使用 react-hooks 更新状态时执行异步代码

Question

我有类似的东西：

const [loading, setLoading] = useState(false);

...

setLoading(true);
doSomething(); // <--- when here, loading is still false. 

设置状态仍然是异步的，那么等待setLoading() 调用完成的最佳方法是什么？

setLoading() 似乎不像以前的 setState() 那样接受回调。

一个例子

基于类

getNextPage = () => {
    // This will scroll back to the top, and also trigger the prefetch for the next page on the way up.
    goToTop();

    if (this.state.pagesSeen.includes(this.state.page + 1)) {
      return this.setState({
        page: this.state.page + 1,
      });
    }

    if (this.state.prefetchedOrders) {
      const allOrders = this.state.orders.concat(this.state.prefetchedOrders);
      return this.setState({
        orders: allOrders,
        page: this.state.page + 1,
        pagesSeen: [...this.state.pagesSeen, this.state.page + 1],
        prefetchedOrders: null,
      });
    }

    this.setState(
      {
        isLoading: true,
      },
      () => {
        getOrders({
          page: this.state.page + 1,
          query: this.state.query,
          held: this.state.holdMode,
          statuses: filterMap[this.state.filterBy],
        })
          .then((o) => {
            const { orders } = o.data;
            const allOrders = this.state.orders.concat(orders);
            this.setState({
              orders: allOrders,
              isLoading: false,
              page: this.state.page + 1,
              pagesSeen: [...this.state.pagesSeen, this.state.page + 1],
              // Just in case we're in the middle of a prefetch.
              prefetchedOrders: null,
            });
          })
          .catch(e => console.error(e.message));
      },
    );
  };

转换为基于函数的

  const getNextPage = () => {
    // This will scroll back to the top, and also trigger the prefetch for the next page on the way up.
    goToTop();

    if (pagesSeen.includes(page + 1)) {
      return setPage(page + 1);
    }

    if (prefetchedOrders) {
      const allOrders = orders.concat(prefetchedOrders);
      setOrders(allOrders);
      setPage(page + 1);
      setPagesSeen([...pagesSeen, page + 1]);
      setPrefetchedOrders(null);
      return;
    }

    setIsLoading(true);

    getOrders({
      page: page + 1,
      query: localQuery,
      held: localHoldMode,
      statuses: filterMap[filterBy],
    })
      .then((o) => {
        const { orders: fetchedOrders } = o.data;
        const allOrders = orders.concat(fetchedOrders);

        setOrders(allOrders);
        setPage(page + 1);
        setPagesSeen([...pagesSeen, page + 1]);
        setPrefetchedOrders(null);
        setIsLoading(false);
      })
      .catch(e => console.error(e.message));
  };

在上面，我们希望按顺序运行每个 setWhatever 调用。这是否意味着我们需要设置许多不同的 useEffect 挂钩来复制这种行为？

Answer 1

useState setter 不会像在 React 类组件中的 setState 那样在状态更新完成后提供回调。为了复制相同的行为，您可以在 React 类组件中使用类似的模式，如 componentDidUpdate 生命周期方法和 useEffect using Hooks

useEffect hooks 将第二个参数作为一个值数组，React 需要在渲染周期完成后监控这些值的变化。

const [loading, setLoading] = useState(false);

...

useEffect(() => {
    doSomething(); // This is be executed when `loading` state changes
}, [loading])
setLoading(true);

编辑

与setState 不同，useState 挂钩的更新程序没有回调，但您始终可以使用useEffect 来复制上述行为。但是你需要确定加载变化

代码的函数式方法如下所示

function usePrevious(value) {
  const ref = useRef();
  useEffect(() => {
    ref.current = value;
  });
  return ref.current;
}

---

const prevLoading = usePrevious(isLoading);

useEffect(() => {
   if (!prevLoading && isLoading) {
       getOrders({
          page: page + 1,
          query: localQuery,
          held: localHoldMode,
          statuses: filterMap[filterBy],
      })
      .then((o) => {
        const { orders: fetchedOrders } = o.data;
        const allOrders = orders.concat(fetchedOrders);

        setOrders(allOrders);
        setPage(page + 1);
        setPagesSeen([...pagesSeen, page + 1]);
        setPrefetchedOrders(null);
        setIsLoading(false);
      })
      .catch(e => console.error(e.message));
   }
}, [isLoading, preFetchedOrders, orders, page, pagesSeen]);

const getNextPage = () => {
    // This will scroll back to the top, and also trigger the prefetch for the next page on the way up.
    goToTop();

    if (pagesSeen.includes(page + 1)) {
      return setPage(page + 1);
    }

    if (prefetchedOrders) {
      const allOrders = orders.concat(prefetchedOrders);
      setOrders(allOrders);
      setPage(page + 1);
      setPagesSeen([...pagesSeen, page + 1]);
      setPrefetchedOrders(null);
      return;
    }

    setIsLoading(true);
  };

Answer 2

等到您的组件重新渲染。

const [loading, setLoading] = useState(false);

useEffect(() => {
    if (loading) {
        doSomething();
    }
}, [loading]);

setLoading(true);

您可以通过以下方式提高清晰度：

function doSomething() {
  // your side effects
  // return () => {  }
}

function useEffectIf(condition, fn) {
  useEffect(() => condition && fn(), [condition])
}

function App() {
  const [loading, setLoading] = useState(false);
  useEffectIf(loading, doSomething)

  return (
    <>
      <div>{loading}</div>
      <button onClick={() => setLoading(true)}>Click Me</button>
    </>
  );
}

Answer 3

创建了一个自定义的useState 钩子，它的工作原理类似于普通的useState 钩子，只是这个自定义钩子的状态更新器函数需要一个回调，该回调将在状态更新和组件重新渲染后执行。

Typescript 解决方案

import { useEffect, useRef, useState } from 'react';

type OnUpdateCallback<T> = (s: T) => void;
type SetStateUpdaterCallback<T> = (s: T) => T;
type SetStateAction<T> = (newState: T | SetStateUpdaterCallback<T>, callback?: OnUpdateCallback<T>) => void;

export function useCustomState<T>(init: T): [T, SetStateAction<T>];
export function useCustomState<T = undefined>(init?: T): [T | undefined, SetStateAction<T | undefined>];
export function useCustomState<T>(init: T): [T, SetStateAction<T>] {
    const [state, setState] = useState<T>(init);
    const cbRef = useRef<OnUpdateCallback<T>>();

    const setCustomState: SetStateAction<T> = (newState, callback?): void => {
        cbRef.current = callback;
        setState(newState);
    };

    useEffect(() => {
        if (cbRef.current) {
            cbRef.current(state);
        }
        cbRef.current = undefined;
    }, [state]);

    return [state, setCustomState];
}

Javascript 解决方案

import { useEffect, useRef, useState } from 'react';

export function useCustomState(init) {
    const [state, setState] = useState(init);
    const cbRef = useRef();

    const setCustomState = (newState, callback) => {
        cbRef.current = callback;
        setState(newState);
    };

    useEffect(() => {
        if (cbRef.current) {
            cbRef.current(state);
        }
        cbRef.current = undefined;
    }, [state]);

    return [state, setCustomState];
}

用法

const [state, setState] = useCustomState(myInitialValue);
...
setState(myNewValueOrStateUpdaterCallback, () => {
   // Function called after state update and component rerender
})

Answer 4

我对此有一个建议。

您可以使用 React Ref 来存储状态变量的状态。然后使用 react ref 更新状态变量。这将渲染页面刷新，然后在异步函数中使用 React Ref。

const stateRef = React.useRef().current
const [state,setState] = useState(stateRef);

async function some() {
  stateRef = { some: 'value' }
  setState(stateRef) // Triggers re-render
  
  await some2();
}

async function some2() {
  await someHTTPFunctionCall(stateRef.some)
  stateRef = null;
  setState(stateRef) // Triggers re-render
}