swift 串行队列操作乱序

Question

我正在尝试实现一个串行队列，但由于某种原因，第二个块在第一个块之前完成（如系统打印中所示。关于为什么会发生这种情况或如何修复它的任何知识都是非常感谢！

SWIFT 代码：

func didAccept(with username: String) {
    
    let serialQueue = DispatchQueue(label: "acceptQueue")
    
    serialQueue.async {
        let url = URL(string: "http://127.0.0.1:5000/get-user-id")!
        var request = URLRequest(url: url, cachePolicy: .useProtocolCachePolicy, timeoutInterval: 10)
        request.httpMethod = "POST"
        request.multipartFormData(parameters: ["username": username])
        let task = URLSession.shared.dataTask(with: request) { (data, response, error) in
            guard let data = data else { return }
            do {
                let requestList = try JSONDecoder().decode(TempId.self, from: data)
                self.friend_id = requestList.friend_id
                print("first: \(self.friend_id)")
            } catch let jsonErr {
                print(jsonErr)
            }
        }
        task.resume()
    }
    
    serialQueue.async {
        let url2 = URL(string: "http://127.0.0.1:5000/accept-decline-request")!
        var request2 = URLRequest(url: url2, cachePolicy: .useProtocolCachePolicy, timeoutInterval: 10)
        request2.httpMethod = "POST"
        request2.multipartFormData(parameters: ["user_id": "17", "friend_id": "\(self.friend_id)", "response": "accept"])
        print("second: \(self.friend_id)")
        let task2 = URLSession.shared.dataTask(with: request2) { (data, response, error) in
            guard let data = data else { return }
        }
        task2.resume()
    }
    
    table.reloadData()
}

输出：

second: -1
first: 21

Answer 1

您可以使用操作和操作队列来满足此要求。您可以设置

maxConcurrentOperationCount = 1

它会一次调用一个请求。因此，在您的第一个任务没有响应之前，永远不会调用第二个请求。一旦第一个请求响应到来，第二个请求将自动添加到下一个队列中。

swift 串行队列操作乱序

swift serial queue operations out of order

concurrency

httpwebrequest

nsoperationqueue

swift