Swift Firebase 实时数据库数据未在函数结束前提取，数据未显示在视图中 [重复]

Question

我对 Swift 还很陌生，所以如果这是显而易见的事情，我深表歉意，但到目前为止我还没有找到答案。

因此，我试图在视图中显示“朋友”列表，并且该数据来自 Firebase 实时数据库。正在拉取数据，但在函数运行完成后才拉取数据。

我也尝试让它与 await async 一起工作，但据我在尝试时看到的，firebase 方法不支持它。

如果您查看下面的函数、视图和打印输出，您就会明白我的意思。

这是我的 firebase 控制器文件中的获取好友功能：

func getFriends(completion: @escaping ([Friend])-> Void) {
        print("start getFriends")
        self.friends = []
        let ref = Database.database().reference(fromURL: "users")
        
        ref.queryOrderedByKey().observeSingleEvent(of: .value) { (snapshot) in
             print("entering data snapshot pull")
            let snapshotValue = snapshot.value as! [String:[String:[String:AnyObject]]]
               
                for (key, _) in snapshotValue {
                    for (_, value) in snapshotValue[key]! {
                        var email = ""
                        var userName = ""
                        var id = ""
                        for (key, value) in value {
                            switch key{
                                case "email":
                                    email = value as? String ?? ""
                                case "userName":
                                    userName = value as? String ?? ""
                                case "id":
                                    id = value as? String ?? ""
                                default:
                                    print("")
                            }
                    }
                        print("--------------------------")
                        print(userName)
                        print(email)
                        print(id)
                        self.friends.append(Friend(userName: userName, email: email, id: id))
                }
            }
             print("leaving data snapshot pull")
             
        }
        print("end getFriends")
        completion(friends)
      }

这是我的看法：

struct ChatListView: View {
    typealias Friend = ChatList<String>.Friend
    @State var friends : [Friend] = []
    @ObservedObject var chatListUI: ChatListUI
    var firebaseController = FirebaseDatabaseController()
    var body: some View {
        Button(action: {
            do {
                try Auth.auth().signOut()
            }
            catch {
                print("cant log out")
            }
        }, label: {
            Text("Log out")
        })
        List {
            ForEach(friends) { Friend in
                Text(Friend.userName)
            }
        }.onAppear(perform: {
            print("start onAppear in view")
            firebaseController.getFriends(){ productsArray in
                self.friends = productsArray
            }
            print("friends count:")
            print(friends.count)
            print("end onAppear  in view")
        })
    }
}

这是输出：

2022-01-05 13:20:32.273898+0100 ChatBuddy[2094:38382] [error] warning:  View context accessed for persistent container ChatBuddy with no stores loaded
CoreData: warning:  View context accessed for persistent container ChatBuddy with no stores loaded
start onAppear in view
start getFriends
end getFriends
friends count:
0
end onAppear  in view
2022-01-05 13:20:32.655201+0100 ChatBuddy[2094:38562] 8.10.0 - [GoogleUtilities/AppDelegateSwizzler][I-SWZ001014] App Delegate does not conform to UIApplicationDelegate protocol.
entering data snapshot pull
--------------------------
ArdianNuhiji
Nuhijiardian@hotmail.com
ItAoaKj5ZTamf3BHikbdYsHmMhq2
--------------------------
Test
Test@hotmail.com
QkpSyxmV5MfqmhPobdOtQI97cdP2
--------------------------
Test2
Test2@hotmail.com
8xSa6Ff73JMJSq1418rgWtil6442
--------------------------
Test1
Test1@hotmail.com
WQ3f9OnmJTOtbxdEXMEB2j4q3bu2
leaving data snapshot pull
2022-01-05 13:20:32.917846+0100 ChatBuddy[2094:38562] [boringssl] boringssl_metrics_log_metric_block_invoke(151) Failed to log metrics

Answer 1

问题在于您的 getFriends 方法。在您的代码中 observeSingleEvent 是一个异步函数，因此仅在该闭包内调用 completion(friends) 即可解决问题。改变方法如下。

func getFriends(completion: @escaping ([Friend])-> Void) {
    print("start getFriends")
    self.friends = []
    let ref = Database.database().reference(fromURL: "users")
    
    ref.queryOrderedByKey().observeSingleEvent(of: .value) { (snapshot) in
        print("entering data snapshot pull")
        let snapshotValue = snapshot.value as! [String:[String:[String:AnyObject]]]
        
        for (key, _) in snapshotValue {
            for (_, value) in snapshotValue[key]! {
                var email = ""
                var userName = ""
                var id = ""
                for (key, value) in value {
                    switch key{
                    case "email":
                        email = value as? String ?? ""
                    case "userName":
                        userName = value as? String ?? ""
                    case "id":
                        id = value as? String ?? ""
                    default:
                        print("")
                    }
                }
                print("--------------------------")
                print(userName)
                print(email)
                print(id)
                self.friends.append(Friend(userName: userName, email: email, id: id))
            }
        }
        print("leaving data snapshot pull")
        // CHANGE -----
        completion(friends)
    }
    print("end getFriends")
    
}