Using GCD to Parse KML With Apple's KMLViewer - kml

Question

I'm using Apple's KMLViewer to load a KML file and display it in a MapView. There are over 50,000 lines of coordinates in the KML file which, of course, causes it to load slowly. In an attempt to speed things up, I'm trying to perform the parsing in another thread using GCD.
I have it working reasonably well as far as it is displaying properly and the speed is acceptable. However, I'm getting intermittent runtime errors when loading the map. I suspect it is because the way I have things laid out, the UI is being updated within the GCD block. Everything I'm reading says the UI should be updated in the main thread or else runtime errors can occur which are intermittent and hard to track down. Well, that's what I'm seeing.
The problem is, I can't figure out how to update the UI in the main thread. I'm still new to iOS programming so I'm just throwing things against the wall to see what works. Here is my code, which is basically Apple's KMLViewerViewController.m with some modifications:
#import "KMLViewerViewController.h"
#implementation KMLViewerViewController
- (void)viewDidLoad
{
[super viewDidLoad];
activityIndicator.hidden = TRUE;
dispatch_queue_t myQueue = dispatch_queue_create("My Queue",NULL);
dispatch_async(myQueue, ^{
// Locate the path to the route.kml file in the application's bundle
// and parse it with the KMLParser.
NSString *path = [[NSBundle mainBundle] pathForResource:#"BigMap" ofType:#"kml"];
NSURL *url = [NSURL fileURLWithPath:path];
kmlParser = [[KMLParser alloc] initWithURL:url];
[kmlParser parseKML];
dispatch_async(dispatch_get_main_queue(), ^{
// Update the UI
// Add all of the MKOverlay objects parsed from the KML file to the map.
NSArray *overlays = [kmlParser overlays];
[map addOverlays:overlays];
// Add all of the MKAnnotation objects parsed from the KML file to the map.
NSArray *annotations = [kmlParser points];
[map addAnnotations:annotations];
// Walk the list of overlays and annotations and create a MKMapRect that
// bounds all of them and store it into flyTo.
MKMapRect flyTo = MKMapRectNull;
for (id <MKOverlay> overlay in overlays) {
if (MKMapRectIsNull(flyTo)) {
flyTo = [overlay boundingMapRect];
} else {
flyTo = MKMapRectUnion(flyTo, [overlay boundingMapRect]);
}
}
for (id <MKAnnotation> annotation in annotations) {
MKMapPoint annotationPoint = MKMapPointForCoordinate(annotation.coordinate);
MKMapRect pointRect = MKMapRectMake(annotationPoint.x, annotationPoint.y, 0, 0);
if (MKMapRectIsNull(flyTo)) {
flyTo = pointRect;
} else {
flyTo = MKMapRectUnion(flyTo, pointRect);
}
}
// Position the map so that all overlays and annotations are visible on screen.
map.visibleMapRect = flyTo;
});
});
}
-(void)viewWillAppear:(BOOL)animated
{
[super viewWillAppear:animated];
activityIndicator.hidden = FALSE;
[activityIndicator startAnimating];
}
#pragma mark MKMapViewDelegate
- (MKOverlayView *)mapView:(MKMapView *)mapView viewForOverlay:(id <MKOverlay>)overlay
{
return [kmlParser viewForOverlay:overlay];
}
- (MKAnnotationView *)mapView:(MKMapView *)mapView viewForAnnotation:(id <MKAnnotation>)annotation
{
return [kmlParser viewForAnnotation:annotation];
}
- (void)mapViewDidFinishLoadingMap:(MKMapView *)mapView
{
[activityIndicator stopAnimating];
activityIndicator.hidden = TRUE;
}
#end
Suggestions?