Detailed explanation of beforeCreate function in Vue documentation

Vue.js is a popular JavaScript framework that provides many features to simplify the web development process. In Vue.js, there are many life cycle hook functions, one of which is very important is the beforeCreate function. This article will introduce in detail the beforeCreate function in the Vue documentation and how to use it correctly.

1. The meaning of the beforeCreate function

In Vue.js, the beforeCreate life cycle hook function is the function called when the vue instance is created. It is called after the instance is created, but before all data properties and events are initialized. The beforeCreate function is used to perform some tasks before the Vue instance is initialized, such as setting the computed properties of the instance or the computed properties of the component.

2. How to use the beforeCreate function

In Vue.js, the beforeCreate function can be used in the following ways:

(1) When defining a Vue instance Add the beforeCreate function to the life cycle hook function:

new Vue({
  beforeCreate: function () {
    // 这里添加beforeCreate函数的任务代码
  },
  // 实例数据和方法
  data: {},
  methods: {}
})

(2) In the Vue component, add the beforeCreate function before the life cycle hook function created:

Vue.component('my-component', {
  beforeCreate: function () {
    // 这里添加beforeCreate函数的任务代码
  },
  created: function () {
    // 这里添加created函数的任务代码
  },
  // 组件数据和方法
  data: {},
  methods: {}
})

3. Use case of the beforeCreate function

(1) Use the beforeCreate function to set the computed property of the Vue instance

new Vue({
  beforeCreate: function () {
    this.myComputedData = this.myData * 2
  },
  data: {
    myData: 10
  },
  computed: {
    myComputedData: 0
  }
})

In this example, we set the computed property myComputedData of the Vue instance in the beforeCreate function. This computed property is twice myData. It is necessary to set the computed properties in the beforeCreate function before the instance data and computed properties are initialized.

(2) Use the beforeCreate function to get data in the Vue component

Vue.component('my-component', {
  beforeCreate: function () {
    this.$http.get('/my-data-url')
    .then(response => {
      this.myData = response.data
    })
  },
  // 组件数据和方法
  data: {
    myData: ''
  },
  methods: {}
})

In this example, we use the Vue-resource plug-in in the beforeCreate function to get the data from the server and store it in the component in the myData data attribute. We cannot use the myData property directly in the component before the component data is initialized, so we use the beforeCreate function to get the data and initialize the component data.

4. Notes on the beforeCreate function

(1) The code in the beforeCreate function will only be executed once before the Vue instance or component is created. So we cannot use this.$watch or this.$on to listen for events in beforeCreate. These logic should be executed in the created function.

(2) This.$el or the DOM element of the component instance cannot be accessed in the beforeCreate function because the DOM has not yet been created.

(3) The beforeCreate function is suitable for tasks before the creation of a Vue instance or component. If you are doing something after creation, you should use the created hook. At this time, the data and method of the Vue instance or component have been initialized. .

In summary, Vue's beforeCreate life cycle hook function is executed before the Vue instance or component is initialized. It is used to perform pre-initialization tasks, such as setting calculated properties or obtaining remote data. When using the beforeCreate function, you should be careful not to do instance-related listening tasks in it. These tasks should be completed in the created hook. Proper use of the beforeCreate function allows us to better understand the life cycle of Vue.js, effectively use Vue.js functions, and improve program efficiency.

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