如何确保在继续执行之前完成异步操作

Question

我正在构建一个iOS应用程序，它的某些代码依赖于从特定任务返回的成功/失败值。此任务涉及来自库的回调。我希望只有在回调返回成功/失败之后才能返回此任务的返回值。但是，由于我编写了顺序代码，即使在回调返回成功/失败之前，也会返回返回值。

我研究了使用模态视图控制器，据我所知，我可以从这个视图控制器执行任务，然后返回代码。

但这也不符合我的要求，因为当启动回调序列的代码被执行时，我不希望显示新的视图控制器。尽管有一个特定的回调需要我提示用户提供信息。我在popover中这样做，并考虑在popover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popover以模式呈现时，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。有没有办法在iOS中做到这一点？

编辑：代码的作用类似于

//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);