# react-native-view-shot  
Capture a React Native view to an image.
## Install
```bash
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](https://facebook.github.io/react-native/docs/linking-libraries-ios.html)).
## Recommended High Level API
```js
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 (
...Something to rasterize...
);
}
}
// alternative
class ExampleCaptureOnMountSimpler extends Component {
onCapture = uri => {
console.log("do something with ", uri);
}
render() {
return (
...Something to rasterize...
);
}
}
// waiting an image
class ExampleWaitingCapture extends Component {
onImageLoad = () => {
this.refs.viewShot.capture().then(uri => {
console.log("do something with ", uri);
})
};
render() {
return (
...Something to rasterize...
);
}
}
// capture ScrollView content
class ExampleCaptureScrollViewContent extends Component {
onCapture = uri => {
console.log("do something with ", uri);
}
render() {
return (
...The Scroll View Content Goes Here...
);
}
}
```
**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
```js
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](https://en.wikipedia.org/wiki/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
```js
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](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 `` 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?
```js
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:
> ```js
> yarn add pngjs
> yarn add zlib
> ```
## Troubleshooting / FAQ
### Saving to a file?
- If you want to save the snapshotted image result to the CameraRoll, just use https://facebook.github.io/react-native/docs/cameraroll.html#savetocameraroll
- If you want to save it to an arbitrary file path, use something like https://github.com/itinance/react-native-fs
- For any more advanced needs, you can write your own (or find another) native module that would solve your use-case.
### 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 `` to actually make them snapshotable! Otherwise that view won't reflect any UI View. ([found by @gaguirre](https://github.com/gre/react-native-view-shot/issues/7#issuecomment-245302844))
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
- To initial iOS work done by @jsierles in https://github.com/jsierles/react-native-view-snapshot
- To React Native implementation of takeSnapshot in iOS by @nicklockwood
- To Windows implementation by @ryanlntn