全局线程同步SWIFT

Question

我做了一个简单的聊天应用。

在主线程上，我接收消息，然后在后台线程上，如果需要，我下载userDatas，如果完成了，那么我向另一个ViewController发送Notification，然后更新一个tableView。 （因此它必须在主线程上）

---

我在使用DispatchQueue.global().async + DispatchQueue.main.async时遇到问题

这是新的代码。使用DispatchQueue.global（）。sync安全吗？

socket.on("message") {
    (data, socketAck) -> Void in
    guard let dataArray = data[0] as? [String: Any] else {
        return
    }

    // Loaded the message
    guard let message = dataArray["message"] as? String, let userId = dataArray["userId"] as? Int
        return
    }

    // Load user data if not loaded
    let loadChatUserData_dispatchGroup = DispatchGroup()
    DispatchQueue.global().async {
        // Created a dispatchGroup so the whole function finished only if this is finished too
        loadChatUserData_dispatchGroup.enter()
        self.chatFunctions.loadChatUserDataIfNeeded(users: [userId]) {
            response in
            if let response = response {
                if response!.type == 0 {
                    print(response)
                    return
                }
            }
            loadChatUserData_dispatchGroup.leave()
        }
    }

    let chatMessageData = ["chatMessage": chatMessage] as [String : Any]

    DispatchQueue.global().async {
        loadChatUserData_dispatchGroup.wait(timeout: .now() + 60)
        DispatchQueue.global().sync {

            // If loadUserDatas is finished, or 60seconds passed, let it update the tableview
            NotificationCenter.default.post(name: .updateChatMessages, object: nil, userInfo: chatMessageData)
        }
    }
}

原始代码（导致我看到tableView错误）：

    .
    .
    // Load user data if not loaded,
    let loadChatUserData_dispatchGroup = DispatchGroup()
    DispatchQueue.global().async {
        loadChatUserData_dispatchGroup.enter()
        self.chatFunctions.loadChatUserDataIfNeeded(users: [userId]) {
            response in
            if let response = response {
                if response!.type == 0 {
                    print(response)
                    return
                }
            }
            loadChatUserData_dispatchGroup.leave()
        }
    }

    let chatMessageData = ["chatMessage": chatMessage] as [String : Any]

    DispatchQueue.global().async {
        loadChatUserData_dispatchGroup.wait(timeout: .now() + 60)
        DispatchQueue.main.async {
            // PROBLEM HERE: Even though this was called on the main thread,
            // it added a closure to be run in the next iteration of the main run loop, therefore it crashes..

            NotificationCenter.default.post(name: .updateChatMessages, object: nil, userInfo: chatMessageData)
        }
    }