ios - Firebase 查询在错误的时间运行

Question

由于某种原因，当我运行以下方法时，第一个查询在该方法完成后运行。我尝试使用分派(dispatch) block 来强制查询先运行，但查询根本不会运行，应用程序只是卡住。如果您知道哪里出了问题，请告诉我。

没有调度组的方法:

func loadConversations() {
    let ref = FIRDatabase.database().reference()
    let convoRef = ref.child("users").child(FIRAuth.auth()!.currentUser!.uid).child("conversations")
    var conversationID = [String]()
    print(1)
    convoRef.queryOrderedByKey().observeSingleEvent(of: .value, with: { (snapshot) in
        let enumerator = snapshot.children
        print(2)
        while let rest = enumerator.nextObject() as? FIRDataSnapshot {
           print(3)
            if let id = rest.value as? String{
                conversationID.append(id)
                print(id)
            }
        }
    })
    print(4)
    print("size: \(conversationID.count)")
    for id in conversationID {
     print(5)
       ref.child("conversations").queryEqual(toValue: id).observeSingleEvent(of: .value, with: { (snapshot) in
      print(6)
            if let convo = snapshot.value as? [String : AnyObject] {
          print(7)
                let conversation = Conversation()
                conversation.conversationID = id
                conversation.name = "Temporary test name"
                self.Conversations.append(conversation)
            }
       })
        ref.removeAllObservers()
    }
    print(8)
    self.conversationTableView.reloadData()
    ref.removeAllObservers()

}

这打印:

1
4
size: 0
8
2
3
-KZyMMzXmkQC_OF0T08_

与调度组:

func loadConversations() {
    let dispatchGroup = DispatchGroup() 
    let ref = FIRDatabase.database().reference()
    let convoRef = ref.child("users").child(FIRAuth.auth()!.currentUser!.uid).child("conversations")
    var conversationID = [String]()
    print(1)


    dispatchGroup.enter()
    convoRef.queryOrderedByKey().observeSingleEvent(of: .value, with: { (snapshot) in

        let enumerator = snapshot.children
        print(2)
        while let rest = enumerator.nextObject() as? FIRDataSnapshot {
           print(3)
            if let id = rest.value as? String{
                conversationID.append(id)
                print(id)
                dispatchGroup.leave()
            }
        }


    })

    print(4)
    dispatchGroup.wait()
    print("size: \(conversationID.count)")

    for id in conversationID {
     print(5)
       ref.child("conversations").queryEqual(toValue: id).observeSingleEvent(of: .value, with: { (snapshot) in
      print(6)
            if let convo = snapshot.value as? [String : AnyObject] {
          print(7)
                let conversation = Conversation()
                conversation.conversationID = id
                conversation.name = "Temporary test name"
                self.Conversations.append(conversation)
            }
       })
    }
    print(8)
    self.conversationTableView.reloadData()
    ref.removeAllObservers()
}

这打印

1
4

但随后它只是卡住并等待。查询永远不会运行。

我不确定为什么查询似乎没有被输入。输入查询时，它工作得很好，但输入得太晚了。任何帮助是极大的赞赏。谢谢!

Answer 1

这仅仅是因为 Firebase 查询是在后台线程上执行的，因为它本质上是一个网络调用。因此响应在您的方法完成后出现，否则 UI 将被阻塞，直到响应来自 Firebase

您需要在查询响应中编写一个闭包，以便在获得响应后立即执行一段代码。

func loadConversations(completion:@escaping (Array<String>) -> Void) -> Void {
    let ref = FIRDatabase.database().reference()
    let convoRef = ref.child("users").child(FIRAuth.auth()!.currentUser!.uid).child("conversations")
    var conversationID = [String]()
    print(1)
    convoRef.queryOrderedByKey().observeSingleEvent(of: .value, with: { (snapshot) in
        let enumerator = snapshot.children
        print(2)
        while let rest = enumerator.nextObject() as? FIRDataSnapshot {
           print(3)
            if let id = rest.value as? String{
                conversationID.append(id)
                print(id)
            }
        }
        completion(conversationID)
    })
}

这会将您的调用发送回从内部调用的任何地方

loadConversations { (array) in

    //do something with this value and execute next query
}