The method of parent-child communication in VueJs components

This article mainly introduces the method of father-son communication in VueJs components. It is very good and has certain reference value. Friends in need can refer to

Component (Father-Child Communication)

1. Summary

Define another component within a component, called a parent-child component.

But it should be noted that: 1. Child components can only be used inside the parent component (written in the parent component template); The data on, the scope of each component instance is independent;

How to complete the communication between father and son, in a simple sentence: props down, events up: the parent component passes data downward to the child component through props , the child component sends to the parent component through events

Pass from parent to child: Props

Pass from child to parent: Child: $emit(eventName) Parent $on(eventName)

Parent accesses child: ref

Let’s explain the three cases below:

2. Pass from father to son: Props The scope of the component instance is isolated. This means that you cannot (and should not) reference the parent component's data directly within the child component's template. To allow the child component to use the data of the parent component, you need to use the props option of the child component

Using Prop to transfer data includes static and dynamic forms. Let’s first introduce static props

1, static props

<script src="https://unpkg.com/vue"></script>
<p id="example">
 <parent></parent>
</p>
<script>
 //要想子组件能够获取父组件的，那么在子组件必须申明:props
 var childNode = {
  template: &#39;<p>{{message}}</p>&#39;,
  props: [&#39;message&#39;]
 }
 //这里的message要和上面props中值一致
 var parentNode = {
  template: `
   <p class="parent">
   <child message="我是"></child>
   <child message="徐小小"></child>
   </p>`,
  components: {
   &#39;child&#39;: childNode
  }
 };
 // 创建根实例
 new Vue({
  el: &#39;#example&#39;,
  components: {
   &#39;parent&#39;: parentNode
  }
 })
</script>

Effect:

## Naming convention:

For attributes declared by props For example, in the parent HTML template, the attribute name needs to be written in underscore

When the child props attribute is declared, it can be written in camel case or underscore; while the child template uses the underscore from the parent When passing variables, you need to use the corresponding camel case

What does the above sentence mean?

<script>
 //这里需要注意的是props可以写成[&#39;my-message&#39;]或者[&#39;myMessage&#39;]都是可以的
 //但是template里的属性名，只能是驼峰式{{myMessage}}，如果也写成{{my-message}}那么是无效的
 var childNode = {
  template: &#39;<p>{{myMessage}}</p>&#39;,
  props: [&#39;myMessage&#39;]
 }

 //这里的属性名为my-message
 var parentNode = {
  template: `
   <p class="parent">
   <child my-message="我是"></child>
   <child my-message="徐小小"></child>
   </p>`,
  components: {
   &#39;child&#39;: childNode
  }
 };
</script>

If myMessage in our childNode is changed to {{my-message}}, look at the running results:

2. Dynamic props

In the template, you need to dynamically bind the data of the parent component to the props of the child template, which is similar to binding to any ordinary HTML feature, using v-bind. Whenever the data of the parent component changes, the change will also be transmitted to the child component

 var childNode = {
  template: &#39;<p>{{myMessage}}</p>&#39;,
  props: [&#39;my-message&#39;]
    }

 var parentNode = {
  template: `
 <p class="parent">
 <child :my-message="data1"></child>
 <child :my-message="data2"></child>
 </p>`,
  components: {
   &#39;child&#39;: childNode
  },
  data() {
   return {
    &#39;data1&#39;: &#39;111&#39;,
    &#39;data2&#39;: &#39;222&#39;
   }
  }
 };

3. Passing numbers

Common mistakes made by beginners One mistake is to use literal syntax to pass a numeric value

<script src="https://unpkg.com/vue"></script>
<p id="example">
 <parent></parent>
</p>
<script>
 var childNode = {
  template: &#39;<p>{{myMessage}}的类型是{{type}}</p>&#39;,
  props: [&#39;myMessage&#39;],
  computed: {
   type() {
    return typeof this.myMessage
   }
  }
 }
 var parentNode = {
  template: `
 <p class="parent">
 <my-child my-message="1"></my-child>
 </p>`,
  components: {
   &#39;myChild&#39;: childNode
  }
 };
 // 创建根实例
 new Vue({
  el: &#39;#example&#39;,
  components: {
   &#39;parent&#39;: parentNode
  }
 })
</script>

The result:

## because it is a Literal prop, its value is the string "1" instead of number. If you want to pass an actual number, you need to use v-bind so that its value is calculated as a JS expression

How to convert String to number? In fact, you only need to change one place.

 var parentNode = {
  template: `
 <p class="parent">
 //只要把父组件my-message="1"改成:my-message="1"结果就变成number类型
 <my-child :my-message="1"></my-child>
 </p>`,
 };

Of course, if you want to pass a string type through v-bind, what should you do?

We can use dynamic props to set the corresponding number in the data attribute 1

var parentNode = {
 template: `
 <p class="parent">
 <my-child :my-message="data"></my-child>
 </p>`,
 components: {
 &#39;myChild&#39;: childNode
 },
 //这里&#39;data&#39;: 1代表就是number类型，&#39;data&#39;: "1"那就代表String类型
 data(){
 return {
  &#39;data&#39;: 1
 }
 }
};

3. Sub-turn Parent: $emit

About the usage of $emit 1. The parent component can use props to pass data to the child component.

2. Subcomponents can use $emit to trigger custom events of parent components.

Child primary key

<template> 
 <p class="train-city"> 
 <span @click=&#39;select(`大连`)&#39;>大连</span> 
 </p> 
</template> 
<script> 
export default { 
 name:&#39;trainCity&#39;, 
 methods:{ 
 select(val) { 
  let data = { 
  cityname: val 
  }; 
  this.$emit(&#39;showCityName&#39;,data);//select事件触发后，自动触发showCityName事件 
 } 
 } 
} 
</script>

Parent component

<template> 
 <trainCity @showCityName="updateCity" :index="goOrtoCity"></trainCity> //监听子组件的showCityName事件。 
<template> 
<script> 
export default { 
 name:&#39;index&#39;, 
 data () { 
 return { 
  toCity:"北京" 
 } 
 } 
 methods:{ 
 updateCity(data){//触发子组件城市选择-选择城市的事件 
  this.toCity = data.cityname;//改变了父组件的值 
  console.log(&#39;toCity:&#39;+this.toCity)  
 } 
 } 
} 
</script>

The result is ：toCity: Dalian

The second case

<script src="https://unpkg.com/vue"></script>

 <p id="counter-event-example">
  <p>{{ total }}</p>
  <button-counter v-on:increment1="incrementTotal"></button-counter>
  <button-counter v-on:increment2="incrementTotal"></button-counter>
 </p>
<script>
 Vue.component(&#39;button-counter&#39;, {
  template: &#39;<button v-on:click="increment">{{ counter }}</button>&#39;,
  //组件数据就是需要函数式，这样的目的就是让每个button-counter不共享一个counter
  data: function() {
   return {
    counter: 0
   } 
  },
  methods: {
   increment: function() {
   //这里+1只对button的值加1，如果要父组件加一，那么就需要$emit事件
    this.counter += 1;
    this.$emit(&#39;increment1&#39;, [12, &#39;kkk&#39;]);
   }
  }
 });
 new Vue({
  el: &#39;#counter-event-example&#39;,
  data: {
   total: 0
  },
  methods: {
   incrementTotal: function(e) {
    this.total += 1;
    console.log(e);
   }
  }
 });
</script>

Detailed explanation:

1: button-counter as the parent primary key , there is a button button in the parent primary key.

2: Both buttons are bound to click events. In the method: this.$emit('increment1', [12, 'kkk']);, then the parent class v-on will be called. The increment1 event being monitored.

3: When the increment1 event is monitored, then incrementTotal is executed. At this time, the value is passed to the parent component and the method of the parent class is called.

4: Pay attention here to v-on:'increment2 corresponding to the second button-counter, and the button inside it corresponds to this.$emit('increment1', [12, 'kkk' ]); So the second button button cannot pass the value to its parent primary key.

Example: If a button is clicked once, both itself and the above will increase by 1, while the second button will only increment itself and will not affect the above.

还有就是第一个按钮每点击一次，后台就会打印一次如下：

四、ref ($refs)用法

ref 有三种用法

1.ref 加在普通的元素上，用this.ref.name 获取到的是dom元素

2.ref 加在子组件上，用this.ref.name 获取到的是组件实例，可以使用组件的所有方法。

3.如何利用v-for 和ref 获取一组数组或者dom 节点

1.ref 加在普通的元素上，用this.ref.name 获取到的是dom元素

<script src="https://unpkg.com/vue"></script>

<p id="ref-outside-component" v-on:click="consoleRef">
 <component-father ref="outsideComponentRef">
 </component-father>
 <p>ref在外面的组件上</p>
</p>
<script>
 var refoutsidecomponentTem = {
  template: "<p class=&#39;childComp&#39;><h5>我是子组件</h5></p>"
 };
 var refoutsidecomponent = new Vue({
  el: "#ref-outside-component",
  components: {
   "component-father": refoutsidecomponentTem
  },
  methods: {
   consoleRef: function() {
    console.log(this.); // #ref-outside-component  vue实例
    console.log(this.$refs.outsideComponentRef); // p.childComp vue实例
   }
  }
 });
</script>

效果：当在p访问内点击一次：

2.ref使用在外面的元素上

<script src="https://unpkg.com/vue"></script>

<!--ref在外面的元素上-->
<p id="ref-outside-dom" v-on:click="consoleRef">
 <component-father>
 </component-father>
 <p ref="outsideDomRef">ref在外面的元素上</p>
</p>
<script>
 var refoutsidedomTem = {
  template: "<p class=&#39;childComp&#39;><h5>我是子组件</h5></p>"
 };
 var refoutsidedom = new Vue({
  el: "#ref-outside-dom",
  components: {
   "component-father": refoutsidedomTem
  },
  methods: {
   consoleRef: function() {
    console.log(this); // #ref-outside-dom vue实例
    console.log(this.$refs.outsideDomRef); // <p> ref在外面的元素上</p>
   }
  }
 });
</script>

效果：当在p访问内点击一次：

3.ref使用在里面的元素上---局部注册组件

<script src="https://unpkg.com/vue"></script>
<!--ref在里面的元素上-->
<p id="ref-inside-dom">
 <component-father>
 </component-father>
 <p>ref在里面的元素上</p>
</p>
<script>
 var refinsidedomTem = {
  template: "<p class=&#39;childComp&#39; v-on:click=&#39;consoleRef&#39;>" +
   "<h5 ref=&#39;insideDomRef&#39;>我是子组件</h5>" +
   "</p>",
  methods: {
   consoleRef: function() {
    console.log(this); // p.childComp vue实例 
    console.log(this.$refs.insideDomRef); // <h5 >我是子组件</h5>
   }
  }
 };
 var refinsidedom = new Vue({
  el: "#ref-inside-dom",
  components: {
   "component-father": refinsidedomTem
  }
 });
</script>

效果：当在click范围内点击一次：

4.ref使用在里面的元素上---全局注册组件

<script src="https://unpkg.com/vue"></script>
<!--ref在里面的元素上--全局注册-->
<p id="ref-inside-dom-all">
 <ref-inside-dom-quanjv></ref-inside-dom-quanjv>
</p>
<script>
 //v-on:input指当input里值发生改变触发showinsideDomRef事件
 Vue.component("ref-inside-dom-quanjv", {
  template: "<p class=&#39;insideFather&#39;> " +
   "<input type=&#39;text&#39; ref=&#39;insideDomRefAll&#39; v-on:input=&#39;showinsideDomRef&#39;>" +
   " <p>ref在里面的元素上--全局注册 </p> " +
   "</p>",
  methods: {
   showinsideDomRef: function() {
    console.log(this); //这里的this其实还是p.insideFather
    console.log(this.$refs.insideDomRefAll); // <input type="text">
   }
  }
 });
 var refinsidedomall = new Vue({
  el: "#ref-inside-dom-all"
 });
</script>

效果：当我第一次输入1时，值已改变出发事件，当我第二次在输入时在触发一次事件，所以后台应该打印两次

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