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| 1 | +--- |
| 2 | +badges: |
| 3 | + - new |
| 4 | +--- |
| 5 | + |
| 6 | +# Suspense <MigrationBadges :badges="$frontmatter.badges" /> |
| 7 | + |
| 8 | +:::warning Experimental |
| 9 | +Suspense is an experimental new feature and the API could change at any time. It is documented here so that the community can provide feedback on the current implementation. |
| 10 | + |
| 11 | +It should not be used in production applications. |
| 12 | +::: |
| 13 | + |
| 14 | +## Introduction |
| 15 | + |
| 16 | +It is common for components to need to perform some kind of asynchronous request before they can be rendered properly. Components often handle this locally and in many cases that is a perfectly good approach. |
| 17 | + |
| 18 | +The `<suspense>` component provides an alternative, allowing for the waiting to be handled further up the component tree rather than in each individual component. |
| 19 | + |
| 20 | +A common use case involves [async components](/guide/component-dynamic-async.html#async-components): |
| 21 | + |
| 22 | +```vue{2-4,6,17} |
| 23 | +<template> |
| 24 | + <suspense> |
| 25 | + <template #default> |
| 26 | + <todo-list /> |
| 27 | + </template> |
| 28 | + <template #fallback> |
| 29 | + <div> |
| 30 | + Loading... |
| 31 | + </div> |
| 32 | + </template> |
| 33 | + </suspense> |
| 34 | +</template> |
| 35 | +
|
| 36 | +<script> |
| 37 | +export default { |
| 38 | + components: { |
| 39 | + TodoList: defineAsyncComponent(() => import('./TodoList.vue')) |
| 40 | + } |
| 41 | +} |
| 42 | +</script> |
| 43 | +``` |
| 44 | + |
| 45 | +The `<suspense>` component has two slots. Both slots only allow for one immediate child node. The node in the `default` slot is shown if possible. If not, the node in the `fallback` slot will be shown instead. |
| 46 | + |
| 47 | +Importantly, the async component doesn't need to be the immediate child of the `<suspense>`. It can be at any depth within the component tree and doesn't need to appear in the same template as the `<suspense>` itself. The content is only considered resolved once all descendants are ready. |
| 48 | + |
| 49 | +The other way to trigger the `fallback` slot is for a descendant component to return a promise from its `setup` function. This is typically implemented using `async` rather than explicitly returning a promise: |
| 50 | + |
| 51 | +```js{2} |
| 52 | +export default { |
| 53 | + async setup() { |
| 54 | + // Be very careful using `await` inside `setup` as |
| 55 | + // most Composition API functions will only work |
| 56 | + // prior to the first `await` |
| 57 | + const data = await loadData() |
| 58 | +
|
| 59 | + // This is implicitly wrapped in a promise because |
| 60 | + // the function is `async` |
| 61 | + return { |
| 62 | + // ... |
| 63 | + } |
| 64 | + } |
| 65 | +} |
| 66 | +``` |
| 67 | + |
| 68 | +## Child Updates |
| 69 | + |
| 70 | +Once a `<suspense>` has resolved the contents of its `default` slot, it can only be triggered again if the `default` root node is replaced. New components nested deeper in the tree are not sufficient to move the `<suspense>` back into a pending state. |
| 71 | + |
| 72 | +If the root node does change it will trigger the `pending` event. However, by default, it won't update the DOM to show the `fallback` content. Instead, it will continue to show the old DOM until the new components are ready. This can be controlled using the `timeout` prop. This value, expressed in milliseconds, tells the `<suspense>` component how long to wait before showing the `fallback`. A value of `0` will show it immediately when the `<suspense>` enters the pending state. |
| 73 | + |
| 74 | +## Events |
| 75 | + |
| 76 | +In addition to the `pending` event, the `<suspense>` component also has `resolve` and `fallback` events. 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. |
| 77 | + |
| 78 | +The events could be used, for example, to show a loading indicator in front of the old DOM while new components are loading. |
| 79 | + |
| 80 | +## Combining with Other Components |
| 81 | + |
| 82 | +It is common to want to use `<suspense>` in combination with the [`<transition>`](/api/built-in-components.html#transition) and [`<keep-alive>`](/api/built-in-components.html#keep-alive) components. The nesting order of these components is important to get them all working correctly. |
| 83 | + |
| 84 | +In addition, these components are often used in conjunction with the `<router-view>` component from [Vue Router](https://next.router.vuejs.org/). |
| 85 | + |
| 86 | +The following example shows how to nest these components so that they all behave as expected. For simpler combinations you can remove the components that you don't need: |
| 87 | + |
| 88 | +```html |
| 89 | +<router-view v-slot="{ Component }"> |
| 90 | + <template v-if="Component"> |
| 91 | + <transition mode="out-in"> |
| 92 | + <keep-alive> |
| 93 | + <suspense> |
| 94 | + <component :is="Component"></component> |
| 95 | + <template #fallback> |
| 96 | + <div> |
| 97 | + Loading... |
| 98 | + </div> |
| 99 | + </template> |
| 100 | + </suspense> |
| 101 | + </keep-alive> |
| 102 | + </transition> |
| 103 | + </template> |
| 104 | +</router-view> |
| 105 | +``` |
| 106 | + |
| 107 | +Vue Router has built-in support for [lazily loading components](https://next.router.vuejs.org/guide/advanced/lazy-loading.html) using dynamic imports. These are distinct from async components and currently they will not trigger `<suspense>`. However, they can still have async components as descendants and those can trigger `<suspense>` in the usual way. |
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