Swift 等待異步任務

Question

在后台線程中，我們有：

defer {
    cleanup()
}

loadData()

if error {
    return
}

processData()

DispatchQueue.main.asyncAfter(deadline: delay) {  //Delay = now + 0-2 seconds
     updateUI()
}

問題是我們要確保 defer cleanUp()代碼在updateUI()之后運行。 就目前而言，這不會發生，因為updateUI()異步運行。

我的想法是在延遲期間休眠/阻塞，而不是異步運行。 一旦updateUI()完成，這將使 defer cleanUp()運行。

你怎么能做到這一點？ 或者，還有更好的方法？

Answer 1

您可以使用信號量告訴清理任務等待updateUI完成：

let semaphore = DispatchSemaphore(value: 1)
defer {
    semaphore.wait()
    cleanup()
}

loadData()

if error {
    // If we exit here, the semaphore would have never been used
    // and cleanup will run immediately
    return
}

processData()
semaphore.wait() // here, we claim the semaphore, making cleanup
                 // wait until updateUI is done
DispatchQueue.main.asyncAfter(deadline: delay) {
     updateUI()
     semaphore.signal()
}

Answer 2

意識到我可以改變代碼的結構：

loadData()

if error {
    log.error("Error")
} else {
   processData()
}

DispatchQueue.main.asyncAfter(deadline: delay) {  //Delay = now + 0-2 seconds
  updateUI()
  cleanup()
}

Answer 3

另一種選擇是使用DispatchGroup() ：

func doWork() {

   let group = DispatchGroup()

   group.enter() //Enter #1
   loadData { result in

      switch (result) {
      case .success(_):
           group.enter()//Enter #2
           processData { group.leave()//Leave #2 }
      case .failure(let error):
           //Do something nice with the error
           print(error)
      }

      group.leave()//Leave #1
    }

    //All the code inside this block will be executed on the mainThread when all tasks will be finished.
     group.notify(queue: .main) { [weak self] in

         guard let strongSelf = self else { return }

         strongSelf.updateUI()
         strongSelf.cleanup()
     }
}

private func updateUI() {
    //All your stuff
}

private func cleanup() {
    //All your stuff
}

private func loadData(completion: (Result<(), Error>) -> ()) {
    //All your stuff
    if error {
       completion(.failure(error))
    }
    else {
       completion(.success(()))
    }
}

private func processData(completion: () -> ()) {
    //All your stuff
    completion()
}