如何在地图上填写iOS 7中的外部叠加圈

- (void)drawMapRect:(MKMapRect)mapRect zoomScale:(MKZoomScalE)zoomScale inContext:(CGContextRef)context
{
    // Fill full map rect with some color.
    CGRect rect = [self rectForMapRect:mapRect];
    CGContextSaveGState(context);
    CGContextAddRect(context,rect);
    CGContextSetFillColorWithColor(context,[UIColor colorWithWhite:0.0 alpha:0.4f].CGColor);
    CGContextFillRect(context,rect);
    CGContextRestoreGState(context);

    // Clip rounded hole.
    CGContextSaveGState(context);
    CGContextSetFillColorWithColor(context,[UIColor whiteColor].CGColor);
    CGContextSetBlendMode(context,kCGBlendModeClear);
    CGContextFillEllipseInRect(context,[self rectForMapRect:[self.overlay boundingMapRect]]);
    CGContextRestoreGState(context);

    // Draw circle
    [super drawMapRect:mapRect zoomScale:zoomScale inContext:context];
}