swift – CGImageSource到CGImage？

let imageManager = PHImageManager.defaultManager()
imageManager.requestImageDataForAsset(self.asset!, options: nil, resultHandler:{
    (data, responseString, imageOriet, info) -> Void in
    let imageData: NSData = data!
    if let imageSource = CGImageSourceCreateWithData(imageData, nil) {
        let imageProperties = CGImageSourceCopyPropertiesAtIndex(imageSource, 0, nil)! as NSDictionary

        //How do I create a CGImage now?

    }
})

我需要提取一个CGImage.遇到麻烦……

let image = CGImageSourceCreateImageAtIndex(imageSource, 0, nil)