错误：未声明的标签

Question

我正在关注本教程：link

但是，在这一部分：

- (void)locationManager:(CLLocationManager *)manager didUpdateToLocation:(CLLocation *)newLocation fromLocation:(CLLocation *)oldLocation
{
    NSLog(@"didUpdateToLocation: %@", newLocation);
    CLLocation *currentLocation = newLocation;

    if (currentLocation != nil) {
        longitudeLabel.text = [NSString stringWithFormat:@"%.8f", currentLocation.coordinate.longitude];
        latitudeLabel.text = [NSString stringWithFormat:@"%.8f", currentLocation.coordinate.latitude];
    }
}

Xcode表示未声明longitudeLabel和latitudeLabel。但它们在ViewController.h中声明如下：

@property (weak, nonatomic) IBOutlet UILabel *latitudeLabel;
@property (weak, nonatomic) IBOutlet UILabel *longitudeLabel;

有人知道如何解决这个问题吗？