在使用针对iOS 8的Xcode 6 beta 6构建Swift程序时，链接器出现错误消息。此代码已在Beta 5中正确编译并运行。

Undefined symbol for architecture x86_64:
__TFSs26_forceBridgeFromObjectiveCU__FTPSs9AnyObject_MQ__Q_", referenced from:
 __TFC8RayTrace14RayTracingPlot15drawFocalPointfS0_FT_T_ in RayTracingPlot.o
ld: symbol(s) not found for architecture x86_64

这是有问题的代码：

private func drawFocalPoint() {
    var attributes = Dictionary<String, AnyObject>()

    let FString: String = "F"
    let distance: CGFloat = focalDistance

    let centerX = CGRectGetMidX(bounds)
    let centerY = CGRectGetMidY(bounds)

    let circleRadius: CGFloat = 4.0
    let focalPointFrame = CGRectMake(0, 0, circleRadius * 2.0, circleRadius * 2.0)
    var path = UIBezierPath(ovalInRect: focalPointFrame)
    let color = UIColor.blackColor()

    let currentContext = UIGraphicsGetCurrentContext()
    CGContextSaveGState(currentContext)
    let shadowColor = UIColor(white:0, alpha:0.75).CGColor
    CGContextSetShadowWithColor(currentContext, CGSizeMake(0, 4), CGFloat(8), shadowColor)

    // Image F
    var imageFPath = UIBezierPath(CGPath: path.CGPath)
    let imageFTransform = CGAffineTransformMakeTranslation((centerX - distance - circleRadius),
        (centerY - circleRadius))
    imageFPath.applyTransform(imageFTransform)
    color.set()
    imageFPath.fill()
    FString.drawAtPoint(CGPointMake(centerX - distance - circleRadius, centerY + 5), withAttributes:attributes)

    CGContextSetShadowWithColor(currentContext, CGSizeMake(0.0, 0.0), CGFloat(0.0), nil) // Clear shadow
    CGContextRestoreGState(currentContext)
}

我希望在此代码中找到错误的提示，以便我可以解决。谢谢。