Detailed explanation of the working principle and usage of keep-alive in vue

Vue.js is a popular front-end framework that provides some convenient functions to optimize performance and improve development efficiency. One of these features is keep-alive, which helps us retain state between components, thereby reducing unnecessary renderings and requests. This article will introduce in detail how keep-alive works and how to use it, and provide some code examples.

1. How keep-alive works

In Vue.js, whenever we switch components, the components will be recreated and rendered. This means that every time you switch a component, the component's state is reset and the data needs to be reloaded. For some relatively stable components, this behavior can lead to unnecessary waste of performance.

And keep-alive The role of the component is to cache components that need to retain their state instead of destroying and recreating them. This way, when switching components, if the component is already cached, it will read the state directly from the cache instead of reloading the data and rendering.

keep-alive works as follows:

1. When a component is loaded for the first time, the component instance will be cached, and the component's vm. $el (the root DOM element of the component instance) is removed from the DOM tree.
2. When switching to another component, the vm.$el of the original component will be put into an array named _inactive and saved.
3. If you switch back to the original component again, the vm.$el of the original component will be taken out from the _inactive array and reinserted into the DOM tree.

It should be noted that since the component is cached, the component's life cycle hook function (such as created, mounted, etc.) is only loaded for the first time It is triggered once when switching, and will not be triggered again during subsequent switching.

2. How to use keep-alive

In Vue.js, we can use the <keep-alive></keep-alive> component to wrap Components that need to be cached. Here are some common usages:

1. Cache a single component:

<template>
  <div>
    <keep-alive>
      <Component></Component>
    </keep-alive>
  </div>
</template>

In this example, the <component></component> component will be cached stand up. When switching to another component and back again, <component></component> the component will read the state from the cache instead of recreating it.

2. Cache multiple components:

<template>
  <div>
    <keep-alive>
      <router-view></router-view>
    </keep-alive>
  </div>
</template>

In this example, all components loaded through the routing configuration will be cached. When switching routes, already loaded components will read their state from the cache.

3. Dynamic caching of components:

<template>
  <div>
    <keep-alive :include="includeComponents">
      <router-view></router-view>
    </keep-alive>
  </div>
</template>

<script>
export default {
  data() {
    return {
      includeComponents: ['ComponentA', 'ComponentB']
    }
  }
}
</script>

In this example, only components contained in the includeComponents array will be cached. Other components will be destroyed and recreated when switching.

4. Cache components with different states before and after:

<template>
  <div>
    <keep-alive :exclude="excludeComponents">
      <router-view></router-view>
    </keep-alive>
  </div>
</template>

<script>
export default {
  data() {
    return {
      excludeComponents: ['ComponentA']
    }
  }
}
</script>

In this example, components in the excludeComponents array will not be cached. This means that after switching to other components, and then switching back, the excluded components will be recreated.

3. Summary

keep-alive The component is a function provided by Vue.js that can help us optimize performance. It works by caching components that need to retain their state, and loading the state directly from the cache when switching, avoiding unnecessary re-rendering and requests. The method of use is simple, you can cache a single component, multiple components, and even dynamically control the cached components.

By rational use of keep-alive components, we can improve application performance and reduce unnecessary resource consumption. I hope this article will help you understand how keep-alive works and how to use it. If you have any questions, please feel free to leave a message to discuss.

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