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An in-depth analysis of the different mechanisms of computed and methods in Vue.js

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May 26, 2018 pm 04:06 PM

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This article introduces you to the different mechanisms of computed and methods in Vue.js. In vue.js, methods and computed are used dynamically as methods. The article also mentions the difference between computed and methods. , interested friends, let’s take a look

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

1. The first and most obvious difference is that When calling, methods should be added with ()

2. We can use methods instead of computed. The two effects are the same, but computed is based on its dependency cache, only when the relevant dependencies change. will be revalued.

With methods, when re-rendering, the function will always be re-called and executed.

In order to facilitate understanding, let’s first upload a 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: &#39;.test2&#39;,
      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. It can be seen that if the computed attribute is used, the information from the first time will be used every time the page is entered, and now will not be triggered again. This is dependent on cache. (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 performance will Better, 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’s 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(&#39;ok&#39;)
           //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。

The above is what I compiled for everyone. I hope it will be helpful to everyone in the future.

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