ios - 如何在iOS中使特定部分功能等待

Question

我正在尝试执行共享操作，在该操作中我使用异步 block 调用函数，但在我的下一个 if 语句中，我需要获取在 block 中完成的值才能继续。这是我的代码，它将突出显示更多细节。我听说过 NSLock 并尝试使用它，但它没有用，可能是我在做一些锁，我对锁不太熟悉。

-(void) shareOperation
{
__block NSString *resultText;
BOOL continueSharing;
 NSLock *conditionLock=[[NSLock alloc] init];
         dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
        dispatch_async(queue, ^{
            [conditionLock lock];
        [self performSomeAsynchronousOperation completionBlock:^(NSError *tError, bool status)
         {
             dispatch_async(dispatch_get_main_queue(),
                            ^{

                                if (status)
                                {
                                    resultText = @"Operation completed. Would you like to continue?";

                                }
                                else
                                {
                                    resultText = @" Operation failed. Would you still like to continue?";
                                }
                                UIAlertView *result = [UIAlertView alertViewWithTitle:nil message:resultText cancelButtonTitle:@"Cancel" otherButtonTitles:[NSArray arrayWithObjects:@"OK",nil] onDismiss:^(int buttonIndex)
                                                       {
                                                           NSLog(@"selected button index: %d",buttonIndex);
                                                           if (buttonIndex == 0)
                                                           {
                                                               continueSharing=YES;
                                                               [conditionLock unlock];
                                                               NSLog(@"We are continuing sharing :)");
                                                           }
                                                       }onCancel:^{
                                                           continueSharing=NO;
                                                            [conditionLock unlock];
                                                           NSLog(@"cancelled");
                                                       }]; [result show];

                            });
         }];
        });
    }


    //should continue only after earlier if block finishes, ie after getting the continueSharing value

    if (continueSharing)
    {
        [self performSomeAnotherAsynchronousOperation];
    }

}

Answer 1

您可以使用信号量(其设计目的是让一个线程可以等待来自另一个线程的信号，这里需要)，而不是使用锁(它只是为了确保不会同时访问某些共享资源)。

因此，您应该创建一个信号量:

dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);

警报 View 完成 block 将发出该信号量的信号:

dispatch_semaphore_signal(semaphore);

以及您要在哪里等待该信号(在 performSomeAnotherAsynchronousOperation 之前):

dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);

我稍微调整了您的代码，但最值得注意的是，通过确保 dispatch_semaphore_wait 对其进行了更改，使其不会阻塞主队列(您永远不想这样做)在后台队列中完成。另请注意 dispatch_semaphore_signal不在 if 内陈述。这导致:

- (void)shareOperation
{
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        __block BOOL continueSharing = NO;
        dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);

        [self performSomeAsynchronousOperationWithCompletionBlock:^(NSError *tError, bool status){
            dispatch_async(dispatch_get_main_queue(),^{
                NSString *resultText;

                if (status)
                    resultText = @"Operation completed. Would you like to continue?";
                else
                    resultText = @"Operation failed. Would you still like to continue?";

                UIAlertView *alertView = [UIAlertView alertViewWithTitle:nil message:resultText cancelButtonTitle:@"Cancel" otherButtonTitles:@[@"OK"] onDismiss:^(int buttonIndex) {
                    NSLog(@"selected button index: %d",buttonIndex);
                    if (buttonIndex == 0) {
                        continueSharing = YES;
                        NSLog(@"We are continuing sharing :)");
                    }
                    dispatch_semaphore_signal(semaphore);
                } onCancel:^{
                    dispatch_semaphore_signal(semaphore);
                    NSLog(@"cancelled");
                }];

                [alertView show];
            });
        }];

        // should continue only after earlier if block finishes, ie after getting the continueSharing value

        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);

        if (continueSharing)
            [self performSomeAnotherAsynchronousOperation];
    });
}

更好的是，您不应该使用任何阻塞机制，如信号量(当然不在主队列上)，而应该简单地让警报 View 的完成 block 直接启动流程的下一步。我了解您说这在您的场景中不实用，但通常是处理这些场景的正确方法。