问如何在photoLibrary或相机中快速裁剪4*3图像

EN

var imgview: UIImageView!
    var imagepicked:UIImage!
    var  minZoomScale:CGFloat!
    let picker = UIImagePickerController()
    @IBOutlet weak var scrollViewSquare: UIScrollView!

    override func viewDidLoad() {
        super.viewDidLoad()
        picker.delegate = self
        scrollViewSquare.delegate = self
    }
func imagePickerController(
        picker: UIImagePickerController,
        didFinishPickingMediaWithInfo info: [String : AnyObject])
    {
        imagepicked = (info[UIImagePickerControllerOriginalImage] as? UIImage)!
        ImageViewInit()
        dismissViewControllerAnimated(false, completion: nil)        
    }

    @IBAction func Pick(sender: AnyObject) {
        picker.allowsEditing = false
        picker.sourceType = .PhotoLibrary
        presentViewController(picker, animated: true, completion: nil)
    }
func ImageViewInit(){
       imgview = UIImageView()
        imgview.frame =  CGRectMake(0, 0, imagepicked.size.width, imagepicked.size.height)
        imgview.image = imagepicked
        imgview.contentMode = .ScaleAspectFit
        imgview.backgroundColor = UIColor.lightGrayColor()
        scrollViewSquare.maximumZoomScale=4;
        scrollViewSquare.minimumZoomScale=0.02;
        scrollViewSquare.bounces=true;
        scrollViewSquare.bouncesZoom=true;
        scrollViewSquare.contentMode = .ScaleAspectFit
        scrollViewSquare.contentSize = imagepicked.size
        scrollViewSquare.autoresizingMask = UIViewAutoresizing.FlexibleWidth
        scrollViewSquare.addSubview(imgview)
        setZoomScale()
    }
    //fit imageview in the scrollview
    func setZoomScale(){
        let imageViewSize = imgview.bounds.size
        let scrollViewSize = scrollViewSquare.bounds.size
        let widthScale = scrollViewSize.width / imageViewSize.width
        let heightScale = scrollViewSize.height / imageViewSize.height
        minZoomScale = max(widthScale, heightScale)
        scrollViewSquare.minimumZoomScale = minZoomScale
        scrollViewSquare.zoomScale = minZoomScale
    }

    func viewForZoomingInScrollView(scrollView: UIScrollView) -> UIView? {
        return imgview
    }
//mapping the image from the scrollview to imageview to get the desired ratio 
    @IBAction func Save(sender: AnyObject) {
            let offset = scrollViewSquare.contentOffset
            let visibleRect: CGRect = CGRectMake(offset.x, offset.y, offset.x+scrollViewSquare.frame.width, offset.y+scrollViewSquare.frame.height)
            let visibleImgRect: CGRect = CGRectMake(offset.x/scrollViewSquare.zoomScale, offset.y/scrollViewSquare.zoomScale, (offset.x+scrollViewSquare.frame.width)/scrollViewSquare.zoomScale, (offset.y+scrollViewSquare.frame.height)/scrollViewSquare.zoomScale)
            print("content offset now\(offset) and visible rect is \(visibleRect)  - zoomlevel \(scrollViewSquare.zoomScale)  now image \(imagepicked.size) and current visible area in image is \(visibleImgRect)")
        }