Swift 完成时总是零分

我正在尝试获取

Firestore中的
地图数据
，如下所示：

我试图获取数据，创建一个
友元对象的数组
，并在
完成处理程序中返回
数组

这就是我所拥有的：

func fetchFriendList(_ id: String, completion: @escaping([Friend]?)->()) {
    var fetchedFriends: [Friend]?
    db.collection(USERS_COLLECTION).document(id).getDocument { (doc, err) in
        if err == nil && doc != nil {
            guard let results = doc?.data()?[USER_FOLLOWING] as? [String: Any] else { return }
            for result in results { // Getting the data in firebase
                if let resultValue = result.value as? [String: Any] { // Getting only the value of the MAP data, we do not need the key.

                    //Getting the fields from the result
                    guard let id = resultValue[FRIEND_ID] as? String else { return }
                    guard let profilePic = resultValue[FRIEND_PROFILE_PIC] as? String else { return }
                    guard let username = resultValue[FRIEND_NAME] as? String else { return }
                    guard let email = resultValue[FRIEND_MAIL] as? String else { return }

                    //Creating a new Friend object from the fields
                    let friend = Friend(id: id, profilePicture: profilePic, username: username, email: email)
                    fetchedFriends?.append(friend)
                }
                completion(fetchedFriends)
            }
        }else {
            print(err!.localizedDescription)
            completion(nil)
        }
    }
}

我试着打印结果，结果值等等，它们不是零。
但是，在尝试附加和打印fetchedFriends数组之后，我得到了nil，并且完成也是nil。
我真的不明白为什么会发生这种情况。
问题是您没有初始化变量fetchedFriends，并且在向其添加数据时使用了可选类型。由于它尚未初始化，它将跳过附加到它。您应该在开始时初始化它。更新后的代码如下

func fetchFriendList(_ id: String, completion: @escaping([Friend]?)->()) {
    var fetchedFriends: [Friend] = []
    db.collection(USERS_COLLECTION).document(id).getDocument { (doc, err) in
        if err == nil && doc != nil {
            guard let results = doc?.data()?[USER_FOLLOWING] as? [String: Any] else { return }
            for result in results { // Getting the data in firebase
                if let resultValue = result.value as? [String: Any] { // Getting only the value of the MAP data, we do not need the key.

                    //Getting the fields from the result
                    guard let id = resultValue[FRIEND_ID] as? String else { return }
                    guard let profilePic = resultValue[FRIEND_PROFILE_PIC] as? String else { return }
                    guard let username = resultValue[FRIEND_NAME] as? String else { return }
                    guard let email = resultValue[FRIEND_MAIL] as? String else { return }

                    //Creating a new Friend object from the fields
                    let friend = Friend(id: id, profilePicture: profilePic, username: username, email: email)
                    fetchedFriends.append(friend)
                }
                completion(fetchedFriends)
            }
        }else {
            print(err!.localizedDescription)
            completion(nil)
        }
    }
}

希望能有帮助