如何在React Hooks useEffect上比较oldValues和newValues？

Question

假设我有3个输入：rate，sendAmount和receiveAmount。我把那3个输入放在useEffect diffing params上。规则是：

• 如果sendAmount更改，我将计算receiveAmount = sendAmount * rate
• 如果receiveAmount更改，我将计算sendAmount = receiveAmount / rate
• 如果费率发生变化，我将在receiveAmount = sendAmount * rate时计算sendAmount > 0或在sendAmount = receiveAmount / rate时计算receiveAmount > 0

这是代码框https://codesandbox.io/s/pkl6vn7x6j来说明问题。

有没有办法像oldValues那样比较newValues和componentDidUpdate，而不是为此情况制作3个处理程序？

谢谢

---

这是我使用usePrevious的最终解决方案 https://codesandbox.io/s/30n01w2r06

在这种情况下，我不能使用多个useEffect，因为每次更改都会导致相同的网络呼叫。这就是为什么我也使用changeCount来跟踪更改的原因。 changeCount还可帮助仅跟踪来自本地的更改，因此我可以防止由于服务器的更改而导致不必要的网络呼叫。

Answer 1

您可以编写一个自定义钩子来为您提供previous props using useRef

function usePrevious(value) {
  const ref = useRef();
  useEffect(() => {
    ref.current = value;
  });
  return ref.current;
}

然后在useEffect

中使用它

const Component = (props) => {
    const {receiveAmount, sendAmount } = props
    const prevAmount = usePrevious({receiveAmount, sendAmount});
    useEffect(() => {
        if(prevAmount.receiveAmount !== receiveAmount) {

         // process here
        }
        if(prevAmount.sendAmount !== sendAmount) {

         // process here
        }
    }, [receiveAmount, sendAmount])
}

但是，如果您要为每个更改ID分别使用两个useEffect，则要更清晰地阅读和理解，您想分别处理它们

Answer 2

如果有人正在寻找使用的TypeScript版本上一个：

import { useEffect, useRef } from "react";

const usePrevious = <T extends {}>(value: T) => {
  const ref = useRef<T>();
  useEffect(() => {
    ref.current = value;
  });
  return ref.current;
};

Answer 3

摆脱已接受的答案，这是不需要自定义钩子的替代解决方案：

const Component = ({ receiveAmount, sendAmount }) => {
  const prevAmount = useRef({ receiveAmount, sendAmount }).current;
  useEffect(() => {
    if (prevAmount.receiveAmount !== receiveAmount) {
     // process here
    }
    if (prevAmount.sendAmount !== sendAmount) {
     // process here
    }
    return () => { 
      prevAmount.receiveAmount = receiveAmount;
      prevAmount.sendAmount = sendAmount;
    };
  }, [receiveAmount, sendAmount]);
};

这假定您实际上需要引用“此处处理”位中任何内容的先前值。否则，除非您的条件不直接进行!==比较，否则这里最简单的解决方案是：

const Component = ({ receiveAmount, sendAmount }) => {
  useEffect(() => {
     // process here
  }, [receiveAmount]);

  useEffect(() => {
     // process here
  }, [sendAmount]);
};

Answer 4

选项1-useCompare挂钩

将当前值与上一个值进行比较是一种常见的模式，它证明了自定义钩子本身是可以隐藏实现细节的。

const useCompare = (val: any) => {
    const prevVal = usePrevious(val)
    return prevVal !== val
}

const usePrevious = (value) {
    const ref = useRef();
    useEffect(() => {
      ref.current = value;
    });
    return ref.current;
}

const Component = (props) => {
  ...
  const hasVal1Changed = useCompare(val1)
  const hasVal2Changed = useCompare(val2);
  useEffect(() => {
    if (hasVal1Changed ) {
      console.log("val1 has changed");
    }
    if (hasVal2Changed ) {
      console.log("val2 has changed");
    }
  });

  return <div>...</div>;
};

Demo

选项2-值更改时运行useEffect

const Component = (props) => {
  ...
  useEffect(() => {
    console.log("val1 has changed");
  }, [val1]);
  useEffect(() => {
    console.log("val2 has changed");
  }, [val2]);

  return <div>...</div>;
};

Demo

Answer 5

对于真正简单的道具比较，您可以使用useEffect和useState轻松检查道具是否已更新。

const myComponent = ({ prop }) => {
  const [propHasChanged, setPropHasChanged] = useState(false)
  useEffect(() => {
    setHasPropChanged(true)
  }, [prop])

  if(propHasChanged){
    ~ do stuff here ~
  }
}

useEffect检查该道具是否已更改，更新状态以说它已经拥有，并允许您的条件代码运行。

Answer 6

使用Ref会在应用中引入一种新的错误。

让我们看看使用$invoice_TaxTotal_SubTotal_Category = new \Bulut\eFaturaUBL\TaxCategory(); $invoice_TaxTotal_SubTotal_Category->TaxExemptionReasonCode = "301"; $invoice_TaxTotal_SubTotal_Category->TaxExemptionReason = "description"; $invoice_TaxTotal_SubTotal_Category_Schema = new \Bulut\eFaturaUBL\TaxScheme(); $invoice_TaxTotal_SubTotal_Category_Schema->Name = "TAX"; $invoice_TaxTotal_SubTotal_Category_Schema->TaxTypeCode = "0015"; $invoice_TaxTotal_SubTotal_Category->TaxScheme = $invoice_TaxTotal_SubTotal_Category_Schema; $invoice_TaxTotal_SubTotal->TaxCategory = $invoice_TaxTotal_SubTotal_Category; $invoice_TaxTotal->TaxSubtotal = $invoice_TaxTotal_SubTotal; $invoice->TaxTotal = $invoice_TaxTotal; 的这个案例，该案例之前有人评论过：

1. prop.minTime：5 ==> ref.current = 5 |设置参考电流
2. prop.minTime：5 ==> ref.current = 5 |新值等于ref.current
3. prop.minTime：8 ==> ref.current = 5 |新值不等于ref.current
4. prop.minTime：5 ==> ref.current = 5 |新值等于ref.current

正如我们在这里看到的那样，我们没有更新内部usePrevious，因为我们正在使用ref

Answer 7

如果您喜欢使用useEffect替换方法：

const usePreviousEffect = (fn, inputs = []) => {
  const previousInputsRef = useRef([...inputs])

  useEffect(() => {
    fn(previousInputsRef.current)
    previousInputsRef.current = [...inputs]
  }, inputs)
}

并像这样使用它：

usePreviousEffect(
  ([prevReceiveAmount, prevSendAmount]) => {
    if (prevReceiveAmount !== receiveAmount) // side effect here
    if (prevSendAmount !== sendAmount) // side effect here
  },
  [receiveAmount, sendAmount]
)

请注意，效果第一次执行第一次时，传递给fn的先前值将与初始输入值相同。这仅对您很重要，如果您想在不变不变的情况下执行某些操作。

Answer 8

由于状态没有与功能组件中的组件实例紧密耦合，因此无法在useEffect中访问先前状态，而无需先保存它，例如，使用useRef。这也意味着状态更新可能在错误的位置被错误地实现了，因为先前的状态在setState更新程序函数中可用。

这是useReducer的好用例，它提供了类似Redux的存储并允许实现各自的模式。状态更新是显式执行的，因此无需弄清楚哪个状态属性被更新；从派遣行动中已经很清楚了。

这是一个example，看起来可能像这样：

function reducer({ sendAmount, receiveAmount, rate }, action) {
  switch (action.type) {
    case "sendAmount":
      sendAmount = action.payload;
      return {
        sendAmount,
        receiveAmount: sendAmount * rate,
        rate
      };
    case "receiveAmount":
      receiveAmount = action.payload;
      return {
        sendAmount: receiveAmount / rate,
        receiveAmount,
        rate
      };
    case "rate":
      rate = action.payload;
      return {
        sendAmount: receiveAmount ? receiveAmount / rate : sendAmount,
        receiveAmount: sendAmount ? sendAmount * rate : receiveAmount,
        rate
      };
    default:
      throw new Error();
  }
}

function handleChange(e) {
  const { name, value } = e.target;
  dispatch({
    type: name,
    payload: value
  });
}

...
const [state, dispatch] = useReducer(reducer, {
  rate: 2,
  sendAmount: 0,
  receiveAmount: 0
});
...

Answer 9

这是我使用的自定义钩子，我相信它比使用usePrevious更直观。

import { useRef, useEffect } from 'react'

// useTransition :: Array a => (a -> Void, a) -> Void
//                              |_______|  |
//                                  |      |
//                              callback  deps
//
// The useTransition hook is similar to the useEffect hook. It requires
// a callback function and an array of dependencies. Unlike the useEffect
// hook, the callback function is only called when the dependencies change.
// Hence, it's not called when the component mounts because there is no change
// in the dependencies. The callback function is supplied the previous array of
// dependencies which it can use to perform transition-based effects.
const useTransition = (callback, deps) => {
  const func = useRef(null)

  useEffect(() => {
    func.current = callback
  }, [callback])

  const args = useRef(null)

  useEffect(() => {
    if (args.current !== null) func.current(...args.current)
    args.current = deps
  }, deps)
}

您将按以下方式使用useTransition。

useTransition((prevRate, prevSendAmount, prevReceiveAmount) => {
  if (sendAmount !== prevSendAmount || rate !== prevRate && sendAmount > 0) {
    const newReceiveAmount = sendAmount * rate
    // do something
  } else {
    const newSendAmount = receiveAmount / rate
    // do something
  }
}, [rate, sendAmount, receiveAmount])

希望有帮助。

Answer 10

我刚刚发布了react-delta，它解决了这种情况。我认为useEffect的职责太多。

职责

1. 它使用Object.is
比较其依赖项数组中的所有值
2. 它根据＃1的结果运行效果/清理回调

打破责任

react-delta将useEffect的职责分解为几个较小的钩子。

职责1

• usePrevious(value)
• useLatest(value)
• useDelta(value, options)
• useDeltaArray(valueArray, options)
• useDeltaObject(valueObject, options)
• some(deltaArray)
• every(deltaArray)

责任2

• useConditionalEffect(callback, boolean)

根据我的经验，这种方法比useEffect / useRef解决方案更加灵活，简洁和简洁。

Answer 11

您可以使用useImmer代替useState并访问该状态。 示例：https://css-tricks.com/build-a-chat-app-using-react-hooks-in-100-lines-of-code/

Answer 12

我不喜欢上面的任何答案，我希望能够传递一组布尔值，如果其中一个为真，则重新渲染

/**
 * effect fires if one of the conditions in the dependency array is true
 */
export const useEffectCompare = (callback: () => void, conditions: boolean[], effect = useEffect) => {
  const shouldUpdate = useRef(false);
  if (conditions.some((cond) => cond)) shouldUpdate.current = !shouldUpdate.current;
  effect(callback, [shouldUpdate.current]);
};

//usage - will fire because one of the dependencies is true.
useEffectCompare(() => {
  console.log('test!');
}, [false, true]);

Answer 13

在你的情况下（简单对象）：

useEffect(()=>{
  // your logic
}, [rate, sendAmount, receiveAmount])

其他情况（复杂对象）

const {cityInfo} = props;
useEffect(()=>{
  // some logic
}, [cityInfo.cityId])