[iOS]How to zoom using the Pinch Gesture

Pocket NoteではUIImage等のUIViewのサイズをピンチ操作で拡大・縮小することができますが、今回はこのピンチ操作でのサイズ変更の方法についてご紹介します。
Pocket Note can be changed the size of UIView,for example UIImage, using pinch gesture and I introduce this method this time.

環境[Environment]：Xcode 8、Swift 3

では早速コードを書いてみましょう。
Let's write code immediately.

import UIKit

class ViewController: UIViewController {

    

    let imageView:UIImageView = UIImageView()

    override func viewDidLoad() {

        super.viewDidLoad()

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

        

        //Pinch Gesture

        let pinchGesture = UIPinchGestureRecognizer(target: self, action: #selector(self.pinchAction(sender:)))

        self.view.addGestureRecognizer(pinchGesture)

        

        let image:UIImage = UIImage(named:"sample.jpg")!

        let scale:CGFloat = image.size.height / image.size.width

        imageView.frame = CGRect(x:0,y:0,width:200,height:200*scale)

        imageView.image = image

        self.view.addSubview(imageView)

    }

    override func didReceiveMemoryWarning() {

        super.didReceiveMemoryWarning()

        // Dispose of any resources that can be recreated.

    }

    

    internal func pinchAction(sender: UIPinchGestureRecognizer){

        let velocity:CGFloat = sender.velocity

        

        var newWidthValue:CGFloat = imageView.frame.width

        var newHeightValue:CGFloat = imageView.frame.height

        let scale:CGFloat = newHeightValue / newWidthValue

        

        if (velocity > 0){

            //拡大(Scale up)

            newWidthValue += 10

            newHeightValue += 10*scale

        }else{

            //縮小(Scale Down)

            if (newWidthValue > 10 && newHeightValue > 10){

                newWidthValue -= 10

                newHeightValue-=10*scale

            }

        }

        imageView.frame = CGRect(x:imageView.frame.minX,y:imageView.frame.minY,width:newWidthValue,height:newHeightValue)

    }

}

まずピンチジェスチャーを受け取る為にUIPinchGestureRecognizerを定義します。

そして発生したイベント内でUIPinchGestureRecognizerのvelocityプロパティの値を見て、プラスの値であれば指が広がった操作なのでサイズを拡大し、マイナスの値であれば指が狭まった操作なのでサイズを縮小します。
At first,define UIPinchGestureRecognizer for receiving the pinch gesture.
And when operator use pinch gesture, get value of UIPinchGestureRecognizer's velocity property.
When it is plus, scale up because fingers were spread.
When it is minus, scale down because fingers were narrowed.

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