Swift 4：如何异步使用URLSessionDataTask但请求是否在一个定时队列中？

3 回答

• 1

  递归解决了这个问题

  import Foundation
  import PlaygroundSupport
  
  // Let asynchronous code run
  PlaygroundPage.current.needsIndefiniteExecution = true
  
  func fetch(urls: [URL]) {
  
      guard urls.count > 0 else {
          print("Queue finished")
          return
      }
  
      var pendingURLs = urls
      let currentUrl = pendingURLs.removeFirst()
  
      print("\(pendingURLs.count)")
  
      let session = URLSession.shared
      let task = session.dataTask(with: currentUrl, completionHandler: { (data, response, error) in
          print("task completed")
          if let _ = error {
              print("error received")
              DispatchQueue.main.async {
                  fetch(urls: pendingURLs)
              }
              return
          }
  
          guard let response = response as? HTTPURLResponse, response.statusCode == 200 else {
              print("server error received")
              DispatchQueue.main.async {
                  fetch(urls: pendingURLs)
              }
              return
          }
          if response.mimeType == "application/json" {
              print("json data parsed")
              DispatchQueue.main.async {
                  fetch(urls: pendingURLs)
              }
          }else {
              print("unknown data")
              DispatchQueue.main.async {
                  fetch(urls: pendingURLs)
              }
          }
      })
  
      //start execution after two seconds
      Timer.scheduledTimer(withTimeInterval: 2, repeats: false) { (timer) in
          print("resume called")
          task.resume()
      }
  }
  
  var urls = [URL]()
  for _ in 0..<100 {
      if let url = URL(string: "https://google.com") {
          urls.append(url)
      }
  }
  
  fetch(urls:urls)
  

  回复于 2024-05-05T22:13:23+08:00
• 0

  最简单的方法是执行递归调用：

  • 想象一下，你的网址中有数组 .

  • 在最初执行循环的位置，将其替换为单个调用 get(url:) . self.get(urls[0])

  • 然后在 self.itemsCount.append(count) 之后立即在响应闭包处添加此行：

  self.urls.removeFirst()
  Timer.scheduledTimer(withTimeInterval: 2, repeats: false) { (_) in
  self.get(url: urls[0])
  }
  

  回复于 2024-05-05T22:13:23+08:00
• 0

  使 DispatchQueue 在线程上运行代码 . 您不需要在主线程上执行此操作 . 所以，

  // make serial queue
  let queue = DispatchQueue(label: "getData")
  
  // for delay
  func wait(seconds: Double, completion: @escaping () -> Void) {
      queue.asyncAfter(deadline: .now() + seconds) { completion() }
  }
  
  // usage
  for url in urls {
    wait(seconds: 2.0) {
        self.get(url: url) { (itemCount) in
           // update UI related to itemCount
        }
    }
  }
  

  顺便说一下，你的 get(url: url) 功能并不是那么好 .

  func get(url: URL, completionHandler: @escaping ([Int]) -> Void) {
      let session = URLSession.shared
      let task = session.dataTask(with: url, completionHandler: { (data, response, error) in
  
          if let error = error {
              print(error.localizedDescription)
              /* Don't need to use main thread
                  DispatchQueue.main.async {
                      print(error.localizedDescription)
                  }
              */
              return
          }
          let data = data!
  
          guard let response = response as? HTTPURLResponse, response.statusCode == 200 else {
              print("Server Error")
              /* Don't need to use main thread
                  DispatchQueue.main.async {
                      print("Server Error")
                  }
              */
              return
          }
          if response.mimeType == "application/json" {
              do {
                  let json = try JSONSerialization.jsonObject(with: data) as! [String: Any]
                  if json["success"] as! Bool == true {
                      if let count = json["total_count"] as? Int {
  
                          self.itemsCount.append(count)
  
                          // append all data that you need and pass it to completion closure
                          DispatchQueue.main.async {
                              completionHandler(self.itemsCount)
                          }
                      }
                  }
              } catch {
                  print(error.localizedDescription)
              }
          }
      })
      task.resume()
  }
  

  我建议你学习 GCD （用于线程）和 escaping closure （用于完成处理程序）的概念 .

  • GCD：https://www.raywenderlich.com/148513/grand-central-dispatch-tutorial-swift-3-part-1

  • 逃离关闭：https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/Closures.html#//apple_ref/doc/uid/TP40014097-CH11-ID546

  回复于 2024-05-05T22:13:23+08:00