I’ve been developing an iPhone app on my job at INRIX that uses MapKit and have run into a number of interesting issues.  One in particular baffled me for some time, and it turns out to be (or at least appear to be) a bug in MKMapView.

The app displays traffic on a map, and on iOS4 it uses an overlay to display the traffic tiles.  As the user moves around or zooms in and out, the traffic tile image must be updated, which means removing the overlay and adding a new one.  This appears to work at most zoom levels until you start zooming in closer.  In those cases, the traffic tile would appear as a big blob – very ugly.

Searching around the net I found a few people that had experienced the same problem.  The work-around is to add a dummy transparent overly.  Sounds silly, but it works.

Here are some code snippets that work for me:

- (void) loadView
{
    // ...
    _mapView = [[MKMapView alloc] init];
    _mapView.delegate = self;
    // ...
    [self.view addSubview: _mapView];
    // ...
    BOOL isOS4orAbove = ([[[UIDevice currentDevice] systemVersion] floatValue] >= 4.0f);
    if (isOS4orAbove)
    {
        // Add a dummy overlay to fix a bug in MKMapView where overlays don't get removed properly.
        // https://devforums.apple.com/message/307581
        MKPolyline * dummyOverlay = [[MKPolyline alloc] init];
        [_mapView addOverlay: dummyOverlay];
        [dummyOverlay release];
    }
    // ...
}

- (MKOverlayView *) mapView: (MKMapView *) mapView viewForOverlay: (id <MKOverlay>) overlay
{
    if ([overlay isKindOfClass: [TileOverlay class]])
    {
        // Return view for traffic tile overlay.
    }
    else if ([overlay isKindOfClass: [MKPolyline class]])
    {
        // This is for a dummy overlay to work around a problem with overlays
        // not getting removed by the map view even though we asked for it to
        // be removed.
        MKOverlayView * dummyView = [[[MKOverlayView alloc] initWithOverlay: overlay] autorelease];
        dummyView.alpha = 0.0;
        return dummyView;
    }
    else
    {
        return nil;
    }
}

The forum post that led me to this solution can be found here.  This appears to be a bug in MapKit.  I’d be interested in hearing if others have encountered this problem and how you solved it.