NSCoding：使Swift类符合NSCoding

Question

我是Swift的新手，我希望在用户从照片库中选择照片后保存图像数据。我正在使用DKImagePickerController（https://github.com/zhangao0086/DKImagePickerController）来选择图像资源。

/**
* An `DKAsset` object represents a photo or a video managed by the `DKImagePickerController`.
*/
public class DKAsset: NSObject {

/// Returns a UIImage that is appropriate for displaying full screen.
private var fullScreenImage: (image: UIImage?, info: [NSObject : AnyObject]?)?

/// Returns the original image.
private var originalImage: (image: UIImage?, info: [NSObject : AnyObject]?)?
...

以上是DKAsset类的简短快照。下面是使类符合NSCoding的方法。

required convenience init?(coder decoder: NSCoder) {
    let fullScreenImage = decoder.decodeObjectForKey("fullScreenImage") as! UIImage
}    

func encodeWithCoder(aCoder: NSCoder) {
    aCoder.encodeObject(fullScreenImage, forKey: "fullScreenImage")
}

但我收到此错误消息＆＃34;无法转换类型＆＃39;（图片：UIImage？，info：[NSObject：AnyObject]？）的值？＆＃39;预期参数类型＆＃39; AnyObject＆＃39;＆＃34;。

我应该如何编码该变量？