为什么在handleChange事件处理程序中，array.length方法总是比输入步骤慢一步？

Question

我正在尝试创建一个基于输入进行过滤的列表，只有在有10个或更少的对象时才显示结果，但是array.length始终滞后于输入。

const [countriesToShow, setCountriesToShow] = useState([])
    const [newSearch, setSearch] = useState('')
    const [showSearched, setShowShearched] = useState(true)
    const [notificationMessage, setNotificationMessage] = useState(null)

    useEffect(() => {
      console.log(countriesToShow.length)
    },[countriesToShow])
  
    const handleSearchChange = (event) => {
      setCountriesToShow(countries.filter(country =>
        country.name.common.toLowerCase().includes(event.target.value.toLowerCase())))
      setSearch(event.target.value)
      if (event.target.value.trim().length === 0) {
        setNotificationMessage(null)
        setShowShearched(false)
      }
      else if (countriesToShow.length <= 10) {
        setNotificationMessage(null)
        setShowShearched(true)
        console.log(countriesToShow.length)
      }
      else {
        setNotificationMessage('list too long')
        setShowShearched(false)
        console.log(countriesToShow.length)
      }
    }

我设法通过Effect Hook帮助正确打印出长度，但是我对如何将其实现到'else if (countriesToShow.length <= 10)'中感到困惑，因为它仍然滞后于输入。

Answer 1

当你调用handleSearchChange然后调用setCountriesToShow来更新状态时：

setCountriesToShow(
  countries.filter(country =>
    country
      .name
      .common
      .toLowerCase()
      .includes(event.target.value.toLowerCase())
  )
)

你触发了重新渲染。新更新的状态值只会在即将到来的重新渲染中变得可用，这就是为什么它落后的原因。

如果你想在下面使用该值，你需要先将它存储在一个变量中：

const handleSearchChange = (event) => {
  const newCountriesToShow = countries.filter(country =>
    country
      .name
      .common
      .toLowerCase()
      .includes(event.target.value.toLowerCase())
  )

  setCountriesToShow(newCountriesToShow)

  setSearch(event.target.value)

  if (event.target.value.trim().length === 0) {
    setNotificationMessage(null)
    setShowShearched(false)
  } else if (newCountriesToShow.length <= 10) {
    setNotificationMessage(null)
    setShowShearched(true)
  } else {
    setNotificationMessage('list too long')
    setShowShearched(false)
  }
}