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 requesting unloaded tiles when I receive a `canDrawMapRect:zoomScale:` call to my overlay view (and returning FALSE in that case), then calling `setNeedsDisplayInMapRect:zoomScale:` once the tile is available. This all generally works, and appears to work perfectly in the simulator. However, on the device I sometimes get a 'hole' in the overlay - a missing tile. I can see that the tile is requested, and that the request completes. I can see that I call `setNeedsDisplayInMapRect:zoomScale:`, and that I am passing the original `MKMapRect` and `MKZoomScale` which were provided in `canDrawMapRect:zoomScale:`. But I can also see that the overlay is never asked to redraw that tile (neither `canDrawMapRect:zoomScale:` nor `drawMapRect:zoomScale:inContext:` is ever again called for that tile). I need to understand why this is happening…

Accepted Answer

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;
}
```

setNeedsDisplayInMapRect doesn't trigger new drawMapRect: call

Solution

Problem