async-components（异步组件）

Async Components

Table of Contents

• When to Use
• When NOT to Use
• Instructions
• Details
• Source

When developing large web applications, performance is paramount. The speed with which a page loads and the responsiveness of its interactive elements can greatly impact user experience. As web applications grow in size and complexity, it can become important to ensure that large bundles of code are loaded only when needed. Enter asynchronous components in Vue.

Components are the fundamental building blocks for constructing the UI. Typically, when we use components, they're automatically loaded and parsed, even if they aren't immediately needed.

When to Use

• Use this when components have large bundle sizes and aren't needed on initial page load
• This is helpful for modals, dialogs, or any UI that is conditionally rendered based on user action

When NOT to Use

• For small components where the async loading overhead (chunk request, parsing) outweighs the bundle savings
• For components that are always visible on initial render — async loading delays their appearance
• When the component is already part of the main chunk and splitting it out wouldn't meaningfully reduce bundle size

Instructions

• Use defineAsyncComponent() with dynamic import() to load components on demand
• Provide loadingComponent and errorComponent options for better user experience
• Combine with v-if to trigger async loading only when the component is actually needed
• Use the delay and timeout options for fine-grained control over loading behavior

Details

Asynchronous components, on the other hand, allow us to define components in a way that they're loaded and parsed only when they're required or when certain conditions are met.

Assume we had a simple modal component that becomes rendered when a button is clicked from the parent. The Modal.vue component file will only contain template and styles that dictate how the modal appears.

<template>
  <div class="modal-mask">
    <div class="modal-container">
      <div class="modal-body">
        <h3>This is the modal!</h3>
      </div>

      <div class="modal-footer">
        <button class="modal-default-button" @click="$emit('close')">OK</button>
      </div>
    </div>
  </div>
</template>

In the parent App component, we can render the modal component and a button that when clicked toggles the visibility of the modal component with the help of a reactive boolean value (showModal).

<template>
  <button id="show-modal" @click="showModal = true">Show Modal</button>
  <Modal v-if="showModal" :show="showModal" @close="showModal = false" />
</template>

<script setup>
  import { ref } from "vue";
  import Modal from "./components/Modal.vue";

  const showModal = ref(false);
</script>

From this example, we can see that the modal component is shown only under a specific circumstance — when the user clicks the Show Modal button. Despite this, the JavaScript bundle associated with the component is loaded automatically when the entire webpage is loaded even before the modal is made visible.

This is fine for the majority of cases. However, under conditions where the bundle size of the modal is really large and/or the application has a multitude of such components, this can lead to a delayed initial load time. With every added bundle, even if it's related to components that are rarely used, the time it takes for the initial page to load grows.

defineAsyncComponent

This is where Vue allows us to divide an app into smaller chunks by loading components asynchronously with the help of the defineAsyncComponent() function.

import { defineAsyncComponent } from "vue";

const AsyncComp = defineAsyncComponent(() => {
  return new Promise((resolve, reject) => {
    // ...load component from the server
    resolve(/* loaded component */);
  });
});

The defineAsyncComponent() function accepts a loader function that returns a Promise that resolves to the imported component. However, instead of defining our async component function like the above, we can leverage dynamic imports to load an ECMAScript module asynchronously.

import { defineAsyncComponent } from "vue";

export const AsyncComp = defineAsyncComponent(() =>
  import("./components/MyComponent.vue")
);

Let's see this in action for our modal example. We'll create a new file titled AsyncModal.js:

import { defineAsyncComponent } from "vue";

export const AsyncModal = defineAsyncComponent(() => import("./Modal.vue"));

In our parent App component, we'll now import and use the AsyncModal asynchronous component in place of the Modal component.

<template>
  <button id="show-modal" @click="showModal = true">Show Modal</button>
  <AsyncModal v-if="showModal" :show="showModal" @close="showModal = false" />
</template>

<script setup>
  import { ref } from "vue";
  import { AsyncModal } from "./components/AsyncModal";

  const showModal = ref(false);
</script>

With this small change, our modal component will now be asynchronously loaded! When our application webpage initially loads, the bundle for the Modal component *is no longer loaded automatically upon page load*. When we click the button to trigger the modal to be shown, the bundle is then asynchronously loaded as the modal component is being rendered.

Loading and error UI

With defineAsyncComponent(), Vue provides developers with more than just a means of asynchronously loading components. It also offers capabilities to display feedback to users during the loading process and handle any potential errors.

loadingComponent

There may be times we may want to provide visual feedback to users while a component is being fetched. To achieve this, defineAsyncComponent() has a loadingComponent option that lets us specify a component to show during the loading phase.

import { defineAsyncComponent } from "vue";
import Loading from "./Loading.vue";

export const AsyncModal = defineAsyncComponent({
  loader: () => import("./Modal.vue"),
  loadingComponent: Loading,
});

As the modal component becomes asynchronously loaded, the user will now be presented with a Loading... message.

errorComponent

In certain conditions (e.g. poor internet connections), there may be chances that the asynchronous component fails to load. The defineAsyncComponent() function offers the errorComponent option to handle such situations, allowing us to specify a component to be displayed when there's a loading error.

import { defineAsyncComponent } from "vue";
import Loading from "./Loading.vue";
import Error from "./Error.vue";

export const AsyncModal = defineAsyncComponent({
  loader: () => import("./Modal.vue"),
  loadingComponent: Loading,
  errorComponent: Error,
});

When the modal component fails to load, the Error component template will be shown.

The defineAsyncComponent() function accepts further options like delay, timeout, suspensible, and onError() which provide developers with more granular control over the asynchronous loading behavior and user experience. Be sure to check out the API documentation for more details on these properties.

The defineAsyncComponent() function can help in breaking down the initial load of a Vue application into manageable chunks by deferring the loading of certain components until they're needed. This can help improve page load times and overall application performance especially when an application has numerous components that have a large bundle size.

Source

• patterns.dev/vue/async-components

References

• Async Components | Vue Documentation