Function sub

packages/react-devtools-shared/src/hook.js:241–244

(event: string, fn: Handler)

  let hasDetectedBadDCE = false;

  function sub(event: string, fn: Handler) {
    hook.on(event, fn);
    return () => hook.off(event, fn);
  }

  function on(event: string, fn: Handler) {
    if (!listeners[event]) {