MKMapView Annotation setCoordinate crashing app - iphone

Question

I am working on a simple GPS-like app. I created a custom object which extends MKAnnotation which also works fine. I can place it etc, but if I put this piece of code [mpm setCoordinate:loc]; in my code, the app opens and freezes while showing the basic gray-grid (not showing any downloaded map and non of my buttons work either then)
Here is my header:
#interface FirstViewController : UIViewController <MKMapViewDelegate>
{
IBOutlet MKMapView *mapView;
MyPlaceMark *mpm;
}
-(void)locationUpdate:(CLLocation *)location;
-(void)locationError:(NSError *)error;
#end
And here is the piece of code where I try to update things:
-(void)locationUpdate:(CLLocation *)location
{
CLLocationCoordinate2D loc = [location coordinate];
[mpm setCoordinate:loc]; // This line messes it up.
[mapView setCenterCoordinate:loc];
if([mapView showsUserLocation] == NO)
[mapView setShowsUserLocation:YES]; // This does not show my position either?
}
If I comment that line out, the app works fine. I need to update the annotation as it will be my marker for showing the users current location. PS: Without that line, it does center my view - so location is a valid/set variable.
My viewDidLoad looks like this:
- (void)viewDidLoad {
mapView = [[MKMapView alloc] initWithFrame:self.view.bounds];
//mapView.showsUserLocation = YES;
[mapView setShowsUserLocation:YES];
mapView.mapType = MKMapTypeStandard;
mapView.delegate = self;
CLLocationCoordinate2D location;
MKCoordinateRegion region;
location.latitude = -33.8;
location.longitude = 18.6;
MKCoordinateSpan span;
span.latitudeDelta = 0.01;
span.longitudeDelta = 0.01;
region.span = span;
region.center = location;
[mapView setRegion:region animated:TRUE];
[mapView regionThatFits:region];
mpm = [[MyPlaceMark alloc] initWithCoordinate:location]; // Creating mpm
[mapView addAnnotation:mpm]; // Adding mpm
[self.view addSubview:mapView];
[super viewDidLoad];
}
Should I re-add mpm after I changed it's location? Or would it just jump to its new location on the map?
So to recap the question: How can I update the custom MKAnnotation's location on my mapview?
Thank you for your time!
EDIT: I think the main reason for the crash is the fact that I do not create a setCoordinate method/function in my custom MKAnnotation. How would I override that but keep it same as original?
The error in log-console:
2011-06-09 14:42:40.377 AeroNav[3706:707] -[MyPlaceMark setCoordinate:]: unrecognized selector sent to instance 0x19e0e0
2011-06-09 14:42:40.493 AeroNav[3706:707] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[MyPlaceMark setCoordinate:]: unrecognized selector sent to instance 0x19e0e0'
...
terminate called after throwing an instance of 'NSException'

Answer 1

[mapView setShowsUserLocation:YES] doesn't update the region that the map shows. You will have to implement the delegate methods didUpdateUserLocation: and call the setRegion:animated: there to move the region to user's location. Until this is called for the first time, the userLocation property of MKMapView is not set.
As for the MKAnnotation thing, MKAnnotation is a protocol. It doesn't have a default implementation. You will have to provide for all the methods and properties that you've agreed to conform to. So declare a property pretty similar to the one in the MKAnnotation protocol.