import UIKit

extension UIImage{

    //根据传入的宽度生成一张按照宽高比压缩的新图片

    func imageWithScale(width:CGFloat) -> UIImage{

       //1.根据 宽度 计算高度

        let height = width * size.height / size.width

      //2.按照宽高比绘制一张新的图片

        let currentSize = CGSize.init(width: width, height: height)

        UIGraphicsBeginImageContext(currentSize)  //开始绘制

         draw(in: CGRect.init(origin: CGPoint.zero, size: currentSize))

        let newImage = UIGraphicsGetImageFromCurrentImageContext()

        UIGraphicsEndImageContext()  //结束上下文

        return newImage!

    }

}

import UIKit

var identifier = "cell"

private var imgAry = [UIImage]()

class ViewController: UIViewController{

    @IBOutlet weak var btn: UIButton!

    @IBOutlet weak var colectionView: UICollectionView!

        override func viewDidLoad() {

        super.viewDidLoad()

        colectionView.register(UICollectionViewCell.self, forCellWithReuseIdentifier: identifier)

        colectionView.delegate = self

        colectionView.dataSource = self

        // Do any additional setup after loading the view, typically from a nib.

    }

    override func didReceiveMemoryWarning() {

        super.didReceiveMemoryWarning()

        // Dispose of any resources that can be recreated.

    }

    //选择 按钮点击事件 弹出相册

    @IBAction func btnAction(_ sender: UIButton) {

        let vc = UIImagePickerController()

        vc.delegate = self

        present(vc, animated: true, completion: nil)

    }

}

extension ViewController:UICollectionViewDelegate,UICollectionViewDataSource,UINavigationControllerDelegate,UIImagePickerControllerDelegate{

    public func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int{

        return imgAry.count

    }

    public func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell{

        let cell = colectionView.dequeueReusableCell(withReuseIdentifier: identifier, for: indexPath)

        let imgView = UIImageView.init(frame: CGRect.init(x: 0, y: 0, width: 100, height: 100))

        if(imgAry.count > 0){

            imgView.image = imgAry[indexPath.item]

        }

        cell.addSubview(imgView)

        return cell

    }

    //选择图片

    public func imagePickerController(_ picker: UIImagePickerController, didFinishPickingMediaWithInfo info: [String : Any]){

        let image = info["UIImagePickerControllerOriginalImage"] as! UIImage  //选择的图片

        let newImage = image.imageWithScale(width: 500)  //按照宽为500的宽高比给图片重新绘制新的图片

        imgAry.append(newImage)

        colectionView.reloadData()

        picker.dismiss(animated: true, completion: nil)

    }

}