Use of vue built-in instructions

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

directive is one of the most commonly used functions in Vue templates. It is prefixed with v-. Its main responsibility is to apply certain behaviors to the DOM accordingly when the value of its expression changes. This article mainly introduces the detailed explanation of Vue's built-in instructions. Friends in need can refer to

Directives are special attributes with v- prefix. Their responsibility is to change the value of the expression when it changes. The associated effects are responsive to the DOM.

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 attributes on HTML elements;

<a>...</a>
<!-- 缩写 -->
<a>...</a>
<p>标题</p>
var app = new Vue({
  el: '#app',
  data: { 
    url: 'www.baidu.com',
　　　　title: 'bind'
  },
}）

2. v-on: used to listen to DOM events; For example: v -on:click v-on:keyup

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

2.1 The expression of @click can be directly Using JavaScript statement, it can also be a function name in the methods option of the Vue instance, and parameters can be passed in the method

<!-- 完整语法 -->
<a>...</a>
<!-- 缩写 -->
<a>...</a>　　　　//是一个方法名
<p>一段文本</p>
<button>点击隐藏文本</button>　　//直接是一个内联的语句
<button>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, used to access native DOM events, can prevent events from bubbling or prevent links from opening

Write an example to prevent bubbling:

  <p>
      </p><p>
        </p><p>阻止冒泡</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 Modifier:

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>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></a>
〈！一修饰符可以串联一〉
<a></a>
〈！一添加事件侦听器时使用事件捕获模式一〉
<p> ... </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> ... </p>
＜ ！一只触发一次，组件同样适用一〉
<p> ... </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><p style="text-align: left;">3. v-model: two-way binding of data; used for form input, etc.; for example: </p><p style="text-align: left;">4. v-show: conditional rendering instruction, set the css style attribute for DOM</p><p style="text-align: left;">5. v-if: conditional rendering instruction, dynamically added in DOM Or delete DOM elements</p><p style="text-align: left;">6. v-else: conditional rendering instruction, must be used in pairs with v-if</p><p style="text-align: left;">7. v-else-if: judge multi-layer conditions, must be paired with v -if used in pairs; </p><p style="text-align: left;">8, v-text: Update the textContent of the element; for example: <span v-text="msg"></span> is equivalent to {{ msg}}  span>;</p><p style="text-align: left;">9. v-html: Update the innerHTML of the element; the tag name will also be included. </p><p style="text-align: left;">10. v-for: loop instruction; for example: </p><pre class="brush:php;toolbar:false"><p>
    </p>

      
• { { book.name } }
    

   var app =new Vue({   el: '#app',   data: {     books: [       {name: ''},       {name: ''},       {name: ''}     ]   } });

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

  <p>
    </p>

      
• {{ index}} - {{book.name })
    

  

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

  <p>
    </p>

      
•         { { index } } - { { key } } : { { value } }       
    

   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>
    <span>{{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.

替换的数组中，含有相同元素的项不会被重新渲染，因此可以大胆地用新数组来替换旧数组，不用担心性能问题。

10.5 过滤与排序

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>
    </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。

13、v-pre：不需要表达式，跳过这个元素以及子元素的编译过程，以此来加快整个项目的编译速度；例如： {{ this will not be compiled }} span>；

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