javascript - 仅在定义对象时如何运行 useEffect （和云 Firestore 查询）？

Question

尝试仅在定义“用户”时运行火库“onSnapshot”。起初，用户是一个空对象，后来它变成了一个带有id，名称等的对象。所以这不应该运行......但似乎这个'if'语句不起作用（firestore onSnapshot甚至会运行如果它未定义）。

如何解决这个问题？

  useEffect(() => {
    if (user) { //object is undefined at first, but gets defined later
      const unsubscribe = firebase
        .firestore()
        .collection('users')
        .doc(user.id) //This seems to run even when 'user' is undefined (an empty object)
        .onSnapshot(snapshot => {
           //...
        })

      return () => {
        unsubscribe()
      }
    }
  }, [])

标签： javascriptreactjsgoogle-cloud-firestore

Answer 1

只需将user作为依赖项包含在您的 useEffect 挂钩中

 useEffect(() => {
    if (user) {
      //object is undefined at first, but gets defined later
      const unsubscribe = firebase
        .firestore()
        .collection("users")
        .doc(user.id) //This seems to run even when 'user' is undefined (an empty object)
        .onSnapshot(snapshot => {
          //...
        });

      return () => {
        unsubscribe();
      };
    }
  }, [user]);