- (void)beginBackgroundCacheForTileSource:(id <RMTileSource>)tileSource southWest:(CLLocationCoordinate2D)southWest northEast:(CLLocationCoordinate2D)northEast minZoom:(float)minZoom maxZoom:(float)maxZoom
{
    if (self.isBackgroundCaching)
        return;

    _activeTileSource = tileSource;
    
    _backgroundFetchQueue = [[NSOperationQueue alloc] init];
    [_backgroundFetchQueue setMaxConcurrentOperationCount:6];
    
    int   minCacheZoom = (int)minZoom;
    int   maxCacheZoom = (int)maxZoom;
    float minCacheLat  = southWest.latitude;
    float maxCacheLat  = northEast.latitude;
    float minCacheLon  = southWest.longitude;
    float maxCacheLon  = northEast.longitude;

    if (maxCacheZoom < minCacheZoom || maxCacheLat <= minCacheLat || maxCacheLon <= minCacheLon)
        return;

    int n, xMin, yMax, xMax, yMin;

    int totalTiles = 0;

    for (int zoom = minCacheZoom; zoom <= maxCacheZoom; zoom++)
    {
        n = pow(2.0, zoom);
        xMin = floor(((minCacheLon + 180.0) / 360.0) * n);
        yMax = floor((1.0 - (logf(tanf(minCacheLat * M_PI / 180.0) + 1.0 / cosf(minCacheLat * M_PI / 180.0)) / M_PI)) / 2.0 * n);
        xMax = floor(((maxCacheLon + 180.0) / 360.0) * n);
        yMin = floor((1.0 - (logf(tanf(maxCacheLat * M_PI / 180.0) + 1.0 / cosf(maxCacheLat * M_PI / 180.0)) / M_PI)) / 2.0 * n);

        totalTiles += (xMax + 1 - xMin) * (yMax + 1 - yMin);
    }

    [_backgroundCacheDelegate tileCache:self didBeginBackgroundCacheWithCount:totalTiles forTileSource:_activeTileSource];

    __block int progTile = 0;

    for (int zoom = minCacheZoom; zoom <= maxCacheZoom; zoom++)
    {
        n = pow(2.0, zoom);
        xMin = floor(((minCacheLon + 180.0) / 360.0) * n);
        yMax = floor((1.0 - (logf(tanf(minCacheLat * M_PI / 180.0) + 1.0 / cosf(minCacheLat * M_PI / 180.0)) / M_PI)) / 2.0 * n);
        xMax = floor(((maxCacheLon + 180.0) / 360.0) * n);
        yMin = floor((1.0 - (logf(tanf(maxCacheLat * M_PI / 180.0) + 1.0 / cosf(maxCacheLat * M_PI / 180.0)) / M_PI)) / 2.0 * n);

        for (int x = xMin; x <= xMax; x++)
        {
            for (int y = yMin; y <= yMax; y++)
            {
                RMTileCacheDownloadOperation *operation = [[RMTileCacheDownloadOperation alloc] initWithTile:RMTileMake(x, y, zoom)
                                                                                                forTileSource:_activeTileSource
                                                                                                   usingCache:self];

                __block RMTileCacheDownloadOperation *internalOperation = operation;

                [operation setCompletionBlock:^(void)
                {
                    dispatch_sync(dispatch_get_main_queue(), ^(void)
                    {
                        if ( ! [internalOperation isCancelled])
                        {
                            progTile++;

                            [_backgroundCacheDelegate tileCache:self didBackgroundCacheTile:RMTileMake(x, y, zoom) withIndex:progTile ofTotalTileCount:totalTiles];

[this then fetches http://tile.openstreetmap.org/%d/%d/%d.png where
the args are zoom, x, y]

                            if (progTile == totalTiles)
                            {
                                 _backgroundFetchQueue = nil;

                                 _activeTileSource = nil;

                                [_backgroundCacheDelegate tileCacheDidFinishBackgroundCache:self];
                            }
                        }

                        internalOperation = nil;
                    });
                }];

                [_backgroundFetchQueue addOperation:operation];
            }
        }