保存ipod库中的音乐 - swift

Question

我正在尝试在我的应用中保存iPod库中的音乐，以后可以使用它。我已经使用媒体选择器获取音乐URL，但我不知道如何将其保存在我的应用程序中并稍后播放。以下是我用来获取音乐网址的代码：

@IBAction func music(sender: AnyObject) {
    var picker = MPMediaPickerController(mediaTypes: MPMediaType.Music)
    picker.delegate = self
    picker.allowsPickingMultipleItems = false
    picker.prompt =  NSLocalizedString("Select one music", comment: "")
    self.presentViewController(picker, animated: false, completion: nil)

}

func mediaPicker(mediaPicker: MPMediaPickerController, didPickMediaItems mediaItemCollection: MPMediaItemCollection) {
    var url = NSURL()

    let items = mediaItemCollection.items
    for item in items {
       url = item.assetURL!
    }

    self.dismissViewControllerAnimated(true, completion: nil)
}
func mediaPickerDidCancel(mediaPicker: MPMediaPickerController) {

   mediaPicker.dismissViewControllerAnimated(true, completion: nil)
}