Expo ImageA cross-platform and performant React component that loads and renders images.
expo-image is a cross-platform React component that loads and renders images.
Main features:
- Designed for speed
- Support for many image formats (including animated ones)
- Disk and memory caching
- Supports BlurHash and ThumbHash - compact representations of a placeholder for an image
- Transitioning between images when the source changes (no more flickering!)
- Implements the CSS
object-fitandobject-positionproperties (seecontentFitandcontentPositionprops) - Uses performant
SDWebImageandGlideunder the hood
Supported image formats
| Format | Android | iOS | Web |
|---|---|---|---|
| WebP | |||
| PNG / APNG | |||
| AVIF | |||
| HEIC | not adopted yet | ||
| JPEG | |||
| GIF | |||
| SVG | |||
| ICO | |||
| ICNS |
Installation
- npx expo install expo-imageIf you are installing this in an existing React Native app, start by installing expo in your project. Then, follow the additional instructions as mentioned by the library's README under "Installation in bare React Native projects" section.
Usage
import { Image } from 'expo-image';
import { StyleSheet, View } from 'react-native';
const blurhash =
'|rF?hV%2WCj[ayj[a|j[az_NaeWBj@ayfRayfQfQM{M|azj[azf6fQfQfQIpWXofj[ayj[j[fQayWCoeoeaya}j[ayfQa{oLj?j[WVj[ayayj[fQoff7azayj[ayj[j[ayofayayayj[fQj[ayayj[ayfjj[j[ayjuayj[';
export default function App() {
return (
<View style={styles.container}>
<Image
style={styles.image}
source="https://picsum.photos/seed/696/3000/2000"
placeholder={{ blurhash }}
contentFit="cover"
transition={1000}
/>
</View>
);
}
const styles = StyleSheet.create({
container: {
flex: 1,
backgroundColor: '#fff',
alignItems: 'center',
justifyContent: 'center',
},
image: {
flex: 1,
width: '100%',
backgroundColor: '#0553',
},
});
API
import { Image } from 'expo-image';
Components
Type: React.PureComponent<ImageProps>
Some props are from React Native Image that Expo Image supports (more or less) for easier migration, but all of them are deprecated and might be removed in the future.
string • Default: undefinedThe text that's read by the screen reader when the user interacts with the image. Sets the the alt tag on web which is used for web crawlers and link traversal.
boolean • Default: falseWhen true, indicates that the view is an accessibility element. When a view is an accessibility element, it groups its children into a single selectable component.
On Android, the accessible property will be translated into the native isScreenReaderFocusable,
so it's only affecting the screen readers behaviour.
boolean • Default: trueWhether the image should be downscaled to match the size of the view container. Turning off this functionality could negatively impact the application's performance, particularly when working with large assets. However, it would result in smoother image resizing, and end-users would always have access to the highest possible asset quality.
Downscaling is never used when the contentFit prop is set to none or fill.
string • Default: undefinedThe text that's read by the screen reader when the user interacts with the image. Sets the the alt tag on web which is used for web crawlers and link traversal. Is an alias for accessibilityLabel.
boolean • Default: trueDetermines if an image should automatically begin playing if it is an animated image.
number • Default: 0The radius of the blur in points, 0 means no blur effect.
This effect is not applied to placeholders.
null | 'none' | 'disk' | 'memory' | 'memory-disk' • Default: 'disk'Determines whether to cache the image and where: on the disk, in the memory or both.
-
'none'- Image is not cached at all. -
'disk'- Image is queried from the disk cache if exists, otherwise it's downloaded and then stored on the disk. -
'memory'- Image is cached in memory. Might be useful when you render a high-resolution picture many times. Memory cache may be purged very quickly to prevent high memory usage and the risk of out of memory exceptions. -
'memory-disk'- Image is cached in memory, but with a fallback to the disk cache.
ImageContentFit • Default: 'cover'Determines how the image should be resized to fit its container. This property tells the image to fill the container
in a variety of ways; such as "preserve that aspect ratio" or "stretch up and take up as much space as possible".
It mirrors the CSS object-fit property.
-
'cover'- The image is sized to maintain its aspect ratio while filling the container box. If the image's aspect ratio does not match the aspect ratio of its box, then the object will be clipped to fit. -
'contain'- The image is scaled down or up to maintain its aspect ratio while fitting within the container box. -
'fill'- The image is sized to entirely fill the container box. If necessary, the image will be stretched or squished to fit. -
'none'- The image is not resized and is centered by default. When specified, the exact position can be controlled withcontentPositionprop. -
'scale-down'- The image is sized as ifnoneorcontainwere specified, whichever would result in a smaller concrete image size.
ImageContentPosition • Default: 'center'It is used together with contentFit to specify how the image should be positioned with x/y coordinates inside its own container.
An equivalent of the CSS object-position property.
ImageDecodeFormat • Default: 'argb'The format in which the image data should be decoded. It's not guaranteed that the platform will use the specified format.
-
'argb'- The image is decoded into a 32-bit color space with alpha channel (https://developer.android.com/reference/android/graphics/Bitmap.Config#ARGB_8888). -
'rgb'- The image is decoded into a 16-bit color space without alpha channel (https://developer.android.com/reference/android/graphics/Bitmap.Config#RGB_565).
Deprecated Provides compatibility for
defaultSourcefrom React Native Image. Useplaceholderprop instead.
null | ImageSourceboolean • Default: falseEnables Live Text interaction with the image. Check official Apple documentation for more details.
Deprecated Provides compatibility for
fadeDurationfrom React Native Image. Instead usetransitionwith the provided duration.
numberboolean • Default: falseWhether this View should be focusable with a non-touch input device and receive focus with a hardware keyboard.
Deprecated Provides compatibility for
loadingIndicatorSourcefrom React Native Image. Useplaceholderprop instead.
null | ImageSource() => voidCalled when the image view successfully rendered the source image.
(event: ImageErrorEventData) => voidCalled on an image fetching error.
(event: ImageLoadEventData) => voidCalled when the image load completes successfully.
() => voidCalled when the image load either succeeds or fails.
(event: ImageProgressEventData) => voidCalled when the image is loading. Can be called multiple times before the image has finished loading. The event object provides details on how many bytes were loaded so far and what's the expected total size.
null | string | number | ImageSource | SharedRef<'image', Record<never, never>> | ImageSource[] | string[]An image to display while loading the proper image and no image has been displayed yet or the source is unset.
ImageContentFit • Default: 'scale-down'Determines how the placeholder should be resized to fit its container. Available resize modes are the same as for the contentFit prop.
null | 'low' | 'normal' | 'high' • Default: 'normal'Priorities for completing loads. If more than one load is queued at a time, the load with the higher priority will be started first. Priorities are considered best effort, there are no guarantees about the order in which loads will start or finish.
null | string • Default: nullChanging this prop resets the image view content to blank or a placeholder before loading and rendering the final image. This is especially useful for any kinds of recycling views like FlashList to prevent showing the previous source before the new one fully loads.
Deprecated Provides compatibility for
resizeModefrom React Native Image. Note that"repeat"option is not supported at all. Use the more powerfulcontentFitandcontentPositionprops instead.
'cover' | 'contain' | 'center' | 'stretch' | 'repeat''live' | 'initial' | 'static' • Default: 'static'Controls the selection of the image source based on the container or viewport size on the web.
If set to 'static', the browser selects the correct source based on user's viewport width. Works with static rendering.
Make sure to set the 'webMaxViewportWidth' property on each source for best results.
For example, if an image occupies 1/3 of the screen width, set the 'webMaxViewportWidth' to 3x the image width.
The source with the largest 'webMaxViewportWidth' is used even for larger viewports.
If set to 'initial', the component will select the correct source during mount based on container size. Does not work with static rendering.
If set to 'live', the component will select the correct source on every resize based on container size. Does not work with static rendering.
null | string | number | ImageSource | SharedRef<'image', Record<never, never>> | ImageSource[] | string[]The image source, either a remote URL, a local file resource or a number that is the result of the require() function.
When provided as an array of sources, the source that fits best into the container size and is closest to the screen scale
will be chosen. In this case it is important to provide width, height and scale properties.
null | string • Default: nullA color used to tint template images (a bitmap image where only the opacity matters). The color is applied to every non-transparent pixel, causing the image’s shape to adopt that color. This effect is not applied to placeholders.
null | number | ImageTransitionDescribes how the image view should transition the contents when switching the image source.
If provided as a number, it is the duration in milliseconds of the 'cross-dissolve' effect.
Inherited Props
Type: React.Element<ImageBackgroundProps>
Inherited Props
Omit<ImageProps, 'style'>
Static Methods
Asynchronously clears all images from the disk cache.
Promise<boolean>
A promise resolving to true when the operation succeeds.
It may resolve to false on Android when the activity is no longer available.
Resolves to false on Web.
Asynchronously clears all images stored in memory.
Promise<boolean>
A promise resolving to true when the operation succeeds.
It may resolve to false on Android when the activity is no longer available.
Resolves to false on Web.
| Parameter | Type | Description |
|---|---|---|
| url | string | The URL of the image to generate a blurhash from. |
| numberOfComponents | [number, number] | {
height: number,
width: number
} | The number of components to encode the blurhash with.
Must be between 1 and 9. Defaults to |
Asynchronously generates a Blurhash from an image.
Promise<null | string>
A promise resolving to the blurhash string.
| Parameter | Type | Description |
|---|---|---|
| cacheKey | string | The cache key for the requested image. Unless you have set a custom cache key, this will be the source URL of the image. |
Asynchronously checks if an image exists in the disk cache and resolves to the path of the cached image if it does.
Promise<null | string>
A promise resolving to the path of the cached image. It will resolve
to null if the image does not exist in the cache.
| Parameter | Type |
|---|---|
| source | string | ImageSource |
| options (optional) | ImageLoadOptions |
Loads an image from the given source to memory and resolves to an object that references the native image instance.
| Parameter | Type | Description |
|---|---|---|
| urls | string | string[] | A URL string or an array of URLs of images to prefetch. |
| cachePolicy (optional) | 'disk' | 'memory' | 'memory-disk' | The cache policy for prefetched images. |
Preloads images at the given URLs that can be later used in the image view.
Preloaded images are cached to the memory and disk by default, so make sure
to use disk (default) or memory-disk cache policy.
Promise<boolean>
A promise resolving to true as soon as all images have been
successfully prefetched. If an image fails to be prefetched, the promise
will immediately resolve to false regardless of whether other images have
finished prefetching.
| Parameter | Type | Description |
|---|---|---|
| urls | string | string[] | A URL string or an array of URLs of images to prefetch. |
| options (optional) | ImagePrefetchOptions | Options for prefetching images. |
Preloads images at the given URLs that can be later used in the image view.
Preloaded images are cached to the memory and disk by default, so make sure
to use disk (default) or memory-disk cache policy.
Promise<boolean>
A promise resolving to true as soon as all images have been
successfully prefetched. If an image fails to be prefetched, the promise
will immediately resolve to false regardless of whether other images have
finished prefetching.
Component Methods
Asynchronously starts playback of the view's image if it is animated.
Promise<void>
Asynchronously stops the playback of the view's image if it is animated.
Promise<void>
Hooks
| Parameter | Type |
|---|---|
| source | string | ImageSource |
| options (optional) | ImageLoadOptions |
| dependencies (optional) | DependencyList |
A hook that loads an image from the given source and returns a reference
to the native image instance, or null until the first image is successfully loaded.
It loads a new image every time the uri of the provided source changes.
To trigger reloads in some other scenarios, you can provide an additional dependency list.
ImageRef | null
Example
import { useImage, Image } from 'expo-image';
import { Text } from 'react-native';
export default function MyImage() {
const image = useImage('https://picsum.photos/1000/800', {
maxWidth: 800,
onError(error, retry) {
console.error('Loading failed:', error.message);
}
});
if (!image) {
return <Text>Image is loading...</Text>;
}
return <Image source={image} style={{ width: image.width / 2, height: image.height / 2 }} />;
}
Classes
Type: Class extends SharedRef<'image'>
An object that is a reference to a native image instance – Drawable on Android and UIImage on iOS. Instances of this class can be passed as a source to the Image component in which case the image is rendered immediately since its native representation is already available in the memory.
ImageRef Properties
numberLogical height of the image. Multiply it by the value in the scale property to get the height in pixels.
booleanWhether the referenced image is an animated image.
null | stringMedia type (also known as MIME type) of the image, based on its format.
Returns null when the format is unknown or not supported.
numberOn iOS, if you load an image from a file whose name includes the @2x modifier, the scale is set to 2.0. All other images are assumed to have a scale factor of 1.0.
On Android, it calculates the scale based on the bitmap density divided by screen density.
On all platforms, if you multiply the logical size of the image by this value, you get the dimensions of the image in pixels.
Types
Specifies the position of the image inside its container. One value controls the x-axis and the second value controls the y-axis.
Additionally, it supports stringified shorthand form that specifies the edges to which to align the image content:
'center', 'top', 'right', 'bottom', 'left', 'top center', 'top right', 'top left', 'right center', 'right top',
'right bottom', 'bottom center', 'bottom right', 'bottom left', 'left center', 'left top', 'left bottom'.
If only one keyword is provided, then the other dimension is set to 'center' ('50%'), so the image is placed in the middle of the specified edge.
As an example, 'top right' is the same as { top: 0, right: 0 } and 'bottom' is the same as { bottom: 0, left: '50%' }.
Type: ImageContentPositionString or object shaped as below:
| Name | Type | Description |
|---|---|---|
| right (optional) | ImageContentPositionValue | - |
| top (optional) | ImageContentPositionValue | - |
| Name | Type | Description |
|---|---|---|
| left (optional) | ImageContentPositionValue | - |
| top (optional) | ImageContentPositionValue | - |
| Name | Type | Description |
|---|---|---|
| bottom (optional) | ImageContentPositionValue | - |
| right (optional) | ImageContentPositionValue | - |
| Name | Type | Description |
|---|---|---|
| bottom (optional) | ImageContentPositionValue | - |
| left (optional) | ImageContentPositionValue | - |
Literal Type: multiple types
A value that represents the relative position of a single axis.
If number, it is a distance in points (logical pixels) from the respective edge.
If string, it must be a percentage value where '100%' is the difference in size between the container and the image along the respective axis,
or 'center' which is an alias for '50%' that is the default value. You can read more regarding percentages on the MDN docs for
background-position that describes this concept well.
Acceptable values are: number | string | 'center'
| Name | Type | Description |
|---|---|---|
| cacheType | 'none' | 'disk' | 'memory' | - |
| source | {
height: number,
isAnimated: boolean,
mediaType: string | null,
url: string,
width: number
} | - |
An object with options for the useImage hook.
| Name | Type | Description |
|---|---|---|
| maxHeight (optional) | number | Only for: iOS If provided, the image will be automatically resized to not exceed this height in pixels, preserving its aspect ratio. |
| maxWidth (optional) | number | Only for: iOS If provided, the image will be automatically resized to not exceed this width in pixels, preserving its aspect ratio. |
| onError (optional) | (error: Error, retry: () => void) => void | Function to call when the image has failed to load. In addition to the error, it also provides a function that retries loading the image. |
| Name | Type | Description |
|---|---|---|
| cachePolicy (optional) | 'disk' | 'memory-disk' | 'memory' | The cache policy for prefetched images. Default: 'memory-disk' |
| headers (optional) | Record<string, string> | A map of headers to use when prefetching the images. |
| Name | Type | Description |
|---|---|---|
| blurhash (optional) | string | A string used to generate the image When using the blurhash, you should also provide |
| cacheKey (optional) | string | The cache key used to query and store this specific image.
If not provided, the |
| headers (optional) | Record<string, string> | An object representing the HTTP headers to send along with the request for a remote image.
On web requires the |
| height (optional) | number | Can be specified if known at build time, in which case the value
will be used to set the default |
| isAnimated (optional) | boolean | Only for: Android iOS Whether the image is animated (an animated GIF or WebP for example). |
| thumbhash (optional) | string | A string used to generate the image For more information, see |
| uri (optional) | string | A string representing the resource identifier for the image, which could be an HTTPS address, a local file path, or the name of a static image resource. |
| webMaxViewportWidth (optional) | number | Only for: Web The max width of the viewport for which this source should be selected.
Has no effect if |
| width (optional) | number | Can be specified if known at build time, in which case the value
will be used to set the default |
An object that describes the smooth transition when switching the image source.
| Name | Type | Description |
|---|---|---|
| duration (optional) | number | The duration of the transition in milliseconds. Default: 0 |
| effect (optional) | 'cross-dissolve' | 'flip-from-top' | 'flip-from-right' | 'flip-from-bottom' | 'flip-from-left' | 'curl-up' | 'curl-down' | null | An animation effect used for transition. Default: 'cross-dissolve'
On Android, only `'cross-dissolve'` is supported.
On Web, `'curl-up'` and `'curl-down'` effects are not supported. |
| timing (optional) | 'ease-in-out' | 'ease-in' | 'ease-out' | 'linear' | Specifies the speed curve of the transition effect and how intermediate values are calculated. Default: 'ease-in-out' |
Generating a blurhash on a server
Images can significantly improve the visual experience, however, they can also slow down app/page loading times due to their large file sizes. To overcome this, you can create placeholder images using blurhash algorithm that provides an immersive experience while deferring the loading of the actual picture until later.
This guide demonstrates how to create a blurhash of an uploaded image on the backend using JavaScript and Express.js. The same techniques and principles apply to other languages and server technologies.
Start by installing a few dependencies: multer for handling multipart requests, sharp for converting files to a data buffer, and the official blurhash JavaScript package.
- npm install multer sharp blurhashNext, import all required functions from installed packages and initialize multer:
// Multer is a middleware for handling `multipart/form-data`.
const multer = require('multer');
// Sharp allows you to receive a data buffer from the uploaded image.
const sharp = require('sharp');
// Import the encode function from the blurhash package.
const { encode } = require('blurhash');
// Initialize `multer`.
const upload = multer();
Assuming the app is a variable that holds a reference to the Express server, an endpoint can be created that accepts an image and returns a JSON response containing the generated blurhash.
app.post('/blurhash', upload.single('image'), async (req, res) => {
const { file } = req;
// If the file is not available we're returning with error.
if (file === null) {
res.status(400).json({ message: 'Image is missing' });
return;
}
// Users can specify number of components in each axes.
const componentX = req.body.componentX ?? 4;
const componentY = req.body.componentY ?? 3;
// We're converting provided image to a byte buffer.
// Sharp currently supports multiple common formats like JPEG, PNG, WebP, GIF, and AVIF.
const { data, info } = await sharp(file.buffer).ensureAlpha().raw().toBuffer({
resolveWithObject: true,
});
const blurhash = encode(
new Uint8ClampedArray(data),
info.width,
info.height,
componentX,
componentY
);
res.json({ blurhash });
});
Additionally, the request can include two parameters: componentX and componentY, are passed through the algorithm. These values can be calculated or hard-coded on the server or specified by the user. However, they must be within the range of 1 to 9 and have an aspect ratio similar to the uploaded image. A value of 9 will give the best results but may take longer to generate the hash.
The process of generating a blurhash can be accomplished in various languages and server technologies, similar to the one using JavaScript. The key step is to locate an encoder for your chosen language, which can often be found in the woltapp/blurhash repository. Once you have the encoder, you will need to obtain a representation of the image. Some libraries use a default image class (for example, the Swift implementation uses UIImage). In other cases, you will have to provide raw byte data. Make sure to check the encoder's documentation to confirm the expected data format.
When working with raw byte data, ensure that the alpha layer is present (each pixel is represented by red, green, blue, and alpha values). Failing to do so will lead to errors such as "width and height must match the pixels array".