setNeedsDisplayInMapRect doesn't trigger new drawMapRect: call

Question

I\'m using a custom MKOverlay/MKOverlayView to completely cover the Google basemap with my own tiles, which are loaded asynchronously. I follow the pattern of

Answer 1

The answer above did not work for me. From the NSLog printout I used I could see that a different thread was being used for despite grabbing the dispatch_get_current_queue() in canDrawMapRect and storing it for later use. This was the case in the iPad 4.3 Simulator at least, I did not attempt on the device.

My solution was less satisfactory and more error prone solution of wait x time before calling.

-(void)setNeedsDisplay:(WebRequest*)webRequest
{  
   [webRequest retain];
   dispatch_after(dispatch_time(DISPATCH_TIME_NOW, 0.25 * NSEC_PER_SEC), dispatch_get_main_queue(), 
   ^{
      [webRequest autorelease];
      [delegate setNeedsDisplayInMapRect:webRequest.request.requestedMapRect 
                              zoomScale:webRequest.request.requestedScale];
   });     
}

Answer 2

I had an issue very similar to the one described here. In my case I couldn't reproduce the desired behaviour (described in http://developer.apple.com/library/ios/documentation/MapKit/Reference/MKOverlayView_class/Reference/Reference.html#//apple_ref/occ/instm/MKOverlayView/setNeedsDisplayInMapRect:zoomScale:) even having the most simple code possible:

- (BOOL)canDrawMapRect:(MKMapRect)mapRect zoomScale:(MKZoomScale)zoomScale {
    NSLog(@"This should trace forever");

    [self setNeedsDisplayInMapRect:mapRect zoomScale:zoomScale];
    return NO;
}

or closer to my original code:

- (BOOL)canDrawMapRect:(MKMapRect)mapRect zoomScale:(MKZoomScale)zoomScale {
    NSLog(@"This should trace forever");

    [SomeAsynchronousRequestWithCompletionHandler:^{
        [self setNeedsDisplayInMapRect:mapRect zoomScale:zoomScale];
    }];
    return NO;
}

In both cases setNeedsDisplayInMapRect:zoomScale: has never been called even once.

The situation changed, when I began running setNeedsDisplayInMapRect:zoomScale: inside a dispatch_async dispatched to the same queue that canDrawMapRect runs on, like:

- (BOOL)canDrawMapRect:(MKMapRect)mapRect zoomScale:(MKZoomScale)zoomScale {

    dispatch_queue_t queue = dispatch_get_current_queue();

    NSLog(@"This should trace forever");

    dispatch_async(queue, ^{
        [self setNeedsDisplayInMapRect:mapRect zoomScale:zoomScale];
    });

    return NO;
}

or with asynchronous job included:

- (BOOL)canDrawMapRect:(MKMapRect)mapRect zoomScale:(MKZoomScale)zoomScale {
    NSLog(@"This should trace forever");

    dispatch_queue_t queue = dispatch_get_current_queue();

    [SomeAsynchronousRequestWithCompletionHandler:^{
        dispatch_async(queue, ^{
            [self setNeedsDisplayInMapRect:mapRect zoomScale:zoomScale];
        });
    }];
    return NO;
}

Using dispatch_async - I can see "This should trace forever" string being traced endlessly. My original problem is also disappeared completely.

LATER UPDATE: Currently, I use dispatch_get_main_queue() to call setNeedsDisplayInMapRect:zoomScale: like

- (BOOL)canDrawMapRect:(MKMapRect)mapRect zoomScale:(MKZoomScale)zoomScale {
    NSLog(@"This should trace forever");

    [SomeAsynchronousRequestWithCompletionHandler:^{
        dispatch_async(dispatch_get_main_queue(), ^{
            [self setNeedsDisplayInMapRect:mapRect zoomScale:zoomScale];
        });
    }];
    return NO;
}