useMutableSource-relay.js

// Somewhere, the Redux store needs to be wrapped in a mutable source object...
const mutableSource = createMutableSource(
  relayEnvironment,
  // Because the state is immutable, it can be used as the "version".
  () => relayEnvironment.getVersion()
);

// It would probably be shared via the Context API...
const MutableSourceContext = createContext(mutableSource);

// Oversimplified example of how Redux could use the mutable source hook:
function useFragment(fragmentSelector) {
  const mutableSource = useContext(MutableSourceContext);

  const getSnapshot = useCallback(
    store => store.lookup(fragmentSelector),
    [fragmentSelector]
  );

  const subscribe = useCallback(
    (store, handleChange) => {
      return store.subscribe(
        fragmentSnapshot, // <------- we need this result from getSnapshot
        () => {
          // The store changed, so let's get an updated snapshot.
          const newSnapshot = getSnapshot(store);

          // Tell React what the snapshot value is after the most recent store update.
          // If it has not changed, React will not schedule any render work.
          handleChange(newSnapshot);
        },
      );
    },
    [getSnapshot]
  );

  return useMutableSource(mutableSource, getSnapshot, subscribe);
}