等待异步方法在 for 循环中完成

我有一个包含三个异步方法的for循环，我想在这3个异步方法完成后进行一些处理。

 -(void)getAllUsersInformations{
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
           for(User *user in users){
              [self getUserInfo:user];
           }
      //Here, I want to reload the table view for example, after finishing the for loop (executing the whole three methods).
    });
}

-(void)getUserInfo:(User*)user{
     [self getInformations:user];
     [self getExperiences:user];
     [self getEducation:user];
}

你有什么技术可以得到这个结果吗？ 非常感谢。

一种 GCD 方法是使用dispatch_group。因此，在启动异步任务之前，请调用dispatch_group_enter，然后当异步任务完成时，调用dispatch_group_leave，然后您可以创建一个dispatch_group_notify当异步任务完成时将调用它。您可以将其与完成块模式结合起来（无论如何，这对于异步方法来说是一个好主意）：

1. If getInformations, getExperiences and getEducation它们本身都是异步方法，您需要的第一件事是某种机制来知道它们何时完成。一个常见的解决方案是为每个实现一个完成块模式。例如：

   // added completionHandler parameter which will be called when the retrieval
   // of the "informations" is done.
   
   - (void)getInformations:(User*)user completionHandler:(void (^)(void))completionHandler {
       // do whatever you were before, but in the asynchronous task's completion block, call this
       // completionHandler()
       //
       // for example
   
       NSURLRequest *request;
   
       [NSURLConnection sendAsynchronousRequest:request queue:nil completionHandler:^(NSURLResponse *response, NSData *data, NSError *connectionError) {
           // handle the request here
   
           // the important thing is that the completion handler should
           // be called _inside_ the this block
   
           if (completionHandler) {
               completionHandler();
           }
       }];
   }
   

   重复此过程getExperiences and getEducation, too.

2. 然后，您可以使用调度组来通知您这三个请求中的每一个何时完成，并调用完成块getUserInfo当这种情况发生时：

   // added completion handler that will be called only when `getInformations`,
   // `getExperiences` and `getEducation` are all done.
   //
   // this takes advantage of the completion block we added to those three
   // methods above
   
   - (void)getUserInfo:(User*)user completionHandler:(void (^)(void))completionHandler {
       dispatch_group_t group = dispatch_group_create();
   
       // start the three requests
   
       dispatch_group_enter(group);
       [self getInformations:user completionHandler:^{
           dispatch_group_leave(group);
       }];
   
       dispatch_group_enter(group);
       [self getExperiences:user completionHandler:^{
           dispatch_group_leave(group);
       }];
   
       dispatch_group_enter(group);
       [self getEducation:user completionHandler:^{
           dispatch_group_leave(group);
       }];
   
       // this block will be called asynchronously only when the above three are done
   
       dispatch_group_notify(group, dispatch_get_main_queue(), ^{
           if (completionHandler) {
               completionHandler();
           }
       });
   }
   

3. 然后你重复这个过程getAllUsersInformations:

   // call new getUserInfo, using dispatch group to keep track of whether
   // all the requests are done
   
   -(void)getAllUsersInformations {
   
       dispatch_group_t group = dispatch_group_create();
   
       for(User *user in users){
           dispatch_group_enter(group);
   
           [self getUserInfo:user completionHandler:^{
               dispatch_group_leave(group);
           }];
       }
   
       dispatch_group_notify(group, dispatch_get_main_queue(), ^{
           [self.tableView reloadData];
       });
   }
   

最后两个想法：

1. 概述了所有这些之后，我必须承认我可能会将这些请求包装在并发/异步自定义中NSOperation子类而不是使用调度组。请参阅《配置并发执行操作》部分并发编程指南 https://developer.apple.com/library/mac/documentation/General/Conceptual/ConcurrencyProgrammingGuide/OperationObjects/OperationObjects.html#//apple_ref/doc/uid/TP40008091-CH101-SW8。这是对代码的更彻底的重构，因此我不会在这里解决这个问题，但它可以让您限制同时运行的这些请求的数量，从而减轻潜在的超时问题。

2. 我不知道有多少用户请求正在进行，但您可能需要考虑在用户信息传入时更新 UI，而不是等待一切完成。这又是对代码的更彻底的重构，但可能会导致一些感觉更灵敏的东西。