如何使用 useContext 更改 Context 的值？

Question

在 React 16.8+ 中使用 useContext 钩子效果很好。您可以毫无问题地创建组件、使用钩子并利用上下文值。

我不确定的是如何将更改应用到 Context Provider 值。

1) useContext 钩子严格来说是使用上下文值的一种方式吗？

2) 有没有推荐的方法，使用 React 钩子来更新子组件的值，然后触发组件重新渲染使用 useContext 钩子与此上下文的任何组件？

const ThemeContext = React.createContext({
  style: 'light',
  visible: true
});

function Content() {
  const { style, visible } = React.useContext(ThemeContext);

  const handleClick = () => {
    // change the context values to
    // style: 'dark'
    // visible: false
  }

  return (
    <div>
      <p>
        The theme is <em>{style}</em> and state of visibility is 
        <em> {visible.toString()}</em>
      </p>
      <button onClick={handleClick}>Change Theme</button>
    </div>
  )
};

function App() {
  return <Content />
};

const rootElement = document.getElementById('root');
ReactDOM.render(<App />, rootElement);

<div id="root"></div>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.8.2/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.8.2/umd/react-dom.production.min.js"></script>

Answer 1

Hooks FAQ 的How to avoid passing callbacks down? 部分讨论了如何使用钩子更新上下文。

如果使用useContext 的组件在树的上方没有Provider，则传递给createContext 的参数将仅是默认值。相反，您可以创建一个 Provider 来提供 style 和 visibility 以及切换它们的功能。

示例

const { createContext, useContext, useState } = React;

const ThemeContext = createContext(null);

function Content() {
  const { style, visible, toggleStyle, toggleVisible } = useContext(
    ThemeContext
  );

  return (
    <div>
      <p>
        The theme is <em>{style}</em> and state of visibility is
        <em> {visible.toString()}</em>
      </p>
      <button onClick={toggleStyle}>Change Theme</button>
      <button onClick={toggleVisible}>Change Visibility</button>
    </div>
  );
}

function App() {
  const [style, setStyle] = useState("light");
  const [visible, setVisible] = useState(true);

  function toggleStyle() {
    setStyle(style => (style === "light" ? "dark" : "light"));
  }
  function toggleVisible() {
    setVisible(visible => !visible);
  }

  return (
    <ThemeContext.Provider
      value={{ style, visible, toggleStyle, toggleVisible }}
    >
      <Content />
    </ThemeContext.Provider>
  );
}

ReactDOM.render(<App />, document.getElementById("root"));

<script src="https://unpkg.com/react@16/umd/react.development.js"></script>
<script src="https://unpkg.com/react-dom@16/umd/react-dom.development.js"></script>

<div id="root"></div>

Answer 2

您可以使用这种方法，无论您有多少嵌套组件，它都可以正常工作。

// Settings Context - src/context/Settings
import React, { useState } from "react";

const SettingsContext = React.createContext();

const defaultSettings = {
  theme: "light",
};

export const SettingsProvider = ({ children, settings }) => {
  const [currentSettings, setCurrentSettings] = useState(
    settings || defaultSettings
  );

  const saveSettings = (values) => {
   setCurrentSettings(values)
  };

  return (
    <SettingsContext.Provider
      value={{ settings: currentSettings, saveSettings }}
    >
      {children}
    </SettingsContext.Provider>
  );
};

export const SettingsConsumer = SettingsContext.Consumer;

export default SettingsContext;

// Settings Hook - src/hooks/useSettings
import { useContext } from "react";
import SettingsContext from "src/context/SettingsContext";

export default () => {
  const context = useContext(SettingsContext);

  return context;
};

// src/index
ReactDOM.render(
  <SettingsProvider settings={settings}>
    <App />
  </SettingsProvider>,
  document.getElementById("root")
);

// Any component do you want to toggle the theme from
// Example: src/components/Navbar
const { settings, saveSettings } = useSettings();

const handleToggleTheme = () => {
  saveSettings({ theme: "light" });
};