Function onMessage

packages/react-devtools-inline/src/backend.js:115–117

({data}: $FlowFixMe)

    wall = {
      listen(fn) {
        const onMessage = ({data}: $FlowFixMe) => {
          fn(data);
        };
        contentWindow.addEventListener('message', onMessage);
        return () => {
          contentWindow.removeEventListener('message', onMessage);