[英]Function variable value resetting after calling this.setState
我对 JavaScript 世界比较陌生,我正在学习 react 并且遇到了一个奇怪的问题,请参阅此代码
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);
}
在这里,我关心的是 newPrice 变量,它用于在添加更多项目时更新状态,它工作正常
问题是在这个之后。 setState
返回 newPrice 再次被重新测试为 0,所以我不能将它用于底部的函数。
是的,我可以直接使用状态变量,但由于setState
执行的 asnyc 性质,我想改为传递变量值。
在控制台中,您可以看到由于setState
异步性质,首先执行外部控制台日志,然后执行内部控制台日志
也许我没有得到一些正在产生这种行为的生命周期反应。
这是状态值,这些值应该无关紧要,但仍然是为了更好的画面
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。
调用setState
后newPrice
等于0
的原因是因为 React 状态更新是异步的。 状态更新后的代码会在setState
实际做某事之前运行,所以在调用this.updatePurchaseable(copyState, newPrice);
的阶段this.updatePurchaseable(copyState, newPrice);
newPrice
所有计算尚未执行。
顺便说一句- 这也是为什么您的console.log
以“相反”顺序打印的原因,每次渲染外部日志都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);
}
this.setState()
被异步调用,所以你不能依赖this.state
在调用this.setState()
后立即引用更新的值。 阅读有关组件状态的常见问题解答。
如果要在状态更新后引用newPrice
的更新值,可以:
componentDidUpdate()
生命周期方法。 请参阅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);
}
}
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);
});
}
ReactDOM.flushSync()
。 请参阅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,我会建议使用componentDidUpdate
生命周期方法,因为这将确保在总价格发生变化时始终调用updatePurchaseable
。 如果您只在事件处理程序中调用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);
}
React 状态更新是异步的,但setState
函数是完全同步的,所以当你调用updatePurchaseable
时newPrice
还没有更新。 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/引用更新的totalPrice
并使用更新的状态调用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
}
});
}
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