当所有依赖项都发生变化时使用效果？

Question

目前useEffect 仅在其中一个依赖项发生更改时被触发。

当两个（或所有）依赖项都发生变化时，我如何更新它/使用它来回火？

Answer 1

当所有依赖项都发生变化时，您需要添加一些逻辑来调用您的效果。这是useEffectAllDepsChange，应该可以实现您想要的行为。

这里的策略是将之前的部门与当前的部门进行比较。如果它们不完全不同，我们将之前的 deps 保存在 ref 中，直到它们不同时才更新它。这允许您在调用效果之前多次更改 deps。

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

// taken from https://usehooks.com/usePrevious/
function usePrevious(value) {
  const ref = useRef();

  useEffect(() => {
    ref.current = value;
  }, [value]);
  
  return ref.current;
}

function useEffectAllDepsChange(fn, deps) {
  const prevDeps = usePrevious(deps);
  const changeTarget = useRef();

  useEffect(() => {
    // nothing to compare to yet
    if (changeTarget.current === undefined) {
      changeTarget.current = prevDeps;
    }

    // we're mounting, so call the callback
    if (changeTarget.current === undefined) {
      return fn();
    }

    // make sure every dependency has changed
    if (changeTarget.current.every((dep, i) => dep !== deps[i])) {
      changeTarget.current = deps;

      return fn();
    }
  }, [fn, prevDeps, deps]);
}

export default function App() {
  const [a, setA] = useState(0);
  const [b, setB] = useState(0);

  useEffectAllDepsChange(() => {
    console.log("running effect", [a, b]);
  }, [a, b]);

  return (
    <div>
      <button onClick={() => setA((prev) => prev + 1)}>A: {a}</button>
      <button onClick={() => setB((prev) => prev + 1)}>B: {b}</button>
    </div>
  );
}

受 Richard 启发的另一种方法更简洁，但缺点是跨更新渲染更多。

function useEffectAllDepsChange(fn, deps) {
  const [changeTarget, setChangeTarget] = useState(deps);

  useEffect(() => {
    setChangeTarget(prev => {
      if (prev.every((dep, i) => dep !== deps[i])) {
        return deps;
      }

      return prev;
    });
  }, [deps]);

  useEffect(fn, changeTarget);
}

Answer 2

我喜欢@AustinBrunkhorst 的灵魂，但你可以用更少的代码来做到这一点。

使用仅在满足您的条件时更新的状态对象，并将其设置在第二个 useEffect 中。

import React, { useEffect, useState } from "react";
import "./styles.css";

export default function App() {
  const [a, setA] = useState(0);
  const [b, setB] = useState(0);
  const [ab, setAB] = useState({a, b});

  useEffect(() => {
    setAB(prev => {
      console.log('prev AB', prev)
      return (a !== prev.a && b !== prev.b) 
        ? {a,b} 
        : prev;  // do nothing
    })
  }, [a, b])

  useEffect(() => {
    console.log('both have changed')
  }, [ab])

  return (
    <div className="App">
      <div>Click on a button to increment its value.</div>
      <button onClick={() => setA((prev) => prev + 1)}>A: {a}</button>
      <button onClick={() => setB((prev) => prev + 1)}>B: {b}</button>
    </div>
  );
}

Answer 3

为了演示如何以各种方式编写钩子，这是我的方法。这不会在初始属性中调用效果。

import React, { useEffect, useRef, useState } from "react";
import "./styles.css";

function usePrevious(state) {
  const ref = useRef();

  useEffect(() => {
    ref.current = state;
  });

  return ref.current;
}

function useAllChanged(callback, array) {
  const previousArray = usePrevious(array);

  console.log("useAllChanged", array, previousArray);

  if (previousArray === undefined) return;

  const allChanged = array.every((state, index) => {
    const previous = previousArray[index];
    return previous !== state;
  });

  if (allChanged) {
    callback(array, previousArray);
  }
}

const randomIncrement = () => Math.floor(Math.random() * 4);

export default function App() {
  const [state1, setState1] = useState(0);
  const [state2, setState2] = useState(0);
  const [state3, setState3] = useState(0);

  useAllChanged(
    (state, prev) => {
      alert("Everything changed!");
      console.info(state, prev);
    },
    [state1, state2, state3]
  );

  const onClick = () => {
    console.info("onClick");
    setState1(state => state + randomIncrement());
    setState2(state => state + randomIncrement());
    setState3(state => state + randomIncrement());
  };

  return (
    <div className="App">
      <p>State 1: {state1}</p>
      <p>State 2: {state2}</p>
      <p>State 3: {state3}</p>

      <button onClick={onClick}>Randomly increment</button>
    </div>
  );
}

Answer 4

您必须跟踪依赖项的先前值，并检查是否只有其中一个发生了变化，或者两者/全部发生了变化。基本实现可能如下所示：

import React from "react";

const usePrev = value => {
  const ref = React.useRef();

  React.useEffect(() => {
    ref.current = value;
  }, [value]);

  return ref.current;
};

const App = () => {
  const [foo, setFoo] = React.useState(0);
  const [bar, setBar] = React.useState(0);
  const prevFoo = usePrev(foo);
  const prevBar = usePrev(bar);

  React.useEffect(() => {
    if (prevFoo !== foo && prevBar !== bar) {
      console.log("both foo and bar changed!");
    }
  }, [prevFoo, prevBar, foo, bar]);

  return (
    <div className="App">
      <h2>foo: {foo}</h2>
      <h2>bar: {bar}</h2>
      <button onClick={() => setFoo(v => v + 1)}>Increment foo</button>
      <button onClick={() => setBar(v => v + 1)}>Increment bar</button>
      <button
        onClick={() => {
          setFoo(v => v + 1);
          setBar(v => v + 1);
        }}
      >
        Increment both
      </button>
    </div>
  );
};

export default App;

这里还有一个CodeSandbox link可以玩。

您可以检查usePrev 挂钩在其他地方的工作方式，例如here。