How to use watchEffect in Vue to monitor responsive data and update DOM

As a front-end framework, one of the cores of Vue is data-driven views. When the data defined in Vue changes, the corresponding view is also updated. In Vue3, in order to optimize performance and response speed, the reactivity system and Composition API are introduced. As part of a responsive system, watchEffect can monitor data changes and update the DOM instantly. This article will introduce in detail how to use watchEffect in Vue to monitor responsive data and update the DOM.

1. Responsive system

In Vue3, the core of the responsive system is ordinary JavaScript objects called "Reactive Objects". By using the reactive function to convert an object into a responsive object, Vue can track and respond to changes in data in this object.

The main APIs used in responsive systems are:

1. reactive: Convert an ordinary JavaScript object into a responsive object.
2. ref: Convert an ordinary JavaScript object into a responsive object, and add the .value attribute to the object. You can directly access the value of the object and modify its value. When using it in a template, you need to use .value to operate.
3. toRefs: Convert a responsive object into a normal JavaScript object, where each property is encapsulated as a ref object.
4. computed: Define a calculated property, and the return value is a responsive object. When the dependence of the calculation process on the responsive object changes, the computed function will automatically recalculate and update.

2. watchEffect

watchEffect is a new API in Vue3. It can automatically infer the dependencies that need to be monitored based on the responsive object. Once the dependent data changes, watchEffect will The callback function will be triggered immediately. The use of watchEffect is similar to computed, but watchEffect does not need to access properties, it only needs to use the reactive object directly in the callback function.

watchEffect has the following characteristics:

1. The execution time of watchEffect is before the Dom is updated, so it can be used directly after updating the data in this function.
2. watchEffect can automatically track changes in responsive data without manually specifying dependencies.
3. watchEffect needs to return a parameterless cleanup function. When watchEffect's related dependencies change and are re-run, the cleanup function will be automatically called to release resources.

Things to note when using watchEffect are:

1. Do not perform excessively time-consuming operations in watchEffect, because they will be performed every time the data is updated.
2. If you need to accurately monitor changes in a certain data, you should also use the watch function or watchEffect's dependent parameters to specify.

3. Use watchEffect to update the DOM

We can use the watchEffect function in Vue's single-file component and update the DOM structure in the callback function. This way, whenever the reactive data in the component changes, we can immediately see the result change.

Below we take a counter as an example to demonstrate the use of the watchEffect function:

<!-- Counter.vue -->
<template>
  <div>
    <button @click="increment">{{ count }}</button>
  </div>
</template>
<script>
import { reactive, watchEffect } from 'vue'

export default {
  setup() {
    const state = reactive({
      count: 0
    })

    watchEffect(() => {
      document.title = `计数器：${state.count}`
    })

    function increment() {
      state.count++
    }

    return {
      count: state.count,
      increment
    }
  }
}
</script>

In this example, we define a responsive object state and use the watchEffect function to monitor its changes. When the count attribute changes, the code in the watchEffect callback function is automatically executed and the browser's tab title is updated.

In addition to updating the browser title, we can implement more data binding operations through the watchEffect function, such as class and style binding, text interpolation and other operations. Through these methods, we can flexibly control the display and hiding of components, style adjustment, etc.

4. Summary

In Vue3, watchEffect is a very practical tool that can update the DOM very quickly when the data changes. When using the watchEffect function, you need to pay attention to the objects it listens to and the code in the callback function to ensure performance and code quality. In addition to watchEffect, Vue3 also provides more responsive functions and Composition APIs, which can be used in combination to meet different needs.

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