只有在方法完成后才继续循环 [英] only continue loop if method has finished

问题描述

我有一个 for / in 循环如下：

for(NSString *paymentId in success){
   [self getPaymentDetails:paymentId];
}

方法 getPaymentDetails 是异步的。如果方法已完成，如何创建完成块以仅继续for / in循环？

The method getPaymentDetails is asynchronous. How do I create a completion block to only continue the for/in loop if the method has finished?

• 详细信息

方法 getPaymentDetails 看起来像这样：

-(void)getPaymentDetails:(NSString *)paymentId{

  PFUser *currentUser = [PFUser currentUser];
  [PFCloud callFunctionInBackground:@"getpaymentdetails"
                     withParameters:@{@"objectid": paymentId, @"userid": currentUser.objectId}
                              block:^(NSDictionary *success, NSError *error) {
                                if(success){
                                  NSDictionary *payment = success;
                                  NSString *amount = [payment objectForKey:@"amount"];
                                  if (![amount isKindOfClass:[NSNull class]]) {
                                    [self.amountArray addObject:amount];
                                  }
                                  else {
                                    [self.amountArray addObject:@""];
                                  }

                                  NSString *currency = [payment objectForKey:@"currency"];
                                  if (![currency isKindOfClass:[NSNull class]]) {
                                    [self.currencyArray addObject:currency];
                                  }
                                  else {
                                    [self.currencyArray addObject:@""];
                                  }

                                  [self.tableView reloadData];

                                } else{

                                  NSLog(@"Error logged getpaymentdetails: %@", error);
                                }
                          }];

}

以及货币已存储在数组中。或者换句话说：当代码块到达特定 paymentId

The definition of "finished" is hereby defined when the amount as well as the currency has been stored in the array. Or in other words: when the code block has reached the end of the method for that specific paymentId

推荐答案

您可以使用信号量进行这种同步。信号量是并发的基本构建块，并且提供非忙等待。 GCD通过 dispatch_semaphore-create ， dispatch_semaphore_signal 和 dispatch_semaphore_wait 。

You can use semaphores for this kind of synchronisation. Semaphores are a basic building block in concurrency and provide among other things non-busy waiting. GCD provides semaphores through dispatch_semaphore-create, dispatch_semaphore_signal and dispatch_semaphore_wait.

在非常一般的大纲中，首先创建一个信号量：

In very general outline first create a semaphore:

dispatch_semaphore_t sema = dispatch_semaphore_create(0);

然后在循环中等待此信号量：

then in your loop wait on this semaphore:

for(NSString *paymentId in success)
{
   [self getPaymentDetails:paymentId];
   dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER); // wait for call to signal completion
}

背景块信号完成：

dispatch_semaphore_signal(sema);

有关API的更多详细信息，请参阅手册（ man command），为信号量找到一本书（或互联网）。

For more details on the API see the manual (man command), for semaphores find a book (or the internet).

HTH

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