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Detailed explanation of the use of computed and methods in Vue.js

php中世界最好的语言Original: 2018-05-09 09:50:573098browse

This time I will bring you a detailed explanation of the use of computed and methods in Vue.js. What are the precautions for using computed and methods in Vue.js. The following is a practical case, let's take a look.

In vue.js, there are two ways to dynamically use methods and computed as methods

1. First of all, the most obvious difference is that when calling, methods must be added ( )

2. We can use methods to replace computed. The effects of both are the same, but computed is based on its dependency cache, and will only be re-valued when the relevant dependencies change.

When using methods, when re-rendering, the function will always be re-called and executed

In order to facilitate understanding, let’s first go to the source code

<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8">
    <title>title</title>
    <script src="https://cdn.bootcss.com/vue/2.4.2/vue.min.js"></script>
  </head>
  <body>
    <p class="test">  <!--computed计算属性-->
      <p>{{now}}</p>
      <p>{{now}}</p>
      <p>{{now}}</p>
      <p>{{now}}</p>
      <hr />      <!--横线分割-->
</p>
    <p class="test2">  <!--methods方法，注意new（）加了括号-->
      <p>{{now()}}</p>
      <p>{{now()}}</p>
      <p>{{now()}}</p>
      <p>{{now()}}</p>
    </p>
  </body>
  <script type="text/javascript">
    var myVue = new Vue({
      el: ".test",
      computed: {
        now: function() {
          var yanshi = 0;
          for(var o = 0; o < 2000; o++) {   //延时
            for(var q = 0; q < 2000; q++) {
              yanshi++;
            }
          }
          return Date.now()
        }
      }
    });
    var vue2 = new Vue({
      el: '.test2',
      methods: {
        now: function() {
          var yanshi = 0;
          for(var o = 0; o < 2000; o++) {
            for(var q = 0; q < 2000; q++) {
              yanshi++;
            }
          }
          return Date.now()
        }
      }
    })
  </script>
</html>

The running results are as above, as you can see If the computed attribute is computed, the information from the first time will be used every time you enter the page, and now will not be triggered again. This is dependent on caching. (When there is a delay, multiple output times are the same)

Then what is the value that will be revalued when the relevant dependencies change? For example, the message variable is called in the calculated attribute of reversedMessage function()

This means that as long as the message has not changed, multiple accesses to the reversedMessage calculated property will immediately return the previous calculation result without having to execute the function again.

Methods are real-time. When re-rendering, the function will always be re-called and executed without caching (multiple output times are different)

It can be said that using computed will have better performance. But if you don't want caching, you can use the methods attribute.

The computed property only has a getter by default, but you can also provide a setter when needed: so in fact, computed can also pass parameters.

ps: Let’s take a look at the difference between vue computed properties computed and methods

在new Vue的配置参数中的computed和methods都可以处理大量的逻辑代码，但是什么时候用哪个属性，要好好区分一下才能做到正确的运用vue。
computed称为计算属性，顾名思义，计算就要返回一个计算的结果，所以，当我们要处理大量的逻辑，但是最后要取得最后的结果的时候可以用computed；
简单示例：
要求：
<input type="text v-model="num1"><input type="text v-model="num2">
现在要返回num1和num2的和；
<script>
  new Vue({
    el:"#box",
    data:{
        num1:0,
        num2:0
        }
    computed:{  
        result:function(){
            return this.num1 + this.num2  
            // 计算属性必须有一个返回值
        }
    }
   })
</script>
methods：是方法的意思，在js中，我们把一些函数叫做方法，一般情况下，要触发这个方法就要执行，要执行就要有一个源来触发，所以就需要一个事件源。这是和computed的一点不同之处；
methods的示例：
要求：
<\button @click="do()">点击弹出<\/button>
<script>
  new Vue({
    el:"#box",
    data:{
        num1:0,
        num2:0
        }
    methods:{  
        do:function(){
           alert('ok')
           //这里根据情况，可以返回有返回值也可以没有返回值。
        }
    }
   })
</script>
对比computed 和 methods：
computed计算的结果如果不发生改变就不会触发result这个函数。而methods中一般都是定义的需要事件触发的一些函数。每次只要触发事件，就会执行对应的方法。如果把computed中的方法写到method中会浪费性能。
computed必须返回一个值页面绑定的才能取得值，而methods中可以只执行逻辑代码，可以有返回值，也可以没有。

I believe you have mastered the method after reading the case in this article, and more How exciting, please pay attention to other related articles on php Chinese website!

Detailed explanation of the use of computed and methods in Vue.js

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