如何在继续执行之前确保完成异步操作

Question

我正在构建一个 iOS 应用程序，其代码的某些部分依赖于从特定任务返回的成功/失败值。这些任务涉及来自库的回调。我希望仅在回调返回成功/失败后才返回此任务的返回值。但是由于我编写了一个顺序代码，因此甚至在回调返回成功/失败之前就返回了返回值。

我研究过使用模态视图控制器，据我了解，我可以让任务从这个视图控制器执行，然后返回代码。

但这也不符合我的要求，因为当执行启动回调序列的代码时，我不希望显示新的视图控制器。虽然有一定的回调需要我提示用户信息。我在弹出框中执行此操作，并且我考虑在弹出框模式中制作视图控制器。但是回调仍然是主线程的一部分，当我的弹出框以模态方式呈现时我不会收到它们（？）。

以我目前对这些概念的理解，我不知道如何继续。在 iOS 中有没有办法做到这一点？

编辑： 代码做了这样的事情

//In CustomTableViewController

-(void) someFunc
{
   ENUM_NAME code = [TaskController startTheTask:args];
   if(code == SUCCEEDED)
   {
      //Do Something
   }
   if(code == FAILED)
   {
      //Do Something Else
   }
}

//In TaskController

-(ENUM_NAME) startTheTask:args
{
   startWorkflow(args); //This function registers callback function with the library. 
   return finalCode; //This is returned even before it is set to SUCCEEDED/FAILED
}

-(void) onCallback:params
{
    MSG_TYPE msg = [params getMsg];

    if(msg == TASK_FAILED)
       finalCode = FAILED;
    if(msg == TASK_SUCCEEDED)
       finalCode = SUCCEEDED;
    if(msg == TASK_SHOW_PROMPT)
    {  
       [PopOverController showPopOver];
    }
}

-(void) onUserInfoAdded
{
    //This is called when Confirm is clicked in the popover
    continueWorkflow(params); //asks for the next callback to happen
}

-(void) onCancleClicked
{
    //This is called when Popover is dismissed without entering Info
    cancleWorkflow(params); //asks for result of the workflow through callback
}

Answer 1

您可以使用 GCD。例如：

    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_group_t group = dispatch_group_create();

    dispatch_group_enter(group);
    dispatch_group_async(group, queue, ^{           
         //put one process here
         dispatch_group_leave(group);  //when done
    });

    dispatch_group_enter(group);
    dispatch_group_async(group, queue, ^{           
         //put another process here
         dispatch_group_leave(group); //when done
    });

    // All updates finished
    dispatch_group_notify(group, dispatch_get_main_queue(), ^{
         // add last steps here after all processess are finished
    });

    dispatch_release(group);

Answer 2

您可以使用信号量来延迟执行，直到块返回：

__block dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
__block NSData *dataFromTheBlock = nil;
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    // block implementation
    // dataFromTheBlock = some data;
    dispatch_semaphore_signal(semaphore);
});
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);