This is documentation for the next SDK version. For up-to-date documentation, see the latest version (SDK 55).

Expo 资源

一个允许下载资源并与其他库一起使用的通用库。

Android

iOS

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Web

Included in Expo Go

GitHub

npm

更新日志

expo-asset 提供了一个与 Expo 资源系统交互的接口。资源是指任何与应用源代码放在一起、且应用在运行时需要的文件。例如图片、字体和声音。Expo 的资源系统与 React Native 的资源系统集成，因此你可以使用 require('path/to/file') 来引用文件。例如，这就是你在 React Native 中为 Image 组件引用静态图片文件的方式。有关更多信息，请查看 React Native 关于静态图片资源的文档。这种引用静态图片资源的方法在 Expo 中开箱即用。

安装

Terminal

- npx expo install expo-asset

If you are installing this in an existing React Native app, make sure to install expo in your project.

在 app config 中进行配置

如果你的项目使用 config plugins（Continuous Native Generation (CNG)），你可以使用 expo-asset 内置的 config plugin 来进行配置。该插件允许你配置一些无法在运行时设置、且需要重新构建新的应用二进制文件后才会生效的属性。如果你的应用不使用 CNG，那么你需要手动配置该库。

Example app.json with config plugin

app.json

{
  "expo": {
    "plugins": [
      [
        "expo-asset",
        {
          "assets": ["path/to/file.png", "path/to/directory"]
        }
      ]
    ]
  }
}

Configurable properties

Name Default Description

Name	Default	Description
`assets`	`[]`	一个资源文件或目录的数组，用于链接到原生项目。路径应相对于项目根目录，这样无论是直接指定文件还是使用目录，文件名都会成为资源名称。支持的文件类型：图片：`.png`、`.jpg`、`.gif` 媒体：`.mp4`、`.mp3`、`.lottie`、`.riv` SQLite 数据库文件：`.db` 3D 模型：`.glb` 注意：要导入现有的数据库文件（`.db`），请参阅 SQLite API 参考中的说明。对于其他文件类型（例如 `.lottie` 或 `.riv`），请参阅如何在 metro 配置中向 `assetExts` 添加文件扩展名。

assets

[]

一个资源文件或目录的数组，用于链接到原生项目。路径应相对于项目根目录，这样无论是直接指定文件还是使用目录，文件名都会成为资源名称。

支持的文件类型：

图片：.png、.jpg、.gif
媒体：.mp4、.mp3、.lottie、.riv
SQLite 数据库文件：.db
3D 模型：.glb

注意：要导入现有的数据库文件（.db），请参阅 SQLite API 参考中的说明。对于其他文件类型（例如 .lottie 或 .riv），请参阅如何在 metro 配置中向 assetExts 添加文件扩展名。

用法

了解更多关于如何使用 expo-asset config plugin 将资源文件嵌入到项目中的内容，请参阅在构建时加载资源。

API

import { Asset } from 'expo-asset';

Hooks

`useAssets(moduleIds)`

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Web

Parameter	Type
moduleIds	`number \| number[]`

Downloads and stores one or more assets locally. After the assets are loaded, this hook returns a list of asset instances. If something went wrong when loading the assets, an error is returned.

Note, the assets are not "reloaded" when you dynamically change the asset list.

Returns:

[Asset[] | undefined, Error | undefined]

Returns an array containing:

on the first position, a list of all loaded assets. If they aren't loaded yet, this value is undefined.
on the second position, an error which encountered when loading the assets. If there was no error, this value is undefined.

Example

const [assets, error] = useAssets([require('path/to/asset.jpg'), require('path/to/other.png')]);

return assets ? <Image source={assets[0]} /> : null;

Classes

`Asset`

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The Asset class represents an asset in your app. It gives metadata about the asset (such as its name and type) and provides facilities to load the asset data.

Asset Properties

`downloaded`

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Type: boolean • Default: false

Whether the asset has finished downloading from a call to downloadAsync().

`hash`

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Read Only • Literal type: union • Default: null

The MD5 hash of the asset's data.

Acceptable values are: string | null

`height`

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Literal type: union • Default: null

If the asset is an image, the height of the image data divided by the scale factor. The scale factor is the number after @ in the filename, or 1 if not present.

Acceptable values are: number | null

`localUri`

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Literal type: union • Default: null

If the asset has been downloaded (by calling downloadAsync()), the file:// URI pointing to the local file on the device that contains the asset data.

Acceptable values are: string | null

`name`

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Type: string

The name of the asset file without the extension. Also without the part from @ onward in the filename (used to specify scale factor for images).

`type`

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Read Only • Type: string

The extension of the asset filename.

`uri`

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Read Only • Type: string

A URI that points to the asset's data on the remote server. When running the published version of your app, this refers to the location on Expo's asset server where Expo has stored your asset. When running the app from Expo CLI during development, this URI points to Expo CLI's server running on your computer and the asset is served directly from your computer. If you are not using Classic Updates (legacy), this field should be ignored as we ensure your assets are on device before running your application logic.

`width`

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Literal type: union • Default: null

If the asset is an image, the width of the image data divided by the scale factor. The scale factor is the number after @ in the filename, or 1 if not present.

Acceptable values are: number | null

Asset Methods

`downloadAsync()`

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Web

Downloads the asset data to a local file in the device's cache directory. Once the returned promise is fulfilled without error, the localUri field of this asset points to a local file containing the asset data. The asset is only downloaded if an up-to-date local file for the asset isn't already present due to an earlier download. The downloaded Asset will be returned when the promise is resolved.

Note: There is no guarantee that files downloaded via downloadAsync persist between app sessions. downloadAsync stores files in the caches directory, so it's up to the OS to clear this folder at its own discretion or when the user manually purges the caches directory. Downloaded assets are stored as ExponentAsset-{cacheFileId}.{extension} within the cache directory. To manually clear cached assets, you can use expo-file-system to delete the cache directory: Paths.cache.delete() or use the legacy API deleteAsync(cacheDirectory).

Returns:

Promise<Asset>

Returns a Promise which fulfills with an Asset instance.

`fromMetadata(meta)`

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Parameter	Type
meta	`AssetMetadata`

Returns:

Asset

`fromModule(virtualAssetModule)`

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Parameter	Type	Description
virtualAssetModule	`string \| number \| { height: number, uri: string, width: number }`	The value of `require('path/to/file')` for the asset or external network URL

Returns the Asset instance representing an asset given its module or URL.

Returns:

Asset

The Asset instance for the asset.

`fromURI(uri)`

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Web

Parameter	Type
uri	`string`

Returns:

Asset

`loadAsync(moduleId)`

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Parameter	Type	Description
moduleId	`string \| number \| number[] \| string[]`	An array of `require('path/to/file')` or external network URLs. Can also be just one module or URL without an Array.

A helper that wraps Asset.fromModule(module).downloadAsync for convenience.

Returns:

Promise<Asset[]>

Returns a Promise that fulfills with an array of Assets when the asset(s) has been saved to disk.

Example

const [{ localUri }] = await Asset.loadAsync(require('./assets/snack-icon.png'));

Types

`AssetDescriptor`

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Web

Property	Type	Description
hash(optional)	`string \| null`	-
height(optional)	`number \| null`	-
name	`string`	-
type	`string`	-
uri	`string`	-
width(optional)	`number \| null`	-

`AssetMetadata`

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Web

Property	Type	Description
fileHashes(optional)	`string[]`	-
fileUris(optional)	`string[]`	-
uri(optional)	`string`	-