等待异步方法在for循环中完成

Question

我有一个包含三个异步方法的for循环,我希望在这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];
}

你有任何技术可以得到这个结果吗？非常感谢你.

Answer 1

一种GCD方法是使用dispatch_group.因此,在启动异步任务之前,调用dispatch_group_enter,然后在异步任务完成时调用dispatch_group_leave,然后可以创建dispatch_group_notify在异步任务完成时调用的任务.您可以将此与完成块模式结合(无论如何,这对于异步方法都是一个好主意):

1. 如果getInformations,getExperiences以及getEducation它们本身,所有的异步方法,您首先需要的是某种机制来知道他们在做的时候.一种常见的解决方案是为每个实现完成块模式.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,并getEducation也.

2. 然后,您可以使用调度组通知您这三个请求中的每个请求何时完成,并在发生getUserInfo时调用完成块:

   // 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];
       });
   }
   

两个最后的想法:

1. 概述了所有这些,我必须承认我可能会将这些请求包装在并发/异步自定义NSOperation子类中,而不是使用调度组.请参阅" 并发编程指南 "的"配置并发执行操作"部分.这是对代码进行更彻底的重构,所以我不会在这里解决这个问题,但是它可以让你限制同时运行的这些请求的数量,从而减少潜在的超时问题.

2. 我不知道有多少这些用户请求正在进行,但您可能需要考虑在用户信息进入时更新UI,而不是等待所有内容完成.这又是对代码进行更彻底的重构,但可能会导致更具响应性的内容.