如何使功能的特定部分在iOS中等待

Question

我正在嘗試執行共享操作，在此我使用異步塊調用函數，但是在下一個if語句中，我需要獲取在塊中完成的值才能繼續。 這是我的代碼，將突出顯示更多細節。 我聽說過NSLock並嘗試使用它，但是它沒有用，可能是我在做鎖操作，我對鎖不太熟悉。

-(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

您可以使用信號量（其設計目的是使一個線程可以等待來自另一個線程的信號，而無需使用鎖（僅用於確保不同時訪問某些共享資源），而無需使用鎖）。

因此，您應該創建一個信號燈：

dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);

警報視圖完成塊將發出該信號燈信號：

dispatch_semaphore_signal(semaphore);

以及您要等待該信號的位置（在performSomeAnotherAsynchronousOperation之前）：

dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);

我對您的代碼進行了一些調整，但最值得注意的是，通過確保在后台隊列中完成dispatch_semaphore_wait來對其進行更改，以使其不會阻塞主隊列（您永遠不想這樣做）。 還要注意， dispatch_semaphore_signal不在if語句內。 結果是：

- (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，您不應使用諸如信號燈之類的任何阻止機制（一定不在主隊列中），而應該讓警報視圖的完成塊直接由進程本身直接啟動下一步。 我了解您說這在您的方案中不切實際，但通常是處理這些方案的正確方法。

Answer 2

您可以使用延遲塊

 dispatch_time_t delayTime = dispatch_time(DISPATCH_TIME_NOW, 0.01 * NSEC_PER_SEC);

    dispatch_after(delayTime, dispatch_get_main_queue(), ^(void){

    })