[React] useEffect – problem: dependency is Object

Let’s say we have a simple app look like this:

import { useEffect, useState } from "react";

function useFetch(config) {
  console.log("useFetch call");
  const [data, setData] = useState(null);
  useEffect(() => {
    if (config.url) {
      fetch(config.url)
        .then((response) => response.json())
        .then((json) => {
          setData(json);
        });
    }
  }, [config]);

  return { data };
}

export default function App() {
  const [url, setUrl] = useState(null);
  const { data } = useFetch({ url });
  return (
    <div className="App">
      <div>Hello</div>
      <div>{JSON.stringify(data)}</div>
      <div>
        <button onClick={() => setUrl("/jack.json")}>Jack</button>
        <button onClick={() => setUrl("/jelly.json")}>Sally</button>
      </div>
    </div>
  );
}

It has a useFetchcustome hook, when any button clicked, it will be invoked because urlchanges will trigger `useFetch` re-run.

  const [url, setUrl] = useState(null);
  const { data } = useFetch({ url });

But the problem here is that, it get stuck in infinity loop:

The reason is we passing {url}to useFetch, and inside useFetch, there is useEffect which deps on {url}.

Solution 1: instead passing object, just pass primitve value:

function useFetch(config) {
  console.log("useFetch call");
  const [data, setData] = useState(null);
  useEffect(() => {
    if (config.url) {
      fetch(config.url)
        .then((response) => response.json())
        .then((json) => {
          setData(json);
        });
    }
  }, [config.url]); // using config.url instead of config object
 
  return { data };
}

But what if we have also callback function inside config object?

export default function App() {
  const [url, setUrl] = useState(null);
  const onSuccess = () => console.log("success");
  const { data } = useFetch({ url, callback: onSuccess });
    
    
...

function useFetch(config) {
  console.log("useFetch call");
  const [data, setData] = useState(null);
  useEffect(() => {
    if (config.url) {
      fetch(config.url)
        .then((response) => response.json())
        .then((json) => {
          setData(json);
          config.callback();
        });
    }
  }, [config.url, config.callback]); // add callback as deps

  return { data };
}

Now again, it become crazy.

Solution to the callback problem, we can use useRefto resolve it:

function useFetch(config) {
  console.log("useFetch call");
  const [data, setData] = useState(null);
  const onSuccessRef = useRef(config.callback);
  useEffect(() => {
    if (config.url) {
      fetch(config.url)
        .then((response) => response.json())
        .then((json) => {
          setData(json);
          onSuccessRef.current?.();
        });
    }
  }, [config.url]);

  return { data };
}

This solution doesn’t work if callback changed… so we need to do

import { useEffect, useRef, useState, useLayoutEffect } from "react";

function useFetch(config) {
  const [data, setData] = useState(null);
  const onSuccessRef = useRef(config.callback);
  useLayoutEffect(() => {
    onSuccessRef.current = config.callback
  }, [config.callback])

  useEffect(() => {
    if (config.url) {
      fetch(config.url)
        .then((response) => response.json())
        .then((json) => {
          setData(json);
          onSuccessRef.current?.();
        });
    }
  }, [config.url]);

  return { data };
}

We use useLayoutEffectto keep ref callback up to date.

To improve, we can create a helper function:

import { useEffect, useRef, useState, useLayoutEffect } from "react";

function useCallbackRef(callback) {
  const callbackRef = useRef(callback);
  useLayoutEffect(() => {
    callbackRef.current = callback;
  }, [callback]);
  return callbackRef;
}

function useFetch(config) {
  const [data, setData] = useState(null);
  const savedOnSuccess = useCallbackRef(config.callback).current;
  useEffect(() => {
    if (config.url) {
      fetch(config.url)
        .then((response) => response.json())
        .then((json) => {
          setData(json);
          savedOnSuccess();
        });
    }
  }, [config.url]);

  return { data };
}

Add cancellation to the useEffect:

import { useEffect, useRef, useState, useLayoutEffect } from "react";

function useCallbackRef(callback) {
  const callbackRef = useRef(callback);
  useLayoutEffect(() => {
    callbackRef.current = callback;
  }, [callback]);
  return callbackRef;
}

function useFetch(config) {
  const [data, setData] = useState(null);
  const savedOnSuccess = useCallbackRef(config.callback).current;
  useEffect(() => {
    let isCancelled = false;
    if (config.url) {
      fetch(config.url)
        .then((response) => response.json())
        .then((json) => {
          if (!isCancelled) {
            setData(json);
            savedOnSuccess();
          }
        });
    }

    return () => {
      isCancelled = true;
    };
  }, [config.url]);

  return { data };
}

原文地址：http://www.cnblogs.com/Answer1215/p/16829497.html