Firestore、onSnapshot() 或 async/await 问题或两者兼有

Question

我正在尝试使用 Firebase Firestore 的 onShapshot() 进行一系列更改。

我无法通过onSnapshot() 检索数据；我可能对async/await 也有麻烦，不太确定...

你能看出哪里有问题吗？

输出应该是（但目前是）：

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.

Answer 1

看看这个answer，它解释了get()和onSnapshot()方法之间的区别。

简而言之：

• 当您使用 get() 时，您只检索一次集合中的所有文档（如“获取并忘记”）。
• 当您使用 onSnapshot() 时，您会不断收听合集。

注意onSnapshot() 不是异步方法，而get() 是=> 不要用await 调用onSnapshot()。

---

由于您的问题，您似乎想通过调用getAllFriends()方法获取好友列表，请执行以下操作：

  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 here 和 here 中可以找到更多可能性。