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react-native-view-shot

Capture a React Native view to an image.

Install

yarn add react-native-view-shot
react-native link react-native-view-shot

Make sure react-native-view-shot is correctly linked in XCode (might require a manual installation, refer to React Native doc).

Recommended High Level API

import ViewShot from "react-native-view-shot";

class ExampleCaptureOnMountManually extends Component {
  componentDidMount () {
    this.refs.viewShot.capture().then(uri => {
      console.log("do something with ", uri);
    });
  }
  render() {
    return (
      <ViewShot ref="viewShot" options={{ format: "jpg", quality: 0.9 }}>
        <Text>...Something to rasterize...</Text>
      </ViewShot>
    );
  }
}

// alternative
class ExampleCaptureOnMountSimpler extends Component {
  onCapture = uri => {
    console.log("do something with ", uri);
  }
  render() {
    return (
      <ViewShot onCapture={this.onCapture} captureMode="mount">
        <Text>...Something to rasterize...</Text>
      </ViewShot>
    );
  }
}

// waiting an image
class ExampleWaitingCapture extends Component {
  onImageLoad = () => {
    this.refs.viewShot.capture().then(uri => {
      console.log("do something with ", uri);
    })
  };
  render() {
    return (
      <ViewShot ref="viewShot">
        <Text>...Something to rasterize...</Text>
        <Image ... onLoad={this.onImageLoad} />
      </ViewShot>
    );
  }
}

// capture ScrollView content
class ExampleCaptureScrollViewContent extends Component {
  onCapture = uri => {
    console.log("do something with ", uri);
  }
  render() {
    return (
      <ScrollView>
        <ViewShot onCapture={this.onCapture} captureMode="mount">
          <Text>...The Scroll View Content Goes Here...</Text>
        </ViewShot>
      </ScrollView>
    );
  }
}

Props:

  • children: the actual content to rasterize.
  • options: the same options as in captureRef method.
  • captureMode (string):
    • if not defined (default). the capture is not automatic and you need to use the ref and call capture() yourself.
    • "mount". Capture the view once at mount. (It is important to understand image loading won't be waited, in such case you want to use "none" with viewShotRef.capture() after Image#onLoad.)
    • "continuous" EXPERIMENTAL, this will capture A LOT of images continuously. For very specific use-cases.
    • "update" EXPERIMENTAL, this will capture images each time React redraw (on did update). For very specific use-cases.
  • onCapture: when a captureMode is defined, this callback will be called with the capture result.
  • onCaptureFailure: when a captureMode is defined, this callback will be called when a capture fails.

captureRef(view, options) lower level imperative API

import { captureRef } from "react-native-view-shot";

captureRef(viewRef, {
  format: "jpg",
  quality: 0.8
})
.then(
  uri => console.log("Image saved to", uri),
  error => console.error("Oops, snapshot failed", error)
);

Returns a Promise of the image URI.

  • view is a reference to a React Native component.
  • options may include:
    • width / height (number): the width and height of the final image (resized from the View bound. don't provide it if you want the original pixel size).
    • format (string): either png or jpg or webm (Android). Defaults to png.
    • quality (number): the quality. 0.0 - 1.0 (default). (only available on lossy formats like jpg)
    • result (string), the method you want to use to save the snapshot, one of:
      • "tmpfile" (default): save to a temporary file (that will only exist for as long as the app is running).
      • "base64": encode as base64 and returns the raw string. Use only with small images as this may result of lags (the string is sent over the bridge). N.B. This is not a data uri, use data-uri instead.
      • "data-uri": same as base64 but also includes the Data URI scheme header.
    • snapshotContentContainer (bool): if true and when view is a ScrollView, the "content container" height will be evaluated instead of the container height.

releaseCapture(uri)

This method release a previously captured uri. For tmpfile it will clean them out, for other result types it just won't do anything.

NB: the tmpfile captures are automatically cleaned out after the app closes, so you might not have to worry about this unless advanced usecases. The ViewShot component will use it each time you capture more than once (useful for continuous capture to not leak files).

captureScreen() Android and iOS Only

import { captureScreen } from "react-native-view-shot";

captureScreen({
  format: "jpg",
  quality: 0.8
})
.then(
  uri => console.log("Image saved to", uri),
  error => console.error("Oops, snapshot failed", error)
);

This method will capture the contents of the currently displayed screen as a native hardware screenshot. It does not require a ref input, as it does not work at the view level. This means that ScrollViews will not be captured in their entirety - only the portions currently visible to the user.

Returns a Promise of the image URI.

  • options: the same options as in captureRef method.

Advanced Examples

Checkout react-native-view-shot-example

Interoperability Table

Snapshots are not guaranteed to be pixel perfect. It also depends on the platform. Here is some difference we have noticed and how to workaround.

Model tested: iPhone 6 (iOS), Nexus 5 (Android).

System iOS Android Windows
View,Text,Image,.. YES YES YES
WebView YES YES1 YES
gl-react v2 YES NO2 NO3
react-native-video NO NO NO
react-native-maps YES NO4 NO3
react-native-svg YES YES maybe?
react-native-camera   NO               YES               NO 3       
  1. Only supported by wrapping a <View collapsable={false}> parent and snapshotting it.
  2. It returns an empty image (not a failure Promise).
  3. Component itself lacks platform support.
  4. But you can just use the react-native-maps snapshot function: https://github.com/airbnb/react-native-maps#take-snapshot-of-map

Performance Optimization

During profiling captured several things that influence on performance:

  1. (de-)allocation of memory for bitmap
  2. (de-)allocation of memory for Base64 output buffer
  3. compression of bitmap to different image formats: PNG, JPG

To solve that in code introduced several new approaches:

  • reusable images, that reduce load on GC;
  • reusable arrays/buffers that also reduce load on GC;
  • RAW image format for avoiding expensive compression;
  • ZIP deflate compression for RAW data, that works faster in compare to Bitmap.compress

more details and code snippet are below.

RAW Images

Introduced a new image format RAW. it correspond a ARGB array of pixels.

Advantages:

  • no compression, so its supper quick. Screenshot taking is less than 16ms;

RAW format supported for zip-base64, base64 and tmpfile result types.

RAW file on disk saved in format: ${width}:${height}|${base64} string.

zip-base64

In compare to BASE64 result string this format fast try to apply zip/deflate compression on screenshot results and only after that convert results to base64 string. In combination zip-base64 + raw we got a super fast approach for capturing screen views and deliver them to the react side.

How to work with zip-base64 and RAW format?

const fs = require('fs')
const zlib = require('zlib')
const PNG = require('pngjs').PNG
const Buffer = require('buffer').Buffer

const format = Platform.OS === 'android' ? 'raw' : 'png'
const result = Platform.OS === 'android' ? 'zip-base64' : 'base64'

captureRef(this.ref, { result, format }).then(data => {
    // expected pattern 'width:height|', example: '1080:1731|'
    const resolution = /^(\d+):(\d+)\|/g.exec(data)
    const width = (resolution || ['', 0, 0])[1]
    const height = (resolution || ['', 0, 0])[2]
    const base64 = data.substr((resolution || [''])[0].length || 0)

    // convert from base64 to Buffer
    const buffer = Buffer.from(base64, 'base64')
    // un-compress data
    const inflated = zlib.inflateSync(buffer)
    // compose PNG
    const png = new PNG({ width, height })
    png.data = inflated
    const pngData = PNG.sync.write(png)
    // save composed PNG
    fs.writeFileSync(output, pngData)
})

Keep in mind that packaging PNG data is a CPU consuming operation as a zlib.inflate.

Hint: use process.fork() approach for converting raw data into PNGs.

Note: code is tested in large commercial project.

Note #2: Don't forget to add packages into your project:

yarn add pngjs
yarn add zlib

Troubleshooting / FAQ

Saving to a file?

The snapshot is rejected with an error?

  • Support of special components like Video / GL views is not guaranteed to work. In case of failure, the captureRef promise gets rejected (the library won't crash).

get a black or blank result or still have an error with simple views?

Check the Interoperability Table above. Some special components are unfortunately not supported. If you have a View that contains one of an unsupported component, the whole snapshot might be compromised as well.

black background instead of transparency / weird border appear around texts?

  • It's preferable to use a background color on the view you rasterize to avoid transparent pixels and potential weirdness that some border appear around texts.

on Android, getting "Trying to resolve view with tag '{tagID}' which doesn't exist"

you need to make sure collapsable is set to false if you want to snapshot a View. Some content might even need to be wrapped into such <View collapsable={false}> to actually make them snapshotable! Otherwise that view won't reflect any UI View. (found by @gaguirre)

Alternatively, you can use the ViewShot component that will have collapsable={false} set to solve this problem.

Getting "The content size must not be zero or negative."

Make sure you don't snapshot instantly, you need to wait at least there is a first onLayout event, or after a timeout, otherwise the View might not be ready yet. (It should also be safe to just wait Image onLoad if you have one). If you still have the problem, make sure your view actually have a width and height > 0.

Alternatively, you can use the ViewShot component that will wait the first onLayout.

Snapshotted image does not match my width and height but is twice/3-times bigger

This is because the snapshot image result is in real pixel size where the width/height defined in a React Native style are defined in "point" unit. You might want to set width and height option to force a resize. (might affect image quality)

Thanks