第一个 fetch 调用返回空数组调用，但在第二个等上工作？

Question

如上所述，每当我获取数据以将值设置为状态时，它只会在我第二次输入收费金额、货币和已签名字段时设置状态。我的主要问题是为什么我第一次在这些字段中输入值时它会获取一个空数组？ 第一次未定义值

代码

setUSDValue() {
const { AmountCharged, Currency, FYSigned} = this.state;

if(AmountCharged && Currency && FYSigned)
{
  fetch(`http://ca-fpscfb2:2000/USDValueExchangeRateValue?rateYear=${FYSigned}&USD=${Currency}&CAD=${Currency}&INR=${Currency}&GBP=${Currency}`)
  .then(response => response.json().then(response =>  
    this.setState(
    { 
      exchangeRateVal: response.data 

    }

    )))
  .then( 
    this.state.exchangeRateVal && this.state.exchangeRateVal.map((rate) =>
    (
      this.setState({
        USDValue: rate.USD
      })
    ))  
  )
}




}

Answer 1

问题是调用this.setState 时状态不会立即更新。如果您需要在状态更新后直接运行代码，请尝试使用回调函数：

setUSDValue() {
const { AmountCharged, Currency, FYSigned} = this.state;

if(AmountCharged && Currency && FYSigned)
{
  fetch(`http://ca-fpscfb2:2000/USDValueExchangeRateValue?rateYear=${FYSigned}&USD=${Currency}&CAD=${Currency}&INR=${Currency}&GBP=${Currency}`)
  .then(response => response.json().then(response =>  
    this.setState(
    { 
      exchangeRateVal: response.data 

    },
    () =>
    this.state.exchangeRateVal.map((rate) =>
    (
      this.setState({
        USDValue: rate.USD
      })
    ))  

    )))
}