Instructions for using vue built-in instructions

This time I will bring you the instructions for using the built-in instructions of vue. What are the precautions of the instructions for using the built-in instructions of vue. The following is a practical case, let’s take a look.

Directives are special attributes with v- prefix. Their responsibility is to reactively apply the associated effects to the DOM when the value of the expression changes.

Built-in instructions

1. v-bind: respond to and update DOM characteristics; for example: v-bind:href v-bind:class v-bind:title etc.

The main usage is to bind attributes and dynamically update the attributes on HTML elements;

<a v-bind:href="url" rel="external nofollow" rel="external nofollow" >...</a>
<!-- 缩写 -->
<a :href="url" rel="external nofollow" rel="external nofollow" >...</a>
<p :title=&#39;title&#39;>标题</p>
var app = new Vue({
  el: '#app',
  data: { 
    url: 'www.baidu.com',
　　　　title: 'bind'
  },
}）

2, v-on: used for monitoring DOM events; For example: v-on:click v-on:keyup

By the way, let’s talk about methods and events

2.1 Expression of @click The formula can use JavaScript statements directly, or it can be a function name in the methods option in the Vue instance. Parameters can be passed in the method

<!-- 完整语法 -->
<a v-on:click="doSomething">...</a>
<!-- 缩写 -->
<a @click="doSomething()">...</a>　　　　//是一个方法名
<p ng-if=&#39;show&#39;>一段文本</p>
<button @click="show=false">点击隐藏文本</button>　　//直接是一个内联的语句
<button v-on:click="count++">Add 1</button>
var app = new Vue({
  el: '#app',
　　data:{
　　　　show: true，
　　　　counter： 0
　　},
  methods: {
    doSomething: function(){
      console.log(this.title);
    },
  }
}）

2.2 Methods and events:

Vue provides a special variable $event, which is used to access native DOM events, which can prevent events from bubbling or prevent links from opening.

Write an example to prevent bubbling:

  <p @click="stopClick1(&#39;stop1&#39;,$event)">
      <p @click="stopClick2(&#39;stop2&#39;,$event)">
        <p @click="stopClick3(&#39;stop3&#39;,$event)">阻止冒泡</p>
      </p>
    </p>
　　</p>
methods:{
    stopClick3: function(message, event){
      console.log(message);
      event.stopPropagation();  //阻止冒泡
    },
    stopClick2: function(message, event){
      console.log(message);
    },
    stopClick1: function(message, event){
      console.log(message);
    }
}

2.3 Modification Symbol:

Add a small dot "." after the @bound event, and then follow it with a suffix to use the modifier.

The above bubbling event can be written as a direct user modifier:

<p @click.stop="stopClick3(&#39;stop3&#39;)">阻止冒泡</p>　　//不用通过$event事件再来写了

Some commonly used modifiers are:

• .stop

• .prevent

• .capture

• .self

• .once

＜ ！一阻止单击事件冒泡一〉
<a @click.stop=”handle "></a>
〈！一修饰符可以串联一〉
<a @click.stop.prevent=” handle ” ></a>
〈！一添加事件侦听器时使用事件捕获模式一〉
<p @click . capture=”handle ”> ... </p>
〈！一只当事件在该元素本身（而不是子元素） 触发时触发回调一〉
<p @click.self=” handle ”> ... </p>
＜ ！一只触发一次，组件同样适用一〉
<p @click.once=” handle ”> ... </p>

When monitoring keyboard events on a form element, also You can use key modifiers, such as calling a method only when a specific key is pressed:

＜ ！一只有在keyCode 是13 时调用vm.submit()一〉
<input @keyup.13 ＝“ submit ”〉

3. v-model: two-way binding of data; used for form input, etc.; for example: < input v-model = "message">

4. v-show: conditional rendering instruction, set the css style attribute for DOM

5. v-if: conditional rendering instruction, dynamically added in DOM Or delete DOM elements

6. v-else: conditional rendering instruction, must be used in pairs with v-if

7. v-else-if: judge multi-layer conditions, must be paired with v -if used in pairs;

8, v-text: Update the textContent of the element; for example: is equivalent to < span>{{ msg}} ;

9. v-html: Update the innerHTML of the element; the tag name will also be included.

10. v-for: loop instruction; for example:

<p id= "app ">
    <ul>
      <li v-for="book in books">{ { book.name } }</li>
    </ul>
  </p>
var app =new Vue({
  el: '#app',
  data: {
    books: [
      {name: '<vue.js实战>'},
      {name: '<javascript语言精粹>'},
      {name: '<javascript高级程序设计>'}
    ]
  }
});

10.1 v-for expressionSupports an optional parameter as the index of the current item when traversing the array , For example:

  <p id="app">
    <ul>
      <li v-for="(book , index) in books ">{{ index}} - {{book.name })</li>
    </ul>
  </p>

10.2 v-For expressionWhen traversing the object attributes, there are two optional parameters, namely key name and index:

  <p id= "app">
    <ul>
      <li v-for="(value , key , index) in user ">
        { { index } } - { { key } } : { { value } }
      </li>
    </ul>
  </p>
var app = new Vue({
  el: '#app',
  data: {
    name: 'Aresn',
    grender: '男',
    age:23
  }
});

10.3 v - The expression of for can also iterate integers:

 <p id="app">
    <span v-for="n in 10">{{n}}</span>
  </p>

10.4 Array update

When we modify the array, Vue will detect the data change, so the view rendered with v-for will also immediately renew.

• push()
• pop()
• shift()
• unshit()
• splice()
• sort()
• reverse ()

These methods will change the original array called by these methods

For example, we will add an item to the data books of the previous example:

app.books.push({
  name: '《css世界》'
});

Some methods will not Change the original array, for example:

• filter()
• concat()
• slice()

They return a new array. When using these non-mutation methods When Vue detects changes in the array, it does not directly re-render the entire list, but maximizes the reuse of DOM elements.

In the replaced array, items containing the same elements will not be re-rendered, so you can boldly replace the old array with a new array without worrying about performance issues.

10.5 Filtering and Sorting

当你不想改变原数组，想通过一个数组的副本来做过滤或排序的显示时， 可以使用计算属性来返回过滤或排序后的数组 ，例如： 

  <p id="app">
    <ul>
      <template v-for="book in filterBooks">
        <li>书名：{{book.name}}</li>
        <li>作者：{{book.author}}</li>
      </template>
    </ul>
  </p>
var app= new Vue({
  el: '#app',
  computed: {
    filterBooks: function(){
      return this.books.filter(function (book) {
        return book.name.match(/JavaScript/);
      })
    },
  }
});

11、v-cloak：不需要表达式，这个指令保持在元素上直到关联实例结束编译； v-cloak 是一个解决初始化慢导致页面闪动的最佳实践 ；

12、v-once：也是一个不需要表达式的指令，作用是定义它的元素或组件只渲染一次，包括元素或组件的所有子节点。

首次渲染后，不再随数据的变化重新渲染，将被视为静态内容； v-once 在业务中也很少使用，当你需要进一步优化性能时，可能会用到。

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