如何在反应应用程序中设置系统偏好暗模式，但还允许用户来回切换当前主题

Question

我有一个在导航上带有主题切换的 React Web 应用程序。我有一个ThemeProvider Context，它具有自动检测用户的系统主题偏好并设置它的逻辑。但是，我觉得用户应该能够在网站上来回切换主题，尽管他们有系统偏好。这是包含所有主题逻辑的 ThemeContext.js 文件，包括 toggle 方法。

import React, { useState, useLayoutEffect } from 'react';

const ThemeContext = React.createContext({
    dark: false,
    toggle: () => {},
});

export default ThemeContext;

export function ThemeProvider({ children }) {
    // keeps state of the current theme
    const [dark, setDark] = useState(false);

    const prefersDark = window.matchMedia('(prefers-color-scheme: dark)')
        .matches;
    const prefersLight = window.matchMedia('(prefers-color-scheme: light)')
        .matches;
    const prefersNotSet = window.matchMedia(
        '(prefers-color-scheme: no-preference)'
    ).matches;

    // paints the app before it renders elements
    useLayoutEffect(() => {
        // Media Hook to check what theme user prefers
        if (prefersDark) {
            setDark(true);
        }

        if (prefersLight) {
            setDark(false);
        }

        if (prefersNotSet) {
            setDark(true);
        }

        applyTheme();

        // if state changes, repaints the app
        // eslint-disable-next-line react-hooks/exhaustive-deps
    }, [dark]);

    // rewrites set of css variablels/colors
    const applyTheme = () => {
        let theme;
        if (dark) {
            theme = darkTheme;
        }
        if (!dark) {
            theme = lightTheme;
        }

        const root = document.getElementsByTagName('html')[0];
        root.style.cssText = theme.join(';');
    };

    const toggle = () => {
        console.log('Toggle Method Called');

        // A smooth transition on theme switch
        const body = document.getElementsByTagName('body')[0];
        body.style.cssText = 'transition: background .5s ease';

        setDark(!dark);
    };

    return (
        <ThemeContext.Provider
            value={{
                dark,
                toggle,
            }}>
            {children}
        </ThemeContext.Provider>
    );
}

// styles
const lightTheme = [
    '--bg-color: var(--color-white)',
    '--text-color-primary: var(--color-black)',
    '--text-color-secondary: var(--color-prussianBlue)',
    '--text-color-tertiary:var(--color-azureRadiance)',
    '--fill-switch: var(--color-prussianBlue)',
    '--fill-primary:var(--color-prussianBlue)',
];

const darkTheme = [
    '--bg-color: var(--color-mirage)',
    '--text-color-primary: var(--color-white)',
    '--text-color-secondary: var(--color-iron)',
    '--text-color-tertiary: var(--color-white)',
    '--fill-switch: var(--color-gold)',
    '--fill-primary:var(--color-white)',
];

因此，当页面加载时，显示用户的系统首选它们，但也允许用户通过单击触发toggle 功能的切换按钮来切换主题。在我当前的代码中，当调用toggle 时，状态更改似乎发生了两次，因此主题保持不变。如何确保toggle 方法正常工作？

这是有问题的web app

Answer 1

虽然 Barry 的解决方案有效，但请注意，您可以通过略读获得相同的结果，而不是添加更多代码：

关键是将用户的偏好设置为初始状态，并在效果中停止检查：

export function ThemeProvider({ children }) {
    /* Because you are setting the initial theme to non-dark, 
    you can assume that your initial state should be dark only 
    when the user's preference is set to dark. */
    const prefersDark = window.matchMedia('(prefers-color-scheme: dark)')
        .matches;

    // True if preference is set to dark, false otherwise.
    const [dark, setDark] = useState(prefersDark);
    /* Note: Initial state is set upon mounting, hence is better 
    to put the <ThemeProvider> up in your tree, close to the root <App> 
    to avoid unmounting it with the result of reverting to the default user 
    preference when and if re-mounting (unless you want that behaviour) */

    useLayoutEffect(() => {
        /* You end up here only when the user takes action 
        to change the theme, hence you can just apply the new theme. */
        applyTheme();
}, [dark]);
...

CodeSandbox example

Answer 2

问题是useLayoutEffect 的整个块在每次dark 值更改时运行。因此，当用户切换dark 时，prefers... if 语句运行，setDark 回到系统偏好设置。

要解决此问题，您需要跟踪用户手动切换主题，然后阻止 prefers... if 语句运行。

在您的ThemeProvider 中执行以下操作：

• 添加状态以监控用户是否使用了切换

const [userPicked, setUserPicked] = useState(false);

• 更新您的toggle 函数：

const toggle = () => {
  console.log('Toggle Method Called');

  const body = document.getElementsByTagName('body')[0];
  body.style.cssText = 'transition: background .5s ease';

  setUserPick(true) // Add this line
  setDark(!dark);
};

• 最后，将useLayout 更新为如下所示：

useLayoutEffect(() => {
  if (!userPicked) { // This will stop the system preferences from taking place if the user manually toggles the them
    if (prefersDark) {
      setDark(true);
    }

    if (prefersLight) {
      setDark(false);
    }

    if (prefersNotSet) {
      setDark(true);
    }
  }

  applyTheme();
}, [dark]);

您的切换组件不必更改。

更新：

Sal 的回答是一个很好的选择。我指出了现有代码中的缺陷以及如何添加它。这指出了如何更有效地执行您的代码。

export function ThemeProvider({ children }) {
  const prefersDark = window.matchMedia('(prefers-color-scheme: dark)').matches;

  const [dark, setDark] = useState(prefersDark);

  useLayoutEffect(() => {
    applyTheme();
  }, [dark]);

  ...

}

Answer 3

为什么不直接使用useEffect？

useEffect(() => {
  const prefersDark = window.matchMedia(
    "(prefers-color-scheme: dark)"
  ).matches;

  if (prefersDark) {
    setIsDark(true);
  }
}, []);

从useEffect访问window的原因：Window is not defined in Next.js React app。