Firestore，onSnapshot()或异步/等待问题或两者

本文介绍了Firestore，onSnapshot()或异步/等待问题或两者的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述

我正在尝试使用Firebase Firestore的 onShapshot().

I'm trying to get an array of changes using Firebase Firestore's onShapshot().

我无法通过 onSnapshot()检索数据；我也可能会遇到 async/await 的麻烦，不太清楚...

I'm having trouble retrieving data through onSnapshot(); I may be in trouble with async/await as well, not quite sure...

您能看到哪里有问题吗?

Can you see where there are problems?

输出应为(但当前为)

1. New friends: ... // via onSnapshot(). Should not be empty, but it is (However, it does get populated afterwards). 2. All friends: ... // Should not be empty, but it is. 3. Fred's friends: ... // Should not be empty, but it is.

代码:

const getAllFriends = async () => { // Gets all friends by connecting to Firestore's onSnapshot stream. const getNewFriends = async () => { // Sets up a onSnapshot() stream, and returns a newFriends array with their names. // Problem: It initially return an empty array, when it shouldn't be empty. let newFriends = []; await db.collection("user").doc("john").collection("friends").onSnapshot(snapshot => { snapshot.docChanges().forEach(change => { newFriends.push({ friend: "Emily" }); }); }); console.log("1. New friends: ", newFriends, newFriends.length); // Length should not be 0. return newFriends; } // John starts with no friends: let friends = []; // John should now have found some friends: let friendChanges = await getNewFriends(); friends = friends.concat(friendChanges); console.log("2. All friends:", friends); // Should contain a few Emilys. return friends; }; let johnFriends = await getAllFriends(); console.log("3. John's friends:", friends); // Should contain a few Emilys.

推荐答案

看看这个答案，它解释了 get()和 onSnapshot()方法之间的区别.

Have a look at this answer which explains the difference between the get() and onSnapshot() methods.

简而言之:

• 当您使用 get()时，您仅会一次检索该集合的所有文档(如一劳永逸").
• 使用 onSnapshot()时，您恒定收听该集合.

• When you use get() you retrieve all the documents of the collection only once (like a "get and forget").
• When you use onSnapshot() you constantly listen to the collection.

请注意， onSnapshot() 不是异步方法，而 get() 是=>不要使用 await 调用 onSnapshot().

由于您的问题，看来您想通过调用 getAllFriends()方法来获取朋友列表，请执行以下操作:

Since, from your question, it seems that you want to get the list of friends by calling the getAllFriends() method, do as follows:

const getAllFriends = async (userName) => { const querySnapshot = await db .collection('user') .doc(userName) .collection('friends') .get(); return querySnapshot; }; let johnFriends = await getAllFriends('john'); johnFriends.forEach(doc => { console.log(doc.id, ' => ', doc.data()); });

在Firestore文档中，此处和此处.