如何在iOS7中使用MKTileOverLay？

我尝试使用wwdc中提供的方法在ios7中使用平铺贴图，但没有显示任何内容。有人有这方面的示例代码吗？下面提供了我的代码

#import "ViewController.h"

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.



    NSString *template = @"http://c.tile.openstreetmap.org/{z}/{x}/{y}.png";
    MKTileOverlay *overLay = [[MKTileOverlay alloc] initWithURLTemplate:template];
    overLay.canReplaceMapContent = YES;
    [self.mapView addOverlay:overLay level:MKOverlayLevelAboveLabels];
    [self.mapView setRegion:MKCoordinateRegionMake(CLLocationCoordinate2DMake(37.776884, -122.387255), MKCoordinateSpanMake(0.25, 0.25))];

}

-(MKTileOverlayRenderer *)mapView:(MKMapView*)mapView rendererForOverlay:(id<MKOverlay>)overlay {

    MKTileOverlayRenderer *renderer = [[MKTileOverlayRenderer alloc] initWithOverlay:overlay];
    renderer.alpha = 0.6;
    return renderer;

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是否已将mapView的委托设置为self？因为如果没有，就不会调用-renderForOverlay