错误：尝试调用实例方法时使用未声明的标识符

Question

我知道这很简单，我忽略了但我不能为我的生活弄清楚出了什么问题。任何人都可以对这种情况有所了解吗？非常感谢。

#import "OnlineLayerViewController.h"

#import "MapBox.h"

#define kNormalMapID @"ianbruton.map-8uhua25t"
#define kRetinaMapID @"ianbruton.map-8uhua25t"

@implementation OnlineLayerViewController

- (void)viewDidLoad
{
    [super viewDidLoad];

    RMMapBoxSource *onlineSource = [[RMMapBoxSource alloc] initWithMapID:(([[UIScreen mainScreen] scale] > 1.0) ? kRetinaMapID : kNormalMapID)];

    RMMapView *mapView = [[RMMapView alloc] initWithFrame:self.view.bounds andTilesource:onlineSource];

    //THIS IS WHERE I'M GETTING THE ERROR
    [showDevicePosition : mapView];

    mapView.zoom = 14;

    mapView.autoresizingMask = UIViewAutoresizingFlexibleHeight | UIViewAutoresizingFlexibleWidth;

    //makes place names legible on retina devices
    mapView.adjustTilesForRetinaDisplay = YES;

    //Creates a point to draw
    RMPointAnnotation *annotation = [[RMPointAnnotation alloc] initWithMapView:mapView
                                                                  coordinate:mapView.centerCoordinate
                                                                  andTitle:@"Hello, world!"];

    //Draws point
    [mapView addAnnotation:annotation];

    [self.view addSubview:mapView];
}

//THIS IS THE METHOD I'M TRYING TO CALL
- (void)showDevicePosition : (RMMapView *) mapView
{
    mapView.centerCoordinate = mapView.userLocation.location.coordinate;
}

@end