React.useContext 显示为未定义

Question

这是我第一次在应用程序中使用 React 上下文挂钩，我的上下文默认值一直显示为“未定义”。

到目前为止的疑难解答：

我已经确定 React.createContext 在一个单独的文件中 (context.js)
我已确保子组件包含在 Provider 中
我正在为 React.createContext()

我的所有代码都可以在下面的这个 CodeSandbox link 中找到：

https://codesandbox.io/s/react-context-troubleshooting-ojnb2?file=/src/child.js

如有任何建议，我将不胜感激！

Answer 1

我已将您的代码更改为以下内容并正常运行。

import React, { useContext } from "react";
import { SelectedBackgroundContext } from "./context";

export default function Child() {
  const  selectedBackground  = useContext(SelectedBackgroundContext);
  // you can comment out line5 above and uncomment line7 below to verify all other code works
  //let selectedBackground = 'https://lp-cms-production.imgix.net/2019-06/81377873%20.jpg?fit=crop&q=40&sharp=10&vib=20&auto=format&ixlib=react-8.6.4'

  const renderSelected = (context) => {
    console.log("Background img src is: " + context); //appears as undefined

    if (context) {
      return (
        <img
          style={{ height: "200px" }}
          src={context}
          key={context + "Thumbnail"}
          alt={"thumbnail of " + context}
        />
      );
    } else {
      return <p>None</p>;
    }
  };

  return (
    <div>
      <p>Background:}</p> {renderSelected(selectedBackground)}
    </div>
  );
}

因为您没有从上下文值传递对象，所以不需要

const  {selectedBackground}  = useContext(SelectedBackgroundContext);

关于解构变量的更多信息 https://www.javascripttutorial.net/es6/javascript-object-destructuring/

Answer 2

在您的 App.js 文件中，您正在传递 value 一个字符串：

import React, { useContext } from "react";
import { SelectedBackgroundContext } from "./context";
import Child from "./child";

function App() {
  const { selectedBackground } = useContext(SelectedBackgroundContext);

  // selectedBackground is just a string, so value = "https://...", which is invalid, the value, in your case, should be an object
  return (
    <SelectedBackgroundContext.Provider value={selectedBackground}>
      <Child />
    </SelectedBackgroundContext.Provider>
  );
}

export default App;

相反，value 需要是一个对象，其中 selectedBackground 属性包含字符串：

import React, { useContext } from "react";
import { SelectedBackgroundContext } from "./context";
import Child from "./child";

function App() {
  const { selectedBackground, selectBackground } = useContext(
    SelectedBackgroundContext
  );
  // alternatively, just collect all context, without destructuring,
  // and pass it to the "value" prop: value={context}
  // const context = useContext(SelectedBackgroundContext);


  // you're also missing the "selectBackground" function, which should be added to this "value" prop
  return (
    <SelectedBackgroundContext.Provider
      value={{ selectedBackground, selectBackground }}
    >
      <Child />
    </SelectedBackgroundContext.Provider>
  );
}

export default App;

由于您使用对象创建了上下文：

{
  selectedBackground:
    "https://lp-cms-production.imgix.net/2019-06/81377873%20.jpg?fit=crop&q=40&sharp=10&vib=20&auto=format&ixlib=react-8.6.4",
  selectBackground: () => {}
}

provider的value属性也应该是对象！

value={{ selectedBackground, selectBackground }}

工作演示：

React.useContext 显示为未定义

React.useContext appearing as undefined

reactjs

react-context

react-hooks

use-context

createcontext