Function useSubscription

packages/react-devtools-shared/src/devtools/views/hooks.js:285–344

({
  getCurrentValue,
  subscribe,
}: {
  getCurrentValue: () => Value,
  subscribe: (callback: Function) => () => void,
})

// Copied from https://github.com/facebook/react/pull/15022
export function useSubscription<Value>({
  getCurrentValue,
  subscribe,
}: {
  getCurrentValue: () => Value,
  subscribe: (callback: Function) => () => void,
}): Value {
  const [state, setState] = useState(() => ({
    getCurrentValue,
    subscribe,
    value: getCurrentValue(),
  }));

  if (
    state.getCurrentValue !== getCurrentValue ||
    state.subscribe !== subscribe
  ) {
    setState({
      getCurrentValue,
      subscribe,
      value: getCurrentValue(),
    });
  }

  useEffect(() => {
    let didUnsubscribe = false;

    const checkForUpdates = () => {
      if (didUnsubscribe) {
        return;
      }

      setState(prevState => {
        if (
          prevState.getCurrentValue !== getCurrentValue ||
          prevState.subscribe !== subscribe
        ) {
          return prevState;
        }

        const value = getCurrentValue();
        if (prevState.value === value) {
          return prevState;
        }

        return {...prevState, value};
      });
    };
    const unsubscribe = subscribe(checkForUpdates);

    checkForUpdates();

    return () => {
      didUnsubscribe = true;
      unsubscribe();
    };
  }, [getCurrentValue, subscribe]);