Suspense

In web development, managing tasks that take time to complete, such as fetching data from a server, is a common challenge. These tasks are known as asynchronous operations because they run separately from the main flow of the program, allowing it to continue functioning while waiting for these tasks to finish. Vue.js has introduced a component in its third version called Suspense to improve the way developers handle these operations. In this topic, you will learn how to use Vue 3's Suspense component to manage asynchronous operations effectively. You'll discover how to display fallback content while waiting for data, and how to transition smoothly to your main content once the data is available.

What is the suspense component?

The Suspense component in Vue is designed to handle asynchronous dependencies seamlessly within your component tree. It serves as a declarative way to specify the loading state for a part of your application, making it easier to manage the user experience during data fetching or any other async operation. When a component within Suspense waits for its dependencies, Suspense can display a fallback content, such as a loading spinner, until the operation completes and the main content is ready to be rendered.

The <Suspense> component emits 3 events: pending, resolve and fallback. The pending event occurs when entering a pending state. The resolve event is emitted when new content has finished resolving in the default slot. The fallback event is fired when the contents of the fallback slot are shown.

Using suspense with async components

Async components in Vue load dynamically and are often associated with code splitting or lazy loading scenarios. To use Suspense with async components, you need to define fallback content and wrap your async component within the Suspense tags. Here's a brief example to illustrate the concept:

<template>
  <Suspense>
    <template #default>
      <AsyncComponent />
    </template>
    <template #fallback>
      <div>Loading...</div>
    </template>
  </Suspense>
</template>

<script setup>
import { defineAsyncComponent } from 'vue';

const AsyncComponent = defineAsyncComponent(() =>
  import('./components/MyAsyncComponent.vue')
 )
</script>

In the snippet above, <AsyncComponent /> loads asynchronously, and during the loading phase, Suspense renders the content specified under the #fallback slot. defineAsyncComponent is a utility called with a function that returns a promise as its argument. This promise comes from calling import(), which is dynamic import syntax allowing you to import a module on demand. Wrapping the dynamic import with defineAsyncComponent tells Vue that this component should load asynchronously, as it facilitates the definition and use of components that should load only when needed, helping to optimize the loading time and performance of a Vue.js application.

On initial render, <Suspense> renders its default slot content in memory. If it encounters any async dependencies during the process, it enters a pending state. During this state, the fallback content displays. When all encountered async dependencies have been resolved, <Suspense> enters a resolved state, and the resolved default slot content displays.

Using suspense with async setup()

Vue 3 introduced the Composition API, which includes the setup() function as a new way to organize component logic. When combined with the async keyword, setup() can return a promise that resolves to the component's reactive state. Suspense handles the asynchronous nature of such a setup function by waiting for the promise to resolve before rendering the component's content.

To streamline component setup, Vue 3 also offers the <script setup> syntax, which simplifies Composition API usage by implicitly returning the reactive state and reducing boilerplate code.

Here's how you can use Suspense with an async setup() method using the <script setup> syntax:

<template>
  <Suspense>
    <template #default>
      <div>{{ userData.name }}</div>
    </template>
    <template #fallback>
      <div>Loading user data...</div>
    </template>
  </Suspense>
</template>

<script setup>
import { ref } from 'vue';
import { fetchUserData } from './api/user';

const userData = ref(null);

(async () => {
  userData.value = await fetchUserData();
})();
</script>

In this example, fetchUserData() is a hypothetical function that fetches user data from an API. The <script setup> block allows for direct use of Composition API functions like ref to create reactive state variables. The async immediately-invoked function expression (IIFE) is used to await the completion of fetchUserData() before assigning the result to userData. During the wait, Suspense displays the fallback content, providing users with immediate feedback until the data is fully loaded.

Conclusion

The Suspense component in Vue is a robust tool for managing asynchronous operations and improving the user experience during data fetching or component loading. By understanding how to use Suspense with both async components and the async setup() method, you can create smoother interactions and more responsive applications. Remember to make use of the fallback content to keep your users informed about the loading state, and enjoy building reactive applications with Vue's robust component system.