The role of watch command in vue

In Vue.js, the watch command is used to listen for data changes and trigger specific processing functions based on the changes. It is used to update the view or perform other operations when the data changes. The specific mechanism includes: specifying the data to be monitored, defining processing functions, and performing operations. Usage scenarios include: dynamically updating views, responding to user interactions, monitoring state changes, and tracking component state changes. watch also supports deep monitoring of nested data.

The role of watch command in Vue.js

In Vue.js, watch Command is used to monitor data changes and execute specific processing functions based on the changes. Its main function is to trigger the corresponding function to update the view or perform other operations when the data changes.

Mechanism of action

1. Specify the data to be monitored: When using the watch command, you need to specify the data to be monitored The data can be the data in the component data, or it can be a calculated attribute.
2. Define the processing function: Then define a processing function, which will be triggered when the monitored data changes.
3. Perform operations: Within the processing function, you can perform corresponding operations based on changes in data, such as updating views, performing asynchronous requests, or triggering other events.

Usage scenarios

watch The command is usually used in the following scenarios:

• When data changes Dynamically update views
• Respond to form input or other user interaction
• Monitor state changes and trigger corresponding actions
• Track component state changes, such as page scrolling or mouse position

Example

The following example shows how to use the watch command:

import Vue from "vue";

export default {
  data() {
    return {
      count: 0,
    };
  },
  watch: {
    count: {
      // 在 count 数据发生改变时触发此函数
      handler(newValue, oldValue) {
        console.log(`count changed from ${oldValue} to ${newValue}`);
      },
      // 仅在 count 数据为偶数时触发此函数
      immediate: true,
    },
  },
};

Deep Watch

Vue.js allows using nested objects or arrays as listening data. For deep monitoring of nested data, you can use the deep option:

watch: {
  obj: {
    handler() {
      // obj 中的任何数据改变都会触发此函数
    },
    deep: true,
  },
};

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