MKOverlay takes long to display from async call

Question

I'm using MKMapView and MKPolyline to display a route shape. When an annotation is placed on my MKMapView and there's another annotation already there, it will asynchronously dispatch a routing call to my directions provider and add the shaped overlay.

Here's the problem. If I use the main thread for the request, it will freeze the MKAnnotation drop while the request is underway, but when it returns, it display the overlay nicely. If instead I dispatch the request to an async queue, there's no freeze in the annotation drop, and when the async calls finalises, it adds the overlay, but it takes a while for the MapView to realise that and actually draw the overlay. Sometimes it takes like 20secs. Here's the code.

//Action handler selector to drop the pin
-(void)dropWayPoint:(WayPoint*)wp prevWayPoint:(WayPoint*)prevWp {
    [self.mapView addWayPoint: wp];
    [self.routeService routeFrom:prevWp to:wp];
}

And the actually route calculation is here:

@implementation RouteService

static dispatch_queue_t queue;

+(RouteService) initRouteService:(id)delegate {
    RouteService *rs = [[RouteService alloc] init];
    //Lots of things happen here, including the queue creation
    ...
    if (!queue) {
        queue = dispatch_queue_create("RouteDispatch.queue", NULL);
    }
    return rs;
}
//Lots of methods...
//then
-(void)routeFrom:(WayPoint*)wp to:(WayPoint*)wpTp {
    dispatch_async(queue, ^{
        //Code here
        DirectionsRouteRequest *rt = [DirectionsRouteRequest buildWithRouteType:@"fastest"];
        [rt addObjectToLocation:[wp coord]];
        [rt addObjectToLocation:[wpTp coord]];
        rt.options.routeType = @"fastest";

        NSLog(@"Dispatching...");

        //Now send the request
        DirectionsResponseType *response = [MapUtil computeDirections:rt];

        Leg *leg = [Leg buildFromResponse:response route: self.route startWayPoint:wp endWayPoint:wpTp];
        MKPolyline *pl = [MapUtil makePolylineWithLocations:[leg routeShape]];
        [self.route.legsHash setObject:pl forKey:leg.legIdStr];

        //Add Overlay here!!!
        [self.mapDeskViewController.mapView addOverlay: pl];
        //Desperately advising map view to redraw itself and show the overlay
        [self.mapDeskViewController.mapView setNeedsDisplay];

        //I can see this being displayed very quickly, meaning 
        NSLog(@"Response, pl_count:%d",pl.pointCount);

        //However it takes a long time after this returns to actually display the overlay.
    });
}

If I take the above code and comment out the async directives, it takes the same time to process it out, provokes an annoying freezing, but the overlay is drawn straight away:

//Lots of methods...
//then
-(void)routeFrom:(WayPoint*)wp to:(WayPoint*)wpTp {
    /* COMMENTED OUT ASYNC
    dispatch_async(queue, ^{
    */
        //Code here
        DirectionsRouteRequest *rt = [DirectionsRouteRequest buildWithRouteType:@"fastest"];
        [rt addObjectToLocation:[wp coord]];
        [rt addObjectToLocation:[wpTp coord]];
        rt.options.routeType = @"fastest";

        NSLog(@"Dispatching...");

        //Now send the request
        DirectionsResponseType *response = [MapUtil computeDirections:rt];

        Leg *leg = [Leg buildFromResponse:response route: self.route startWayPoint:wp endWayPoint:wpTp];
        MKPolyline *pl = [MapUtil makePolylineWithLocations:[leg routeShape]];
        [self.route.legsHash setObject:pl forKey:leg.legIdStr];

        //Add Overlay here!!!
        [self.mapDeskViewController.mapView addOverlay: pl];
        //Desperately advising map view to redraw itself and show the overlay
        [self.mapDeskViewController.mapView setNeedsDisplay];

        //I can see this being displayed very quickly, meaning 
        NSLog(@"Response, pl_count:%d",pl.pointCount);
    /*
    COMMENTED OUT ASYNC
    });
    */
}

Any idea why it takes a while for MKMapView to realise it needs to draw the overlay when it's done asynchronously?

Thanks Aurelio

Answer 1

You need to update the map view (and all UI) on the main thread. So, within your dispatch_async block and after receiving your response:

   // Create another block that gets queued up in the main_queue, a default serial queue
   dispatch_async(dispatch_get_main_queue(), ^{

       Leg *leg = [Leg buildFromResponse:response route: self.route startWayPoint:wp endWayPoint:wpTp];
       MKPolyline *pl = [MapUtil makePolylineWithLocations:[leg routeShape]];
       [self.route.legsHash setObject:pl forKey:leg.legIdStr];

       //Add Overlay here!!!
       [self.mapDeskViewController.mapView addOverlay: pl];
       //Desperately advising map view to redraw itself and show the overlay
       [self.mapDeskViewController.mapView setNeedsDisplay];

       //I can see this being displayed very quickly, meaning 
       NSLog(@"Response, pl_count:%d",pl.pointCount);
    });