如何为几个 URLSession.DataTaskPublisher 制作串行队列？答案

【问题标题】：How to make serial queue for a few URLSession.DataTaskPublisher?如何为几个 URLSession.DataTaskPublisher 制作串行队列？
【发布时间】：2021-03-20 10:06:30
【问题描述】：

我有几个 datataskpublisher，它们从不同的应用程序屏幕执行对服务器的请求。如何让它们串行运行？

下面是一个粗略的例子

这是一个执行请求和附加逻辑的服务。

class Service {

   var cacheResult: Data?
   var cacnellable: AnyCancellable?
   static let shared = Service()


   func performPost() -> AnyPublisher<Data, URLError> {
      let task = URLSession.shared.dataTaskPublisher(for: URL(string: "postURL")!)
         .map { (data, _) in data}
         .multicast(subject: PassthroughSubject())

      cacnellable = task.eraseToAnyPublisher().sink(
         receiveCompletion: {_ in },
         receiveValue: { data in
            self.cacheResult = data
         })

      return task.autoconnect().eraseToAnyPublisher()
   }

   func performGet() -> AnyPublisher<Data, URLError> {
      let task = URLSession.shared.dataTaskPublisher(for: URL(string: "getURL")!)
         .map { (data, _) in data}
         .multicast(subject: PassthroughSubject())

      cacnellable = task.eraseToAnyPublisher().sink(
         receiveCompletion: {_ in },
         receiveValue: { data in
            self.cacheResult = data
         })

      return task.autoconnect().eraseToAnyPublisher()
   }

}

这是视图

final class GetVC: UIViewController {

   let service = Service.shared
   var cancellable: AnyCancellable?

   override func viewWillAppear(_ animated: Bool) {
      super.viewWillAppear(true)
      cancellable = service.performGet()
         .sink(receiveCompletion: {_ in},
               receiveValue: { data in
                  print("Present data")
               })
   }

}

final class PostVC: UIViewController {

   let service = Service.shared
   var cancellable: AnyCancellable?

   lazy var button: UIButton = {
      let btn = UIButton(primaryAction: UIAction(handler: { _ in
         self.cancellable = self.service.performPost()
            .sink(receiveCompletion: {_ in },
                  receiveValue: { data in
                     print("Some logic")
                  })
      }))
      return btn
   }()
}

我需要请求永远不会并行发送到服务器

如果“发布任务”完成，“获取任务”开始

谢谢！

【问题讨论】：

标签： multithreading nsurlsession combine nsurlsessiondatatask

【解决方案1】：

我做出了这个决定

class Service {

   var cacheResult: Data?

   var cancellables = Set<AnyCancellable>()

   var taskSubject = PassthroughSubject<AnyPublisher<Data, URLError>, Never>()
   var taskInProcess: Bool = false
   var tasks: [AnyPublisher<Data, URLError>] = [] {
      didSet {
         if !taskInProcess, let firstTask = tasks.first {
            taskInProcess = true
            taskSubject.send(firstTask)
         }
      }
   }
   static let shared = Service()


   init() {

      taskSubject
         .flatMap { $0 }
         .sink(receiveCompletion: { _ in },
               receiveValue: { _ in
                  self.taskInProcess = false
                  self.tasks.removeFirst()
               })
         .store(in: &cancellables)

   }

   func performPost() -> AnyPublisher<Data, URLError> {
      let task = URLSession.shared.dataTaskPublisher(for: URL(string: url)!)
         .map { (data, _) in data}
         .multicast(subject: PassthroughSubject())

      tasks.append(task.autoconnect().eraseToAnyPublisher())

      return task.eraseToAnyPublisher()
   }

   func performGet() -> AnyPublisher<Data, URLError> {
      let task = URLSession.shared.dataTaskPublisher(for: URL(string: url)!)
         .map { (data, _) in data}
         .multicast(subject: PassthroughSubject())

      tasks.append(task.autoconnect().eraseToAnyPublisher())

      return task.eraseToAnyPublisher()
   }

}

【讨论】：