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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?

<p>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。</p> <pre class="brush:php;toolbar:false;">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) } }</pre> <p>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。</p>
P粉596161915P粉596161915471 天前391

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  • P粉765684602

    P粉7656846022023-08-16 09:53:17

    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)
      }
    }

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