useTransition polyfill

useTransition.tsx

import { unstable_withSuspenseConfig, useState, useCallback } from 'react'

// based on https://github.com/facebook/relay/commit/1befdc085bceef5c78f8eb8c2117459ce4b9d7c7#diff-c8dcd2c1e7d048e3b69a8538434cbca4

function useSuspenseTransition(config: {|timeoutMs: number|}) {
  const [isPending, setPending] = useState(false);	
  const startTransition = useCallback(	
    (callback: () => void) => {	
      setPending(true);	
      Scheduler.unstable_next(() => {	
        unstable_withSuspenseConfig(() => {	
          setPending(false);	
          callback();	
        }, config);	
      });	
    },	
    [config, setPending],	
  );	
  return [startTransition, isPending];	
}