我的自定义MKOverlayView没有被调用

Question

尝试在MKMapkit上执行WMS磁贴作为自定义叠加层。我创建了一个MKOverlay模式，从我们的服务器检索一个tile并将它们存储在对象的NSCache中：

- (CLLocationCoordinate2D)coordinate
{
    return MKCoordinateForMapPoint(MKMapPointMake(MKMapRectGetMidX(boundingMapRect),
                                                  MKMapRectGetMidY(boundingMapRect)));
}

- (MKMapRect)boundingMapRect
{
    return boundingMapRect;
}

- (NSArray *)tilesInMapRect:(MKMapRect)rect zoomScale:(MKZoomScale)scale
{
    NSInteger z = zoomScaleToZoomLevel(scale);

    // Number of tiles wide or high (but not wide * high)
    NSInteger tilesAtZ = pow(2, z);

    NSInteger minX = floor((MKMapRectGetMinX(rect) * scale) / TILE_SIZE);
    NSInteger maxX = floor((MKMapRectGetMaxX(rect) * scale) / TILE_SIZE);
    NSInteger minY = floor((MKMapRectGetMinY(rect) * scale) / TILE_SIZE);
    NSInteger maxY = floor((MKMapRectGetMaxY(rect) * scale) / TILE_SIZE);

    //NSLog(@"minX: %d maxX: %d minY: %d, maxY: %d", minX, maxX, minY, maxY);

    NSMutableArray *tiles = nil;

    for (NSInteger x = minX; x <= maxX; x++) {
        for (NSInteger y = minY; y <= maxY; y++) {
            // As in initWithTilePath, need to flip y index to match the gdal2tiles.py convention.
            NSInteger flippedY = abs(y + 1 - tilesAtZ);

            NSString *tileKey = [[NSString alloc] initWithFormat:@"%d+%d+%d", z, x, flippedY];
            //NSLog(@"tilekey: %@", tileKey);

            if ([self.tileCache objectForKey:tileKey])
            {
                if (!tiles) {
                    tiles = [NSMutableArray array];
                }

                MKMapRect frame = MKMapRectMake((double)(x * TILE_SIZE) / scale,
                                                (double)(y * TILE_SIZE) / scale,
                                                TILE_SIZE / scale,
                                                TILE_SIZE / scale);

                ImageTile *tile = [[ImageTile alloc] initWithFrame:frame path:[self.tileCache objectForKey:tileKey]];
                [tiles addObject:tile];
            }
        }
    }
    return tiles;
}

我有其他方法可以找到google tile（x，y，z）并进行WMS调用。

我查看过苹果公司的TileMap示例并借鉴了他们的代码。

我创建了一个MKOverlayView并覆盖了canDrawMapRect和drawMapRect：

//EDIT: added the init method
- (id)initWithOverlay:(id <MKOverlay>)overlay
{
    if (self = [super initWithOverlay:overlay]) {
        tileAlpha = 0.75;
    }
    return self;
}

- (BOOL)canDrawMapRect:(MKMapRect)mapRect
             zoomScale:(MKZoomScale)zoomScale
{
    TileOverlay *tileOverlay = (TileOverlay *)self.overlay;
    NSArray *tilesInRect = [tileOverlay tilesInMapRect:mapRect zoomScale:zoomScale];

    if ([tilesInRect count] > 0) 
    {
        return YES;
    } 
    else 
    {
        [tileOverlay retrieveWMSTileWithMapRect:mapRect zoomScale:zoomScale];
        return NO;
    }
}

- (void)drawMapRect:(MKMapRect)mapRect
          zoomScale:(MKZoomScale)zoomScale
          inContext:(CGContextRef)context
{    
    TileOverlay *tileOverlay = (TileOverlay *)self.overlay;
    NSArray *tilesInRect = [tileOverlay tilesInMapRect:mapRect zoomScale:zoomScale];

    CGContextSetAlpha(context, tileAlpha);

    for (ImageTile *tile in tilesInRect) {
        // For each image tile, draw it in its corresponding MKMapRect frame
        CGRect rect = [self rectForMapRect:tile.imageFrame];
        UIImage *image = [[UIImage alloc] initWithData:tile.imageData];
        CGContextSaveGState(context);
        CGContextTranslateCTM(context, CGRectGetMinX(rect), CGRectGetMinY(rect));
        CGContextScaleCTM(context, 1/zoomScale, 1/zoomScale);
        CGContextTranslateCTM(context, 0, image.size.height);
        CGContextScaleCTM(context, 1, -1);
        CGContextDrawImage(context, CGRectMake(0, 0, image.size.width, image.size.height), [image CGImage]);
        CGContextRestoreGState(context);
    }
}

在我的MKMapKitViewController中，我创建了TileOverlay对象并将其添加到地图中：

- (void)viewDidLoad
{
    [super viewDidLoad];

    self.mapView.delegate = self;

    self.mapView.mapType = MKMapTypeStandard;

    TileOverlay *tileOverlay = [[TileOverlay alloc] init];
    [self.mapView addOverlay:tileOverlay];
}

我还实现了mapView：viewForOverlay方法：

- (MKOverlayView *)mapView:(MKMapView *)mapView viewForOverlay:(id <MKOverlay>)overlay
{    
    NSLog(@"viewForOverlay fired");
    TileOverlayView *view = [[TileOverlayView alloc] initWithOverlay:overlay];
    view.tileAlpha = 0.6;
    return view;
}

问题是从不调用canDrawRect / drawRect。我错过了什么让这个工作？