如何在Swift中創建自定義完成塊

Question

我正在嘗試做一些後期處理

a。一旦我的一個異步功能已經完成

b。一旦我所有的異步功能都完成了。

不幸的是，我在下面的代碼中遇到了競爭條件。

func foo(stuff: AnArrayOfObjects, completed: (NSError?)->()) { 
    // STEP 1 OF CREATING AN OVERALL COMPLETION BLOCK: Create a dispatch group. 
    let loadServiceGroup: dispatch_group_t = dispatch_group_create() 

    // Define errors to be processed when everything is complete. 
    // One error per service; in this example we'll have two 
    let configError: NSError? = nil 
    let preferenceError: NSError? = nil 

    // some more preprocessing/variable declarations here. E.g.: 
    var counter = 0 

    // STEP 2 OF CREATING AN OVERALL COMPLETION BLOCK: Adding tasks to a dispatch group 
    dispatch_group_enter(loadServiceGroup) 

    // here i may start MULTPILE functions that are asynchronous. For example: 
    for i in stuff { 
     estore.fetchRemindersMatchingPredicate(remindersPredicate) { 
      // MARK: Begininning of thread 

      // does something here. E.g., count the elements: 
      counter += 1 

      // update the UI      
      dispatch_async(dispatch_get_main_queue()) { 
       self.sendChangedNotification()  // reloads the tableview. 
       // WARNING: I CAN'T JUST SHOW THE counter RESULTS HERE BECAUSE IT MIGHT NOT BE DONE YET. IT IS ASYNCHRONOUS, IT MIGHT STILL BE RUNNING. 
      } 
     } 

     // STEP 3 OF CREATING AN OVERALL COMPLETION BLOCK: Leave dispatch group. This must be done at the end of the completion block. 
     dispatch_group_leave(loadServiceGroup) 

     // MARK: End of thread 
    } 

    // STEP 4 OF CREATING AN OVERALL COMPLETION BLOCK: Acting when the group is finished 
    dispatch_group_notify(loadServiceGroup, dispatch_get_main_queue(), { 
     // do something when asychrounous call block (above) has finished running. E.g.: 
     print("number of elements: \(counter)") 

     // Assess any errors 
     var overallError: NSError? = nil; 

     if configError != nil || preferenceError != nil { 
      // Either make a new error or assign one of them to the overall error. 
      overallError = configError ?? preferenceError 
     } 

     // Call the completed function passed to foo. This will contain additional stuff that I want executed in the end. 
     completed(overallError) 
    }) 
} 

Answer 1

說實話，我並不完全明白你在做什麼。不過，我總是使用一個計數器變量來確定是否所有的異步函數都完成了。

func foo(completionHandler: (success: Bool) -> Void) { 
    var numberOfTasks = 0 
    for data in datasource { 
     // do something preparing 
     numberOfTasks += 1 
    } 

    var numberOfDones = 0 
    objc_sync_enter(numberOfDones) 
    data.foo(completionHandler:(){ 
     // do something handling outcome 
     numberOfDones += 1 
     if numberOfDones == numberOfTasks { 
      completionHandler(true) 
     } 
    }) 
    objc_sync_exit(numberOfDones) 
} 

的機制，我們知道要完成的任務的總數，以及每個任務中，我們可以捕捉完成事件，因此我們增加了numberOfDones。因此，無論何時numberOfDones == numberOfTasks，我們都會知道這是最後一個，並且已經完成。

根據你的代碼，我試圖將這個想法應用於它。

func foo(stuff: AnArrayOfObjects, completed: ([NSError]?)->()) { 
    // STEP 1 OF CREATING AN OVERALL COMPLETION BLOCK: Create a dispatch group. 
    let loadServiceGroup: dispatch_group_t = dispatch_group_create() 

    // Define errors to be processed when everything is complete. 
    // One error per service; in this example we'll have two 
    let configError: NSError? = nil 
    let preferenceError: NSError? = nil 

    // some more preprocessing/variable declarations here. E.g.: 
    var counter = 0 

    // STEP 2 OF CREATING AN OVERALL COMPLETION BLOCK: Adding tasks to a dispatch group 
    dispatch_group_enter(loadServiceGroup) 

    // here i may start MULTPILE functions that are asynchronous. For example: 
    for i in stuff { 
     estore.fetchRemindersMatchingPredicate(remindersPredicate) { 
      // MARK: Begininning of thread 

      // does something here. E.g., count the elements: 
      counter += 1 

      // update the UI 
      dispatch_async(dispatch_get_main_queue()) { 
       self.sendChangedNotification()  // reloads the tableview. 
       // WARNING: I CAN'T JUST SHOW THE counter RESULTS HERE BECAUSE IT MIGHT NOT BE DONE YET. IT IS ASYNCHRONOUS, IT MIGHT STILL BE RUNNING. 
      } 
     } 

     // STEP 3 OF CREATING AN OVERALL COMPLETION BLOCK: Leave dispatch group. This must be done at the end of the completion block. 
     dispatch_group_leave(loadServiceGroup) 

     // MARK: End of thread 
    } 

    var numberOfDones = 0 
    var errors = [NSError]() 

    // STEP 4 OF CREATING AN OVERALL COMPLETION BLOCK: Acting when the group is finished 
    objc_sync_enter(numberOfDones) 
    dispatch_group_notify(loadServiceGroup, dispatch_get_main_queue(), { 
     // do something when asychrounous call block (above) has finished running. E.g.: 
     print("number of elements: \(counter)") 

     // Assess any errors 
     var overallError: NSError? = nil; 

     if configError != nil || preferenceError != nil { 
      // Either make a new error or assign one of them to the overall error. 
      errors.append(configError ?? preferenceError!) 
     } 

     numberOfDones += 1 
     if numberOfDones == counter { 
      // Call the completed function passed to foo. This will contain additional stuff that I want executed in the end. 
      completed(errors) 
     } 
    }) 
    objc_sync_exit(numberOfDones) 
} 

編輯：

由於@CouchDeveloper評論我的答案。我制定了另一個通過同步計數器變量使線程安全的解決方案。答案是更新，下面是解決

https://gist.github.com/qiuyujx/7173ea663308cc03f07e8a5c09cf4cba