画像をダブルタップとピンチイン・ピンチアウトで拡大・縮小する Swift3編 ref: http://qiita.com/yonezawaizumi/items/bd3f53b2f4d80f815357

ViewController.swift

class ViewController: UIViewController, UIScrollViewDelegate {

    @IBOutlet var myImageView: UIImageView!
    @IBOutlet var myScrollView: UIScrollView!

    override func viewDidLoad() {
        super.viewDidLoad()

        self.myScrollView.delegate = self

        let doubleTapGesture = UITapGestureRecognizer(target: self, action:#selector(self.doubleTap))
        doubleTapGesture.numberOfTapsRequired = 2
        self.myImageView.addGestureRecognizer(doubleTapGesture)
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
    }

    func viewForZooming(in scrollView: UIScrollView) -> UIView? {
        //print("pinch")
        return self.myImageView
    }

    func doubleTap(gesture: UITapGestureRecognizer) -> Void {
        //print(self.myScrollView.zoomScale)
        if (self.myScrollView.zoomScale < self.myScrollView.maximumZoomScale) {
            let newScale = self.myScrollView.zoomScale * 3
            let zoomRect = self.zoomRectForScale(scale: newScale, center: gesture.location(in: gesture.view))
            self.myScrollView.zoom(to: zoomRect, animated: true)
        } else {
            self.myScrollView.setZoomScale(1.0, animated: true)
        }
    }

    func zoomRectForScale(scale:CGFloat, center: CGPoint) -> CGRect{
        let size = CGSize(
            width: self.myScrollView.frame.size.width / scale,
            height: self.myScrollView.frame.size.height / scale
        )
        return CGRect(
            origin: CGPoint(
                x: center.x - size.width / 2.0,
                y: center.y - size.height / 2.0
            ),
            size: size
        )
    }
}