Swift 调度队列异步调用

我在另一个网络请求中的for循环中发出一个网络请求。我使用的是核心数据，但我相当确定这不是核心数据问题，而是异步问题

Firebase请求中的2条print语句正确地打印数据，但如果没有DispatchQueue，数组将返回为空（在网络请求完成之前）

以下是坠机的照片：

以下是代码本身：

var userReps = [UserRepresentation]()

// Fetch all Friends -> update Core Data accordingly
func fetchFriendsFromServer(completion: @escaping (Error?) -> Void = { _ in}){
    let backgroundContext = CoreDataStack.shared.container.newBackgroundContext()

    // 1. Fetch all friends from Firebase
    FirebaseDatabase.UserDatabaseReference.child(CoreUserController.shared.userPhoneNumber).child(UserKeys.UserFriends).child(UserKeys.UserAcceptedFriends).observe(.value) { (data) in
        if let dictionary = data.value as? [String: String] {
            var userReps = [UserRepresentation]()
            let group = DispatchGroup()

            group.enter()
            for friend in dictionary {
                let friendName = friend.value
                let friendId = friend.key

                FirebaseDatabase.UserDatabaseReference.child(friendId).observe(.value, with: { (data) in
                    if let dictionary = data.value as? [String: Any] {
                        guard let gender = dictionary[UserKeys.UserGender] as? String else {return}
                        guard let bio = dictionary[UserKeys.UserBio] as? String else {return}
                        guard let status = dictionary[UserKeys.UserStatus] as? String else {return}
                        guard let avatarUrl = dictionary[UserKeys.UserAvatarUrlKey] as? String else {return}

                        let friendRepresentation = UserRepresentation(avatarUrl: avatarUrl, name: friendName, number: friendId, gender: gender, status: status, bio: bio)
                        userReps.append(friendRepresentation)
                        print("HERE, friends fetched: ", friendRepresentation)
                        print("HERE, reps fetched: ", userReps)
                        group.leave()
                    }
                })
            }

            group.notify(queue: .main) {
                // 2. Update Core Data value with Firebase values
                self.updateFriends(with: userReps, in: backgroundContext)

                // 3. Save Core Data background context
                do {
                    try  CoreDataStack.shared.save(context: backgroundContext)
                } catch let error {
                    print("HERE. Error saving changes to core data: \(error.localizedDescription)")
                }
            }
        }
    }
}

任何帮助都会大有帮助

let group = DispatchGroup()

是一个局部变量，在这里使用observe

FirebaseDatabase.UserDatabaseReference.child(friendId).observe(.value, with: { (data) in

在函数解除分配后，它将重新调用它，或者使其成为一个实例变量，或者使其成为单个变量

 FirebaseDatabase.UserDatabaseReference.child(friendId).observeSingleEvent(of:.value) { (data) in

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在for循环中也输入enter

for friend in dictionary {  
  group.enter()

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可能需要很长时间才能注意到问题的起因。多谢各位