React useDelayedState Hook

延迟创建一个有状态值，直到满足某个条件。

• 使用 useState() 钩子创建一个包含实际 state 和布尔值 loaded 的有状态值。
• 使用 useEffect() 钩子在 condition 或 loaded 改变时更新有状态值。
• 创建一个名为 updateState 的函数，只有在 loaded 为真时才更新 state 值。

const useDelayedState = (initialState, condition) => {
  const [{ state, loaded }, setState] = React.useState({
    state: null,
    loaded: false,
  });

  React.useEffect(() => {
    if (!loaded && condition) setState({ state: initialState, loaded: true });
  }, [condition, loaded]);

  const updateState = newState => {
    if (!loaded) return;
    setState({ state: newState, loaded });
  };

  return [state, updateState];
};

const App = () => {
  const [branches, setBranches] = React.useState([]);
  const [selectedBranch, setSelectedBranch] = useDelayedState(
    branches[0],
    branches.length
  );

  React.useEffect(() => {
    const handle = setTimeout(() => {
      setBranches(['master', 'staging', 'test', 'dev']);
    }, 2000);
    return () => {
      handle && clearTimeout(handle);
    };
  }, []);

  return (
    <div>
      <p>Selected branch: {selectedBranch}</p>
      <select onChange={e => setSelectedBranch(e.target.value)}>
        {branches.map(branch => (
          <option key={branch} value={branch}>
            {branch}
          </option>
        ))}
      </select>
    </div>
  );
};

ReactDOM.createRoot(document.getElementById('root')).render(
  <App />
);

ReactDOM.createRoot(document.getElementById('root')).render( ); ```