Return value from inside block (Objective-C)

Question

I've been trying to get a value from inside a block for a few hours now, I can't understand how to use the handlers on completion and literally everything.
Here's my code:

+ (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]);
    }];

}

So I'm trying to NSLog the returnString from the downloadUserID method. I first tried returning, then I realized you can't do a return from inside a block. So now I've been trying to do it with the :(void(^)(NSString *result))handler to try and access it from another class method.

So I'm calling downloadUserID from the getUserID method, and trying to log the returnString string. It just keeps going to nil. It just prints From get userID and nothing else.

How do I access the returnString that's inside the block of the downloadUserID method?

Answer 1

You can do this if you write such a wrapper. In this situation, you need a while loop that will wait for a response from the block.

Method which shoud return value of enum

- (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;
}

Method which calls delegate's method with block. And waits answer from it. Here I use while loop. Just check every 0.5sec answer from block

- (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;
}

Delegate's method in ViewController

- (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];

    });
}

Answer 2

The problem is not the block itself, the problem is realizing that the block is executed asynchronously.

In your code, at the time you call handler(returnString); the block is probably still executing on another thread, so there's no way you can catch the value at this point.

Probably what you want to do is move that line inside the block (probably at the end, before the closing braces).