swift - 从 Firebase 同步读取

Question

我需要从 Firebase 读取一个值，然后在总共两个对象的事务中将其与多个其他值一起写入 Firebase。

我正在创建一个聊天室，因此当发送消息时，我正在为两个联系人创建一个聊天室，每个人都有自己的聊天室。我的代码:

private func CreateChatRoom(creatorID: String, creatorName: String ,contactID: String, contactName: String)
    {

        var creatorImageString: String = ""
        var contactImageString: String = ""
        ReadContactImage(contactID: contactID)
        {
            success in

            if success
            {
                contactImageString = self.tempContactImg
            }
        }
        ReadContactImage(contactID: creatorID)
        {
            success in

            if success
            {
                creatorImageString = self.tempContactImg
            }
        }

        let infoForCreator = [Constants.Chat.ChatRoomsLite.CONTACT_NAME: contactName,
                              Constants.Chat.ChatRoomsLite.CONTACT_ID: contactID,
                              Constants.Chat.ChatRoomsLite.NUM_OF_UNREAD_MSGS : 0,
                              Constants.Chat.ChatRoomsLite.CONTACT_IMG_URL: contactImageString] as [String : Any]


        let infoForContact = [Constants.Chat.ChatRoomsLite.CONTACT_NAME: creatorName,
                              Constants.Chat.ChatRoomsLite.CONTACT_ID: creatorID,
                              Constants.Chat.ChatRoomsLite.NUM_OF_UNREAD_MSGS : 0,
                              Constants.Chat.ChatRoomsLite.CONTACT_IMG_URL: creatorImageString] as [String : Any]

        let childUpdates = ["\(creatorID)/\(contactID)/": infoForCreator,
                            "\(contactID)/\(creatorID)/": infoForContact
                           ]

        Constants.refs.databaseChatsLite.updateChildValues(childUpdates)
    }

    private func ReadContactImage(contactID: String, completion: @escaping (Bool) -> ())
    {
        Constants.refs.databaseUsers.child(contactID).child(Constants.Account.AccountFields.USER_IMAGE_STR).observeSingleEvent(of: .value, with: {(snapshot) in

            self.tempContactImg = (snapshot.value as? String)!
            completion(true)
        })
    }

    var tempContactImg : String = "";

我在这里读到函数“ReadContactImage”应该同步运行，但事实并非如此。所以我留下了空的联系人图像。

我想过在同一个函数中读取两个图像，但是 CreateChatRoom 也需要同步，所以基本上我遇到了同样的问题。

有谁知道如何正确处理这个问题吗？

是否有更简单的方法可以做到这一点？

编辑:

如果写入数据库是异步的，我会在这里得到一个异常:

func AddChatToCollections(chatAsDictionary: NSDictionary!)
    {
        if chatAsDictionary == nil
        {
            return
        }
        let contactName = chatAsDictionary[Constants.Chat.ChatRoomsLite.CONTACT_NAME] as! String
        let contactImg = chatAsDictionary[Constants.Chat.ChatRoomsLite.CONTACT_IMG_URL] as! String
        //let lastMsg = chatAsDictionary["lastMessage"] as! String
        let newMsgs = chatAsDictionary[Constants.Chat.ChatRoomsLite.NUM_OF_UNREAD_MSGS] as! Int
        let contactID = chatAsDictionary[Constants.Chat.ChatRoomsLite.CONTACT_ID] as! String

        let chatToAdd = PrivateChatLiteObject(chattingWith: contactName, ContactID: contactID, unreadMessages: newMsgs, LastMSG: "", ContactImageStr: contactImg)


        chatsDictionary[contactID] = chatToAdd
        chatsIndex.append(contactID)
    }

当尝试使用字典中的信息时，该信息取自 Firebase。 该函数从这里调用:

private func populateActiveChats()
    {
        let loggedOnUserID = Auth.auth().currentUser?.uid
        let ref = Constants.refs.databaseChatsLite.child(loggedOnUserID!)

        // Retrieve the products and listen for changes
        ref.observe(.value, with:
                { (snapshot) in

                    for child in snapshot.children.allObjects as! [DataSnapshot]
                    {

                        if (self.chatsDictionary.keys.contains(child.key) == false)
                        {
                            let chatValueDictionary = child.value as? NSDictionary
                            self.AddChatToCollections(chatAsDictionary: chatValueDictionary)
                            self.DispatchQueueFunc()
                        }


                    }
            })
    }

当我打开聊天页面时，从 viewDidLoad() 调用它。

Thread 1: Fatal error: Unexpectedly found nil while unwrapping an Optional value

因为 chatAsDictionary[CONTACT_NAME] 不存在，因为当 chatAsDictionary 从 Firebase 获取数据时，它还没有从异步函数写入那里

Answer 1

您调用的这两种方法都会从 Firebase 异步加载数据。在对 ReadContactImage 的两次调用完成之前，您无法构造 infoForCreator(等)。

一个简单的方法是嵌套调用:

var creatorImageString: String = ""
var contactImageString: String = ""
ReadContactImage(contactID: contactID)
{
    success in

    if success
    {
        contactImageString = self.tempContactImg
        ReadContactImage(contactID: creatorID)
        {
            success in

            if success
            {
                creatorImageString = self.tempContactImg

                let infoForCreator = [Constants.Chat.ChatRoomsLite.CONTACT_NAME: contactName,
                      Constants.Chat.ChatRoomsLite.CONTACT_ID: contactID,
                      Constants.Chat.ChatRoomsLite.NUM_OF_UNREAD_MSGS : 0,
                      Constants.Chat.ChatRoomsLite.CONTACT_IMG_URL: contactImageString] as [String : Any]


let infoForContact = [Constants.Chat.ChatRoomsLite.CONTACT_NAME: creatorName,
                      Constants.Chat.ChatRoomsLite.CONTACT_ID: creatorID,
                      Constants.Chat.ChatRoomsLite.NUM_OF_UNREAD_MSGS : 0,
                      Constants.Chat.ChatRoomsLite.CONTACT_IMG_URL: creatorImageString] as [String : Any]

let childUpdates = ["\(creatorID)/\(contactID)/": infoForCreator,
                    "\(contactID)/\(creatorID)/": infoForContact
                   ]

                   Constants.refs.databaseChatsLite.updateChildValues(childUpdates)
            }
        }
    }
}

或者，您可以保留一个计数器:

var creatorImageString: String = ""
var contactImageString: String = ""
var completedCount = 0;
ReadContactImage(contactID: contactID)
{
    success in

    if success
    {
        contactImageString = self.tempContactImg
        completedCount++
        if completedCount == 2
        {
            createDatabaseNode(contactImageString, creatorImageString)
        }
    }
}
ReadContactImage(contactID: creatorID)
{
    success in

    if success
    {
        creatorImageString = self.tempContactImg
        completedCount++
        if completedCount == 2
        {
            createDatabaseNode(contactImageString, creatorImageString)
        }
    }
}

然后 createDatabaseNode 是一个函数，其中包含用于填充数据结构并调用 updateChildValues 的代码。