只有在完成所有同步请求后才发出信令通知

Question

当所有异步请求都完成时收到通知。

我在下面的循环中多次呼叫registerTrack。我只想触发syncGroup.notify时全部请求已完成。从看这个代码，在registerTrack每次成功完成之后，它会去notify。只有在所有registerTrack操作完成后，我如何才能使其显示notify？

我的代码如下：

var fail = false 
    let syncGroup = DispatchGroup() //used so we can determine if all the register requests have finished 

    for track in unsyncedTracks { 
     syncGroup.enter() 

     if fail { 
      syncGroup.leave() 
      break //might save an aync request from getting executed on failure 
     } 

     registerTrack(track: track, withCompletion: { (success: Bool) ->() in 

      if success { 
       self.debug.log(tag: "RecordViewController", content: "Registered track: \(track.name)") 
       syncGroup.leave() 
      } 
      else { 
       fail = true 
       syncGroup.leave() 
      } 
     }) 
    } 

    //all requests complete 
    syncGroup.notify(queue: .main) { 
     if fail { 
      complete(false) 
     } 
     else { 
      self.debug.log(tag: "RecordViewController", content: "Finished registering all unsynced tracks") 
      complete(true) 
     } 
    } 

编辑：

基于使用GCD并发队列的建议，我改变了我的代码：

var failed = false 
    //work item to loop through and call registerTrack on each track 
    let registerTracksWorkItem = DispatchWorkItem { 
     for track in unsyncedTracks { 
      if failed { //exit early on failure, potentially save a network request 
       break 
      } 

      self.registerTrack(track: track, withCompletion: { (success: Bool) ->() in 

       if success { 
        self.debug.log(tag: "RecordViewController", content: "Registered track: \(track.name)") 
       } 
       else { 
        failed = true 
       } 
      }) 
     } 
    } 

    //handler for when all registerTrack calls are complete 
    registerTracksWorkItem.notify(queue: DispatchQueue.main) { 
     if failed { 
      self.debug.log(tag: "RecordViewController", content: "At least one registerTrack call failed") 
      complete(false) 
     } 
     else { 
      self.debug.log(tag: "RecordViewController", content: "Finished registering all unsynced tracks") 
      complete(true) 
     } 
    } 

    //execute the work item 
    let queue = DispatchQueue.global() 
    queue.async(execute: registerTracksWorkItem) 

此代码不等待registerTrack完成，而是执行它们并调用notify事件。 :(

编辑3：

所以这个作品它使用一个计数器进行检查，看其是否在最后unsyncedTracks对象

var fail = false 
    let syncGroup = DispatchGroup() //used so we can determine if all the register requests have finished 

    //used to make sure notify is only triggered when all registerTracks are complete 
    let unsyncedTracksCount = unsyncedTracks.count 
    var completeCounter = 0 

    syncGroup.enter() 
    for track in unsyncedTracks { 

     registerTrack(track: track, withCompletion: { (success: Bool) ->() in 

      if success { 
       self.debug.log(tag: "RecordViewController", content: "Registered track: \(track.name)") 
      } 
      else { 
       fail = true 
      } 

      completeCounter = completeCounter + 1 
      if completeCounter == unsyncedTracksCount { 
       syncGroup.leave() 
      } 
     }) 
    } 

    //all requests complete 
    syncGroup.notify(queue: .main) { 
     if fail { 
      complete(false) 
     } 
     else { 
      self.debug.log(tag: "RecordViewController", content: "Finished registering all unsynced tracks") 
      complete(true) 
     } 
    } 

我能做到这一点（它也出现工作。 ）？我加了，如果失败，leave()如果。

var fail = false 
    let syncGroup = DispatchGroup() //used so we can determine if all the register requests have finished 

    //used to make sure notify is only triggered when all registerTracks are complete 
    let unsyncedTracksCount = unsyncedTracks.count 
    var completeCounter = 0 

    syncGroup.enter() 
    for track in unsyncedTracks { 
     if fail { //might save an aync request from getting executed on failure 
      syncGroup.leave() 
      break 
     } 

     registerTrack(track: track, withCompletion: { (success: Bool) ->() in 

      if success { 
       self.debug.log(tag: "RecordViewController", content: "Registered track: \(track.name)") 
      } 
      else { 
       fail = true 
      } 

      completeCounter = completeCounter + 1 
      if completeCounter == unsyncedTracksCount { 
       syncGroup.leave() 
      } 
     }) 
    } 

    //all requests complete 
    syncGroup.notify(queue: .main) { 
     if fail { 
      complete(false) 
     } 
     else { 
      self.debug.log(tag: "RecordViewController", content: "Finished registering all unsynced tracks") 
      complete(true) 
     } 
    } 

Answer 1

那么，问题是关系到你的fail变量，for循环结构。你一个在循环执行之前将fail指定为false。当您的代码进入循环并注册.enter()时，它会检查if fail条件，并在fail为true时调用.leave()。基本上，当其中一个呼叫registerTrack()将fail设置为true您的代码处于未确定状态，因为registerTrack()是异步的，并且您不知道哪个.enter()调用将标志设置为true。 当.enter() count等于.leave() count（例如，两行失败）时，CGD决定您的队列已完成，并且“过早地”调用.notify()。

我建议您删除标记fail（代码将按预期工作，但将对所有unsyncedTracks执行registerTrack()）并重新考虑您的解决方案和逻辑。例如，DispatchWorkItem.cancel()可以解决您的问题。