如何在swift中将COpaquePointer转换为某种类型(特别是CGContext？)

我在 swift中有以下代码,不能编译：

class CustomView: NSView {
  override func drawRect(dirtyRect: NSRect) {
    var contextPointer: copaquePointer = NSGraphicsContext.currentContext()!.graphicsPort()
    var context: CGContext? = contextPointer as? CGContext
    CGContextSetRGBFillColor(context,1,1)
    CGContextFillRect(context,CGRectMake(0,100,100))
    CGContextFlush(context)
  }
}

如何将copaquePointer转换为CGContext？

从Developer Preview 5开始,NSGraphicsContext现在具有CGContext属性.它还没有记录,但它在头文件中：

@property (readonly) CGContextRef CGContext NS_RETURNS_INNER_POINTER NS_AVAILABLE_MAC(10_10);