javascript - 我怎样才能调用该函数一次？

Question

我有一个 React Native 项目，我需要实时从 Firebase 数据库读取数据。我尝试:

const [numberTrains, setNumberTrains] = useState(0)
 function Read(){
    const starCountRef = ref(db, 'users/' + user.localId);
    onValue(starCountRef, (snapshot) => {
      const data = snapshot.val();
      setNumberTrains(data.countTrains);   
    });
  }
  Read()
    console.log(numberTrains)

但它抛出异常:错误:重新渲染次数过多。React 限制渲染次数以防止无限循环。输出如下所示:

 LOG  0
 LOG  1
 LOG  1
 LOG  1
 LOG  1
 LOG  1
 LOG  1
 LOG  1
 LOG  1
 LOG  1
 LOG  1
 LOG  1
 LOG  1
 LOG  1
 LOG  1
 LOG  1
 LOG  1
 LOG  1
 LOG  1
 LOG  1
 LOG  1
 LOG  1
 LOG  1
 LOG  1
 LOG  1
 LOG  1

如何只读取numberTrain中的数据一次？

Answer 1

使用useEffect

const [numberTrains, setNumberTrains] = useState(0)

function Read(){
  const starCountRef = ref(db, 'users/' + user.localId);
  onValue(starCountRef, (snapshot) => {
    const data = snapshot.val();
    setNumberTrains(data.countTrains);   
  });
}

React.useEffect(()=>{
  Read();
  console.log(numberTrains)
},[]);