Why is the interface in Vue called twice?

When developing with Vue, we often encounter the problem of the interface being called twice. This is caused by the execution order of the Vue component life cycle function (LifeCycle Hook) and the rendering mechanism of Vue.

First of all, we need to understand the life cycle functions of Vue components, such as created, mounted, updated and other functions. When a component is created, these life cycle functions are automatically called in a certain order. The execution order of these functions is as follows:

1. created function: This function is called immediately after the Vue instance is created;
2. mounted function: This function is called after the Vue instance is mounted to the DOM;
3. updated function: This function is called after the Vue instance is updated.

Through the execution sequence of the above life cycle functions, we can find that the mounted function is executed after the DOM rendering is completed, and interface requests are generally made in the mounted function. However, Vue's rendering mechanism will cause the mounted function to be called once during the initial rendering and when the data is updated. This is why the interface is called twice.

In Vue, view rendering is asynchronous. When the data is updated, Vue will update the data first, and then update the view in the next event loop. This results in that when making an interface request in the mounted function, the first call occurs when the component is first rendered, and the second call occurs when the component is re-rendered after the data is updated.

So how can we avoid the problem of the interface being called twice? There are several methods:

1. Use the v-if directive to avoid frequent rendering of components

When making an interface request in the mounted function, we can judge based on the conditions. Only when The interface request is initiated only when the component is rendered for the first time. This can be achieved by using the v-if directive, for example:

<template>
  <div v-if="!isFetched">
    <!-- 接口请求相关的DOM结构 -->
  </div>
</template>

<script>
export default {
  data() {
    return {
      isFetched: false,
      responseData: null
    }
  },
  mounted() {
    if (!this.isFetched) {
      // 发起接口请求
      // 请求成功后将isFetched置为true，将responseData更新为接口返回值
      this.isFetched = true;
    }
  }
}
</script>

Using the v-if directive can ensure that the component will only initiate an interface request when it is rendered for the first time, avoiding the problem of the interface being called twice.

2. Initiate an interface request in the created function

In Vue's life cycle function, the created function is called immediately after the instance is created, earlier than the mounted function . Therefore, we can initiate an interface request in the created function to avoid the problem of the interface being called twice in the mounted function.

<template>
  <div>
    <!-- 接口请求相关的DOM结构 -->
  </div>
</template>

<script>
export default {
  data() {
    return {
      responseData: null
    }
  },
  created() {
    // 发起接口请求
    // 请求成功后将responseData更新为接口返回值
  }
}
</script>

Making an interface request in the created function can ensure that the component will only call the interface once, instead of making two requests.

Summary:

The problem of the interface being called twice is caused by the Vue component life cycle function and rendering mechanism. We can use the v-if directive or initiate an interface request in the created function, etc. method to solve the problem. In actual development, we should choose the most appropriate way to solve the problem of calling the interface twice according to the specific situation to achieve the best performance and user experience.

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