Question

我一直试图从里面一个块中获取一个值几个小时，我无法理解如何在完成时使用处理程序以及几乎所有内容。
这是我的代码：

+ (void)downloadUserID:(void(^)(NSString *result))handler {
    //Now redirect to assignments page

    __block NSMutableString *returnString = [[NSMutableString alloc] init]; //'__block' so that it has a direct connection to both scopes, in the method AND in the block


    NSURL *homeURL = [NSURL URLWithString:@"https://mistar.oakland.k12.mi.us/novi/StudentPortal/Home/PortalMainPage"];
    NSMutableURLRequest *requestHome = [[NSMutableURLRequest alloc] initWithURL:homeURL];
    [requestHome setHTTPMethod:@"GET"]; // this looks like GET request, not POST

    [NSURLConnection sendAsynchronousRequest:requestHome queue:[NSOperationQueue mainQueue] completionHandler:^(NSURLResponse *homeResponse, NSData *homeData, NSError *homeError) {
         // do whatever with the data...and errors
         if ([homeData length] > 0 && homeError == nil) {
             NSError *parseError;
             NSDictionary *responseJSON = [NSJSONSerialization JSONObjectWithData:homeData options:0 error:&parseError];
             if (responseJSON) {
                 // the response was JSON and we successfully decoded it

                 //NSLog(@"Response was = %@", responseJSON);
             } else {
                 // the response was not JSON, so let's see what it was so we can diagnose the issue

                 returnString = (@"Response was not JSON (from home), it was = %@", [[NSMutableString alloc] initWithData:homeData encoding:NSUTF8StringEncoding]);
                 //NSLog(returnString);
             }
         }
         else {
             //NSLog(@"error: %@", homeError);
         }
    }];
    //NSLog(@"myResult: %@", [[NSString alloc] initWithData:myResult encoding:NSUTF8StringEncoding]);
    handler(returnString);
}

- (void)getUserID {
    [TClient downloadUserID:^(NSString *getIt){
        NSLog([NSString stringWithFormat:@"From get userID %@", getIt]);
    }];

}

所以我试图从returnString方法NSLog downloadUserID。我第一次尝试返回，然后我意识到你不能从一个块内返回。所以现在我一直试图用:(void(^)(NSString *result))handler来尝试从另一个类方法中访问它。

所以我从downloadUserID方法调用getUserID，并尝试记录returnString字符串。它只是持续到零。它只打印From get userID而没有别的。

如何访问returnString方法块内的downloadUserID？

Answer 1

问题不在于block本身，问题在于意识到该块是异步执行的。

在您的代码中，当您调用handler(returnString);时，该块可能仍在另一个线程上执行，因此此时无法捕获该值。

你想要做的就是在阻挡区内移动该线（可能在结束时，在结束括号之前）。

Answer 2

如果编写这样的包装器，则可以执行此操作。在这种情况下，您需要一个while循环来等待该块的响应。

应该返回枚举值的方法

- (RXCM_TroubleTypes) logic_getEnumValueOfCurrentCacheProblem
{
    RXCM_TroubleTypes result = RXCM_HaveNotTrouble;
    NetworkStatus statusConnection = [self network_typeOfInternetConnection];

    RXCM_TypesOfInternetConnection convertedNetStatus = [RXCM convertNetworkStatusTo_TypeOfInternetConnection:statusConnection];


    BOOL isAllowed = [self someMethodWith:convertedNetStatus];
    if (isAllowed){
        return RXCM_HaveNotTrouble;
    }else { 
        return RXCM_Trouble_NotSuitableTypeOfInternetConnection;
    }

   return result;
}

使用block调用委托人方法的方法。并等待答案。在这里我使用while循环。只需检查每隔0.5秒的答案

- (BOOL) isUserPermissioned:(RXCM_TypesOfInternetConnection)newType
{
    __block BOOL isReceivedValueFromBlock = NO;
    __block BOOL result = NO;
    __block BOOL isCalledDelegateMethod = NO;

    dispatch_queue_t aQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0);
    dispatch_sync(aQueue,^{

        while (!isReceivedValueFromBlock) {
            NSLog(@"While");
            if (!isCalledDelegateMethod){
                [self.delegate rxcm_isAllowToContinueDownloadingOnNewTypeOfInternetConnection:newType
                                                                                   completion:^(BOOL isContinueWorkOnNewTypeOfConnection) {
                                                                                       result = isContinueWorkOnNewTypeOfConnection;
                                                                                       isReceivedValueFromBlock = YES;
                                                                                   }];
                isCalledDelegateMethod = YES;
            }
            [NSThread sleepForTimeInterval:0.5];

        }
    });
    return result;
}

ViewController中的委托方法

- (void) rxcm_isAllowToContinueDownloadingOnNewTypeOfInternetConnection:(RXCM_TypesOfInternetConnection)newType
                                                             completion:(void(^)(BOOL isContinueWorkOnNewTypeOfConnection))completion
{
    __weak ViewController* weak = self;

    dispatch_async(dispatch_get_main_queue(), ^{
        UIAlertController *alert = [UIAlertController alertControllerWithTitle:@"Alert"
                                                                       message:@"to continue download on the new type of connection"
                                                                preferredStyle:UIAlertControllerStyleAlert];

        UIAlertAction *ok = [UIAlertAction actionWithTitle:@"YES" style:UIAlertActionStyleDefault handler:^(UIAlertAction * _Nonnull action) {
            completion(YES);
        }];

        UIAlertAction *cancel = [UIAlertAction actionWithTitle:@"NO" style:UIAlertActionStyleCancel handler:^(UIAlertAction * _Nonnull action) {
            completion(NO);
        }];

        [alert addAction:cancel];
        [alert addAction:ok];
        [weak presentViewController:alert animated:YES completion:nil];

    });
}

内部块的返回值（Objective-C）

2 个答案: