I have a react web app with a theme toggle on the navigation. I have a ThemeProvider Context
that has logic to auto detects a user's System theme preference and sets it. However, I feel a user should be able to toggle themes back and forth on the website despite their system preference. Here is the ThemeContext.js
file with all the theme logic including the toggle
method.
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)',
];
So when the page loads, show the user's system preferred them but also allow user to toggle themes by clicking a toggle button that fires the toggle
function. In my current code, when toggle
is called, it seems that state changes occur twice and therefore theme remains unchanged. How do I ensure the toggle
method works correctly?
Here is the web app in question
Although Barry's solution is working, note that instead of adding more code, you could achieve the same result by skimming it:
The key is to set the user's preference as initial state and stop checking it in the effect:
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]);
...
The problem is that the whole block of useLayoutEffect
runs every the dark
value changes. So when the user toggles dark
, the prefers...
if statements run and setDark
back to the system preference.
To solve this you'll need to keep track of the user manually toggling the theme and then preventing the prefers...
if statements from running.
In your ThemeProvider
do the following:
const [userPicked, setUserPicked] = useState(false);
toggle
function: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);
};
useLayout
to look like this: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]);
Your toggle component shouldn't have to change.
Update:
Sal's answer is a great alternative. Mine points out the flaw in existing code and how to add to it. This points out how to which your code more effectively.
export function ThemeProvider({ children }) {
const prefersDark = window.matchMedia('(prefers-color-scheme: dark)').matches;
const [dark, setDark] = useState(prefersDark);
useLayoutEffect(() => {
applyTheme();
}, [dark]);
...
}
Why don't use simply useEffect
?
useEffect(() => {
const prefersDark = window.matchMedia(
"(prefers-color-scheme: dark)"
).matches;
if (prefersDark) {
setIsDark(true);
}
}, []);
The reason to access window
from useEffect
: Window is not defined in Next.js React app .
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