expo/src/takeSnapshotAsync.js

// @flow

import invariant from 'invariant';
import * as React from 'react';
import { NativeModules, findNodeHandle } from 'react-native';

const { RNViewShot } = NativeModules;

type ReactNativeNodeHandle = number;

type SnapshotOptions = {
  width?: number,
  height?: number,
  format?: 'png' | 'jpg' | 'jpeg' | 'webm',
  quality?: number,
  result?: 'file' | 'base64' | 'data-uri',
};

export default async function takeSnapshotAsync(
  node: ReactNativeNodeHandle | React.Component<*>,
  options?: SnapshotOptions
): Promise<string> {
  let handle = typeof node === 'number' ? node : _getNodeHandle(node);
  return RNViewShot.takeSnapshot(handle, options);
}

function _getNodeHandle(component: React.Component<*>): ReactNativeNodeHandle {
  let handle = findNodeHandle(component);
  invariant(
    handle != null,
    `Could not find the React node handle for component to snapshot: %s`,
    component
  );
  return handle;
}