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

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;
}