How to use keep-alive to optimize performance in Vue?

When developing web applications, we are all concerned about the performance of the application. One common scenario is to click on a page and then return to the previous page. During this process, the page needs to be reloaded. This is very unfriendly to the user experience and also wastes server resources and user traffic. To avoid this situation, we can use the keep-alive provided in Vue for caching, thereby improving the performance of the application.

What is keep-alive?

keep-alive is a built-in component of Vue.js, used to cache already rendered components to avoid repeated rendering. In Vue, each component is an independent instance. When we switch components, the original component will be destroyed and re-rendered, and then a new component will be generated. This process is no problem for simple components, but for some complex components, it may take a long time to complete rendering.

keep-alive is equivalent to a cache, which allows components to avoid re-rendering and improve application performance. When the component is rendered for the first time, keep-alive caches it. When we switch to other components and return to the cached component again, keep-alive will display the cached component directly on the page instead of re-rendering it again.

How to use keep-alive in Vue?

Using keep-alive is very simple. We only need to wrap the components that need to be cached in a tag.

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

In this example, we wrap the tag in a tag. In this way, every time you switch routes, the cached components will be retained, ensuring that the previously rendered state will not be lost, and also avoiding repeated rendering. This method is very effective for components that need to be reused frequently, such as navigation bars, menus, etc.

Keep-alive’s life cycle methods

In addition to providing a caching mechanism, keep-alive also provides some life cycle methods that can help us manage cached components.

• activated – called when the cache component is activated
• deactivated – called when the cache component is deactivated

Both life cycle methods are It will only be called when the component is cached. Therefore, these two methods are not commonly used when there is only one component that needs to be cached. But when there are multiple components, these two methods allow us to deal with the interaction between different cache components.

For example, we have two components A and B, both of which are wrapped in a tag. When component A is activated, we can use the activated method to process some data that needs to be reloaded; when component B is deactivated, we can use the deactivated method to process some data that needs to be cleared.

The following is an example:

// 在 A 组件中
activated() {
  // 在 A 被激活时重新加载数据
  this.loadData()
},
// 在 B 组件中
deactivated() {
  // 在 B 被停用时清除数据
  this.clearData();
}

Notes

Although keep-alive provides a good caching mechanism, we still need to pay attention to some details when using it.

Don’t overuse keep-alive

Although keep-alive can cache the components we want to reuse, the caching process also requires a certain amount of memory and processor resources. When we cache too many components, it will cause application performance degradation and even cause memory leaks. Therefore, when using keep-alive, we need to control the number of caches and only cache frequently used components.

Don’t use keep-alive with v-for

When using the v-for directive, each component will be rendered once. When we use keep-alive, these components may be cached, but their data and state are independent of each other. For example, we render a list in v-for. When we delete one of the components, we can only delete one component from the list, and all components in the cache will be deleted, which will cause us to have some errors.

Don’t use asynchronous requests in keep-alive

Some problems may occur when using asynchronous requests in keep-alive. For example, when we return a cached component, the asynchronous request may not have completed yet, which may cause the cached component to render incompletely or throw an exception. Therefore, in keep-alive, it is best not to use asynchronous requests.

Summary

keep-alive is a very useful component provided by Vue.js. It can help us cache rendered components, improve application performance and improve user experience. When using keep-alive, we need to pay attention to some issues and control the number of caches to prevent performance problems.

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