调用 this.setState 后函数变量值重置

Question

我对 JavaScript 世界比较陌生，我正在学习 react 并遇到了一个奇怪的问题 查看此代码

addIngredientHandler = (type) => {

    let oldCount  = this.state.ingredients[type];
    let copyState = {...this.state.ingredients};

    let newPrice = 0;

    copyState[type] = oldCount + 1;

    this.setState( (prevState, prevProps) => {

        newPrice = prevState.totalPrice + PRICES_OF_INGREDIENTS[type];

        newPrice =  Math.round(newPrice * 100) / 100;

        console.log('newprice inside setState: ' + newPrice);
        
        return { ingredients: copyState, totalPrice:  newPrice}
        

    } );

    console.log('newprice outside setState: ' + newPrice);

    this.updatePurchaseable(copyState, newPrice);


}

这里我关心的是 newPrice 变量，它用于在添加更多项目时更新状态，效果很好

问题是在 this.setState return 之后 newPrice 再次被重新测试为 0 所以我不能将它用于底部的功能。

是的，我可以直接使用状态变量，但由于setState 执行的异步性质，我想改为传递变量值。

在控制台中，由于setState 的异步性质，您可以看到首先执行外部控制台日志，然后执行内部日志

也许我没有得到一些产生这种行为的生命周期反应。

这里是状​​态值，值应该不重要，但仍然是为了更好的图片

state = {
    ingredients: {
        salad: 0,
        bacon: 0,
        meat: 0,
        cheese: 0,
    },
    purchasable: false,

    totalPrice: 0

}

任何提示都有帮助，感谢阅读。

Answer 1

在调用setState 之后newPrice 等于0 的原因是因为React 状态更新是异步。状态更新后的代码会在setState实际做某事之前运行之前，所以在调用this.updatePurchaseable(copyState, newPrice);的阶段所有newPrice的计算都还没有被执行。

顺便说一句 - 这也是为什么您的 console.log 以“反向”顺序打印的原因，每次渲染外部日志都会在内​​部日志之前打印。

对于这个特定的代码示例，我建议您尝试将现在在 setState 回调中的所有计算移到它之外，甚至移到不同的函数中。

试试这个 -

calculateNewPrice = (totalPrice, type) => {
    newPrice = totalPrice + PRICES_OF_INGREDIENTS[type];
    newPrice =  Math.round(newPrice * 100) / 100;
}

addIngredientHandler = (type) => {
    const { totalPrice } = this.state;
    
    let oldCount  = this.state.ingredients[type];
    let copyState = {...this.state.ingredients};
    copyState[type] = oldCount + 1;

    const newPrice = calculateNewPrice(totalPrice, type);

    this.setState({ ingredients: copyState, totalPrice:  newPrice });
    
    this.updatePurchaseable(copyState, newPrice);
}

Answer 2

this.setState() 被异步调用，因此您不能依赖 this.state 在调用 this.setState() 后立即引用更新的值。阅读FAQ on component state。

如果你想在状态更新后引用newPrice的更新值，你可以：

1. 使用componentDidUpdate() 生命周期方法。见https://reactjs.org/docs/react-component.html#componentdidupdate。

addIngredientHandler = (type) => {
  let oldCount = this.state.ingredients[type];
  let copyState = { ...this.state.ingredients };

  let newPrice = 0;

  copyState[type] = oldCount + 1;

  this.setState((prevState) => {
    newPrice = prevState.totalPrice + PRICES_OF_INGREDIENTS[type];
    newPrice = Math.round(newPrice * 100) / 100;

    return { ingredients: copyState, totalPrice: newPrice }
  });
}

componentDidUpdate(prevProps, prevState) {
  if (prevState.totalPrice !== this.state.totalPrice) {
    this.updatePurchaseable(this.state.ingredients, this.state.totalPrice);
  }
}

1. 使用this.setState() 的第二个参数。请参阅https://reactjs.org/docs/react-component.html#setstate 上的文档。

addIngredientHandler = (type) => {
  let oldCount = this.state.ingredients[type];
  let copyState = { ...this.state.ingredients };

  let newPrice = 0;

  copyState[type] = oldCount + 1;

  this.setState((prevState) => {
    newPrice = prevState.totalPrice + PRICES_OF_INGREDIENTS[type];
    newPrice = Math.round(newPrice * 100) / 100;

    return { ingredients: copyState, totalPrice: newPrice }
  }, () => {
    this.updatePurchaseable(this.state.ingredients, this.state.totalPrice);
  });
}

1. 使用ReactDOM.flushSync()。见https://github.com/reactwg/react-18/discussions/21。

import { flushSync } from 'react-dom';

addIngredientHandler = (type) => {
  let oldCount = this.state.ingredients[type];
  let copyState = { ...this.state.ingredients };

  let newPrice = 0;

  copyState[type] = oldCount + 1;

  flushSync(() => {
    this.setState((prevState) => {
      newPrice = prevState.totalPrice + PRICES_OF_INGREDIENTS[type];
      newPrice = Math.round(newPrice * 100) / 100;

      return { ingredients: copyState, totalPrice: newPrice }
    });
  });

  this.updatePurchaseable(copyState, newPrice);
}

---

如果我要编写此方法，我建议使用componentDidUpdate 生命周期方法，因为这将确保总价格变化时始终调用updatePurchaseable。如果您只在事件处理程序内部调用updatePurchaseable，那么如果价格在该处理程序之外发生变化，您最终可能会遇到错误。

addIngredientHandler = (type) => {
  this.setState(prevState => {
    let totalPrice = prevState.totalPrice + PRICES_OF_INGREDIENTS[type];
    totalPrice = Math.round(totalPrice * 100) / 100;

    return {
      ingredients: {
        ...prevState.ingredients,
        [type]: prevState.ingredients[type] + 1,
      },
      totalPrice,
    };
  });
}

componentDidUpdate(prevProps, prevState) {
  const { totalPrice, ingredients } = this.state;

  if (prevState.totalPrice === totalPrice) {
    /*
    
    Bail early. This is a personal code style preference. It may 
    make things easier to read as it keeps the main logic on the 
    "main line" (un-nested / unindented)
    
    */
    return;
  }

  /*

  If `updatePurchaseable` is a class method then you don't need to
  pass state to it as it will already have access to `this.state`.

  If `updatePurchaseable` contains complicated business logic,
  consider pulling it out into its own module to make it easier 
  to test.
  
  */
  this.updatePurchaseable(ingredients, totalPrice);
}

Answer 3

React 状态更新是异步，但setState 函数完全是同步，所以newPrice当您致电updatePurchaseable 时尚未更新。将所有额外的“状态更新后”逻辑移动到 componentDidUpdate 生命周期方法中，以便您可以访问/引用更新后的 totalPrice 并使用更新后的状态调用 updatePurchaseable。

componentDidUpdate(prevProps, prevState) {
  if (prevState.totalPrice !== this.state.totalPrice) {
    const { ingredients, totalPrice } = this.state;
    console.log('newprice outside setState: ' + totalPrice);

    this.updatePurchaseable(ingredients, totalPrice);
  }
}

addIngredientHandler = (type) => {
  this.setState((prevState, prevProps) => {
    let newPrice = prevState.totalPrice + PRICES_OF_INGREDIENTS[type];
    newPrice =  Math.round(newPrice * 100) / 100;
    return {
      ingredients: {
        ...prevState.ingredients,
        [type]: prevState.ingredients[type] + 1,
      }, 
      totalPrice:  newPrice
    }
  });
}