如何在 Next.js 中更改 localStorage 值时更改状态？

Question

How to change state when localStorage value changed. For example, I have a language switching button, like French and English, when I click English, it will be storing to localStorage , when I click English it will also.

When I click the French the whole project need to see in French, also when I click English, want to do like that, it So how can I change state when I update localStorage ?

<button onclick={()=>localStorage.setItem("language",'english')}>English</button>
<button onclick={()=>localStorage.setItem("language",'french')}>French</button>

let language;
if (typeof window !== "undefined") {
  if (localStorage.getItem("language") === null) {
    language = "english";
  }

  if (localStorage.getItem("language") !== null) {
    language = localStorage.getItem("language");
  }
}

const [langu, setLangua] = useState(language);

console.log(langu);

Answer 1

One way to achieve this that wouldn't change that much your current structure is first to change your buttons to this:

<button
  onclick={() => {
    localStorage.setItem("language", "english");
    window.dispatchEvent(new Event("storage"));
  }}
>
  English
</button>
<button
  onclick={() => {
    localStorage.setItem("language", "french");
    window.dispatchEvent(new Event("storage"));
  }}
>
  French
</button>

And then set up inside the component where you have setLangua and langu an useEffect that would listen to changes in the localStorage and update the state:

useEffect(() => {
  const listenStorageChange = () => {
    if (localStorage.getItem("language") === null) {
      setLangua("english");
    } else {
      setLangua(localStorage.getItem("language"));
    }
  };
  window.addEventListener("storage", listenStorageChange);
  return () => window.removeEventListener("storage", listenStorageChange);
}, []);

Answer 2

you need set it in useEffect hook, with empty dependences, it will run only when the component mount.

const [langu,setLangua] = useState(language)
useEffect(() => {
let language = ""
if (typeof window !== 'undefined') {
    if ( localStorage.getItem("language")  === null) {
        language = "english"
    }

    if ( localStorage.getItem("language")  !== null) {
        language = localStorage.getItem("language")
    }
}
setLanguage(language)
}, [])

Answer 3

You can setLangua at the same time as putting it in local storage. Or you can subscribe to local storage changes with the useEffect hook.

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

    const [userLang, setUserLang] = useState('english')

    const getLangFromLocalStorage = useCallback(() => {
        return localStorage.getItem('userLang');
    }, []);

    useEffect(() => {
      function checkUserLang() {
        const value = getLangFromLocalStorage()
    
        // Do with value what you want
    
        if (value) {
          setUserLang(value)
        }
      }
    
      window.addEventListener('storage', checkUserLang)
    
      return () => {
        window.removeEventListener('storage', checkUserLang)
      }
    }, [])

    // Set userLang initially when component did mount

    useEffect(() => {
        const value = getLangFromLocalStorage();
        if (value) {
            setUserLang(value);
        }
    }, []);

Note: This won't work on the same page that is making the changes — it is really a way for other pages on the domain using the storage to sync any changes that are made. Pages on other domains can't access the same storage objects.