我尝试使用Firebase Firestore的onShapshot()获取一系列更改。
通过onSnapshot()检索数据时遇到问题;我可能也遇到了async/await的麻烦，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.

验证码：

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.