用prop default来解决{{childObject.items[0]}}
parent.vue
<template> <div> 父组件 <child :child-object="asyncObject"></child> </div> </template> <script> import child from './child' export default { data: () => ({ asyncObject: undefined // 这里使用null反而报0的错 }), components: { child }, created () { }, mounted () { // setTimeout模拟异步数据 setTimeout(() => { this.asyncObject = {'items': [1, 2, 3]} console.log('parent finish') }, 2000) } } </script>
child.vue
<template> <div> 子组件<!--1--> <p>{{childObject.items[0]}}</p> </div> </template> <script> export default { props: { childObject: { type: Object, default () { return { items: '' } } } }, data: () => ({ }), created () { console.log(this.childObject) // {item: ''} } } </script>
在说用vuex解决方法的时候,首先看看案例三
案例三
main.js
import Vue from 'vue' import App from './App' import router from './router' import VueBus from 'vue-bus' import index from './index.js' Vue.use(VueBus) Vue.config.productionTip = false import Vuex from 'vuex' Vue.use(Vuex) const store = new Vuex.Store({ modules: { index } }) /* eslint-disable no-new */ new Vue({ el: '#app', store, router, template: '<App/>', components: { App } })
index.js
const state = { asyncData: '' } const actions = { asyncAction ({ commit }) { setTimeout(() => { commit('asyncMutation') }, 2000) } } const getters = { } const mutations = { asyncMutation (state) { state.asyncData = {'items': [1, 2, 3]} } } export default { state, actions, getters, mutations }
parent.vue
<template> <div> 父组件 <child></child> </div> </template> <script> import child from './child' export default { data: () => ({ }), components: { child }, created () { this.$store.dispatch('asyncAction') }, mounted () { } } </script>
child.vue
<template> <div> 子组件 <p>{{$store.state.index.asyncData.items[0]}}</p> </div> </template> <script> export default { data: () => ({ }), created () { }, methods: { } } </script>
{{$store.state.index.asyncData.items[0]}}可以取到改变的值,但是过程中还是出现这样的报错,原因同上
[Vue warn]: Error in render function: "TypeError: Cannot read property '0' of undefined"
所以这里的解决方法是:vuex结合computed、mapState或者合computed、mapGetters
parent.vue
<template> <div> 父组件 <child></child> </div> </template> <script> import child from './child' export default { data: () => ({ }), components: { child }, created () { this.$store.dispatch('asyncAction') }, mounted () { } } </script>
child.vue
<template> <div> 子组件 <p>{{item0}}</p> <p>{{item}}</p> </div> </template> <script> import { mapState, mapGetters } from 'vuex' export default { data: () => ({ test: '' }), computed: { ...mapGetters({ item: 'getAsyncData' }), ...mapState({ item0: state => state.index.asyncData }) }, created () { }, methods: { } } </script>
index.js
const state = { asyncData: '' } const actions = { asyncAction ({ commit }) { setTimeout(() => { commit('asyncMutation', {'items': [1, 2, 3]})// 作为参数,去调用mutations中的asyncMutation方法来对state改变 }, 2000) } } const getters = { getAsyncData: state => state.asyncData } const mutations = { asyncMutation (state, params) { state.asyncData = params.items[0] // 此时params={'items': [1, 2, 3]}被传过来赋值给asyncData,来同步更新asyncData的值,这样html就可以拿到asyncData.items[0]这样的值了 } } export default { state, actions, getters, mutations }
注意上面的
.... commit('asyncMutation', {'items': [1, 2, 3]}) ... state.asyncData = params.items[0]
如果写成这样的话
commit('asyncMutation') state.asyncData = {'items': [1, 2, 3]}
首先asyncAction是个异步的操作,所以asyncData默认值为空,那么还是导致,child.vue这里报0的错
<template> <div> 子组件 <p>{{item0}}</p> <p>{{item}}</p> </div> </template>
不过根据以上的案例,得出来一个问题就是异步更新值的问题,就是说开始的时候有个默认值,这个默认值会被异步数据改变,比如说这个异步数据返回的object,如果你用props的方式去传递这个数据,其实第一次传递的空值,第二次传递的是更新后的值,所以就出现{{childObject.items[0]}}类似这种取不到值的问题,既然说第一次是空值,它会这样处理''.items[0],那么我们是不是可以在html判断这个是不是空(或者在computed来判断是否为默认值),所以把案例二的child.vue
<template> <div> <p>{{childObject != '' ? childObject.items[0]: ''}}</p> </div> </template> <script> export default { props: ['childObject'], data: () => ({ }), created () { console.log(this.childObject) // 空值 }, methods: { } } </script>
这样是可以通过不报错的,就是created是空值,猜想上面vuex去stroe也可以也可以这样做
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