iOS SDK: MapKit MKPolyLine not showing

Question

I'm trying to display a polyline on my map, but the line doesn't show up. I tried a lot of things, but noting seems to work.

I checked the Core Data functions, and it is returning data, so that is not the problem. It must me somewhere in the mappoint creation or the dwawing on the map (I guess). I'm sure it must be a little mistake somewhere, but I can't find it.

My code:

- (void)viewDidLoad
{
    [super viewDidLoad];

    AppDelegate *appDelegate = [[UIApplication sharedApplication] delegate];
    mapView.delegate = self;
}

- (void)createLine
{
    AppDelegate *appDelegate = [[UIApplication sharedApplication] delegate];
    NSManagedObjectContext *context = [appDelegate managedObjectContext];

    NSEntityDescription *entityDescription = [NSEntityDescription entityForName:@"Logs" inManagedObjectContext:context];
    NSFetchRequest *request = [[NSFetchRequest alloc] init];
    [request setEntity:entityDescription];

    NSError *error;
    NSArray *logs = [context executeFetchRequest:request error:&error];

    int logsCount = [logs count];
    MKMapPoint points[logsCount];

    // loop logs
    for (int i = 0; i < logsCount; i++)
    {
        MKMapPoint point;
        point = MKMapPointMake([[[logs objectAtIndex:i] valueForKey:@"lat"] doubleValue], [[[logs objectAtIndex:i] valueForKey:@"lng"] doubleValue]);

        points[i] = point;
    }

    MKPolyline *routeLine = [MKPolyline polylineWithPoints:points count:logsCount];
    [mapView addOverlay:routeLine];
}

- (MKOverlayView *)mapView:(MKMapView *)mapView viewForOverlay:(id <MKOverlay>)overlay
{
    MKOverlayView *mapOverlayView = [[MKOverlayView alloc] initWithOverlay:overlay];
    return mapOverlayView;
}

Answer 1

There are two issues with the code shown:

1. The polyline is being created using MKMapPoints which are incorrectly set to latitude/longitude values. An MKMapPoint is not degrees of latitude/longitude. It is an x/y transformation of a lat/long on the flat map projection. Either convert the latitude/longitude values to MKMapPoints using MKMapPointForCoordinate or just use CLLocationCoordinate2D instead. Just using CLLocationCoordinate2D when you have the lat/long is much easier to code and understand.
2. In viewForOverlay, the code is creating an empty MKOverlayView which is invisible. Create an MKPolylineView instead (a subclass of MKOverlayView that draws MKPolylines) and set its strokeColor.

For the first issue, use:

• CLLocationCoordinate2D instead of MKMapPoint,
• CLLocationCoordinate2DMake instead of MKMapPointMake,
• and polylineWithCoordinates instead of polylineWithPoints

For the second issue, here's an example:

- (MKOverlayView *)mapView:(MKMapView *)mapView viewForOverlay:(id <MKOverlay>)overlay
{
    if ([overlay isKindOfClass:[MKPolyline class]])
    {
        MKPolylineView *mapOverlayView = [[MKPolylineView alloc] initWithPolyline:overlay];
        //add autorelease if not using ARC
        mapOverlayView.strokeColor = [UIColor redColor];
        mapOverlayView.lineWidth = 2;
        return mapOverlayView;
    }

    return nil;
}

A couple of other things:

• Instead of valueForKey:@"lat", I would use objectForKey:@"lat" (same for @"lng").
• Make sure the map view's delegate is set otherwise the viewForOverlay delegate method will not get called even with all the other changes.