Question

I have some concurrent operation for the data processing. During the processing I need to retrieve the reverse geocoding for the location. It is known that - (void)reverseGeocodeLocation:(CLLocation *)location completionHandler:(CLGeocodeCompletionHandler)completionHandler also performs geocoding request in background thread and returns immediately after the call. When geocoded finishes request it executes the completion handler on the main thread. How can I block my concurrent operation until the geocoder retrieves the result?

__block CLPlacemark *_placemark

- (NSDictionary *)performDataProcessingInCustomThread
{
    NSMutableDictionary *dict = [NSMutableDictionary alloc] initWithCapacity:1];
    // some operations

    CLLocation *location = [[CLLocation alloc] initWithLatitude:40.7 longitude:-74.0];
    [self proceedReverseGeocoding:location];

    // wait until the geocoder request completes

    if (_placemark) {
        [dict setValue:_placemark.addressDictionary forKey:@"AddressDictionary"];

    return dict;
}

- (void)proceedReverseGeocoding:(CLLocation *)location
{
    CLGeocoder *geocoder = [[CLGeocoder alloc] init];
    [geocoder reverseGeocodeLocation:location completionHandler:^(NSArray *placemarks, NSError *error) {
        if ([error code] == noErr) {
            _placemark = placemarks.lastObject;
        }
    }];
}

Answer 1

To achieve this we can use dispatch_semaphore_t. First of all we need to add semaphore to the class:

dispatch_semaphore_t semaphore;

When we are processing data and ready to receive the geocoding data, we should create dispatch semaphore and start to wait a signal:

semaphore = dispatch_semaphore_create(0);
[self proceedReverseGeocoding:location];
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);

At the end of CLGeocodeCompletionHandler all we need is to send the signal to resume the data processing:

dispatch_semaphore_signal(semaphore);

After this the data processing will continue