Swift 2 MKMapViewDelegate rendererForOverlay 可选性

Question

在 Swift 1.2 中我有这个:

class UVC: NSViewController, MKMapViewDelegate {
    // ...

    // **************************************
    // MARK: MapView Delegate
    // **************************************
    func mapView(mapView: MKMapView, rendererForOverlay overlay: MKOverlay) -> MKOverlayRenderer! {
        if overlay is OSGBTiles {
            return OSGBTilesRenderer(tileOverlay: overlay as! OSGBTiles)
        } else if overlay is ESRI {
            return ESRIRenderer(shapeFileOverlay: overlay as! ESRI)
        } else if overlay is MKTileOverlay {
            return MKTileOverlayRenderer(overlay: overlay)
        } else {
            print("Unknown overlay")
        }
        return nil
    }
}

Swift 2 已将 mapView:rendererForOverlay 的定义更改为现在返回 MKOverlayRenderer 而不是 MKOverlayRenderer!，并且现在不允许我返回返回 nil(毫不奇怪)因为 MKOverlayRenderer 不是 NilLiteralConvertible。但是，文档(在 Xcode 7 中)仍然说:

Return Value

The renderer to use when presenting the specified overlay on the map. If you return nil, no content is drawn for the specified overlay object.

如果传递了一个我不认识的叠加层，我应该返回什么？

Answer 1

根据最新的MKMapView.h，现在新的函数声明是:

// Current renderer for overlay; returns nil if the overlay is not shown.
@available(iOS 7.0, *)
func rendererForOverlay(overlay: MKOverlay) -> MKOverlayRenderer?