Question

我已经设置了一个基本示例项目，该项目使用Context来存储页面标题，但是当我设置它时，组件不会重新呈现。

主要文件：

Context.js

import React from 'react'

const Context = React.createContext({})

export default Context

AppWrapper.js

import React from 'react'
import App from './App'
import Context from './Context'

function AppWrapper () {
  return (
    <Context.Provider value={{page: {}}}>
      <App />
    </Context.Provider>
  )
}

export default AppWrapper

App.js

import React, { useContext } from 'react';
import Context from './Context';
import Home from './Home';

function App() {
  const { page } = useContext(Context)
  return (
    <>
      <h1>Title: {page.title}</h1>
      <Home />
    </>
  );
}

export default App;

Home.js

import React, { useContext } from 'react'
import Context from './Context'

function Home () {
  const { page } = useContext(Context)
  page.title = 'Home'

  return (
    <p>Hello, World!</p>
  )
}

export default Home

full code

我在做什么错了？

Answer 1

像对待组件一样思考React上下文，如果要更新值并显示它，则需要使用state。在这种情况下，您将在AppWrapper处呈现上下文提供程序，而在该处需要跟踪状态。

import React, {useContext, useState, useCallback, useEffect} from 'react'

const PageContext = React.createContext({})

function Home() {
  const {setPageContext, page} = useContext(PageContext)
  // essentially a componentDidMount
  useEffect(() => {
    if (page.title !== 'Home')
      setPageContext({title: 'Home'})
  }, [setPageContext])
  return <p>Hello, World!</p>
}

function App() {
  const {page} = useContext(PageContext)
  return (
    <>
      <h1>Title: {page.title}</h1>
      <Home />
    </>
  )
}

function AppWrapper() {
  const [state, setState] = useState({page: {}})
  const setPageContext = useCallback(
    newState => {
      setState({page: {...state.page, ...newState}})
    },
    [state, setState],
  )
  const getContextValue = useCallback(
    () => ({setPageContext, ...state}),
    [state, updateState],
  )
  return (
    <PageContext.Provider value={getContextValue()}>
      <App />
    </PageContext.Provider>
  )
}

编辑-更新了链接存储库中的工作解决方案

我重命名了一些东西，使其更具体一些，我不建议通过上下文传递setState，因为这可能会使组件中的局部状态产生混乱和冲突。而且我也省略了答案不需要的代码段，只是我更改的部分

src / AppContext.js

export const updatePageContext = (values = {}) => ({ page: values })
export const updateProductsContext = (values = {}) => ({ products: values })

export const Pages = {
  help: 'Help',
  home: 'Home',
  productsList: 'Products list',
  shoppingCart: 'Cart',
}

const AppContext = React.createContext({})

export default AppContext

src / AppWrapper.js

const getDefaultState = () => {
  // TODO rehydrate from persistent storage (localStorage.getItem(myLastSavedStateKey)) ?
  return {
    page: { title: 'Home' },
    products: {},
  }
}

function AppWrapper() {
  const [state, setState] = useState(getDefaultState())

  // here we only re-create setContext when its dependencies change ([state, setState])
  const setContext = useCallback(
    updates => {
      setState({ ...state, ...updates })
    },
    [state, setState],
  )

  // here context value is just returning an object, but only re-creating the object when its dependencies change ([state, setContext])
  const getContextValue = useCallback(
    () => ({
      ...state,
      setContext,
    }),
    [state, setContext],
  )
  return (
    <Context.Provider value={getContextValue()}>
      ...

src / App.js

...
import AppContext, { updateProductsContext } from './AppContext'

function App() {
  const [openDrawer, setOpenDrawer] = useState(false)
  const classes = useStyles()
  const {
    page: { title },
    setContext,
  } = useContext(Context)

  useEffect(() => {
    fetch(...)
      .then(...)
      .then(items => {
        setContext(updateProductsContext({ items }))
      })
  }, [])

src / components / DocumentMeta.js

这是一个新组件，您可以使用它以声明性的方式更新页面名称，从而减少每个视图中的代码复杂性/冗余性

import React, { useContext, useEffect } from 'react'
import Context, { updatePageContext } from '../Context'

export default function DocumentMeta({ title }) {
  const { page, setContext } = useContext(Context)

  useEffect(() => {
    if (page.title !== title) {
      // TODO use this todo as a marker to also update the actual document title so the browser tab name changes to reflect the current view
      setContext(updatePageContext({ title }))
    }
  }, [title, page, setContext])
  return null
}

aka用法类似于<DocumentMeta title="Whatever Title I Want Here" />

src / pages / Home.js

每个视图现在只需要导入DocumentMeta和Pages“枚举”来更新标题，而不是每次都拉入上下文并手动进行操作。

import { Pages } from '../Context'
import DocumentMeta from '../components/DocumentMeta'

function Home() {
  return (
    <>
      <DocumentMeta title={Pages.home} />
      <h1>WIP</h1>
    </>
  )
}

注意：其他页面需要复制主页的内容

请记住，这不是我在生产环境中要做的事情，我会写一个更通用的助手来将数据写入缓存，从而可以在性能，深度合并等方面做更多的事情。这应该是一个很好的起点。

Answer 2

这是您需要的工作版本。

import React, { useState, useContext, useEffect } from "react";
import "./styles.css";

const Context = React.createContext({});

export default function AppWrapper() {
  // creating a local state
  const [state, setState] = useState({ page: {} });

  return (
    <Context.Provider value={{ state, setState }}> {/* passing state to in provider */}
      <App />
    </Context.Provider>
  );
}

function App() {
  // getting the state from Context
  const { state } = useContext(Context);
  return (
    <>
      <h1>Title: {state.page.title}</h1>
      <Home />
    </>
  );
}

function Home() {
  // getting setter function from Context
  const { setState } = useContext(Context);
  useEffect(() => {
    setState({ page: { title: "Home" } });
  }, [setState]);

  return <p>Hello, World!</p>;
}

进一步了解Hooks API Reference。

反应上下文不更新

2 个答案: