Zooming an UIImageView that contains subviews

Question

I have an UIImageView object in UIScrollView(to zoom). In addition, UIImageView also contains subviews. I am using following function to zoom in UIImageView.

func viewForZooming(in scrollView: UIScrollView) -> UIView? {
    return self.imageView
}

It works perfectly fine for UIImageView.

The problem is when zooming these subviews, it also zooms these subviews the same way. Is there a way to disable zooming for these subviews?

Answer 1

There are many ways to achieve this. I would say the easiest one is to simply back-scale your views. This is what I used as a demo:

class ViewController: UIViewController {

    @IBOutlet private var scrollView: UIScrollView?
    private var imageView: UIImageView?
    private var annotations: [UIView] = [UIView]()

    override func viewDidLoad() {
        super.viewDidLoad()

        let imageView = UIImageView(image: UIImage(named: "background"))
        scrollView?.addSubview(imageView)
        scrollView?.contentSize = imageView.bounds.size
        scrollView?.maximumZoomScale = 5.0
        self.imageView = imageView

        scrollView?.delegate = self

        applyAnnotationView({
            let view = UIView(frame: CGRect(x: 0.0, y: 0.0, width: 10.0, height: 10.0))
            view.backgroundColor = UIColor.red
            view.layer.cornerRadius = view.bounds.width*0.5
            return view
        }(), at: CGPoint(x: 100.0, y: 100.0))

        applyAnnotationView({
            let view = UIView(frame: CGRect(x: 0.0, y: 0.0, width: 10.0, height: 10.0))
            view.backgroundColor = UIColor.green
            view.layer.cornerRadius = view.bounds.width*0.5
            return view
        }(), at: CGPoint(x: 200.0, y: 200.0))
    }

    func applyAnnotationView(_ view: UIView, at position: CGPoint) {
        view.center = position
        annotations.append(view)
        imageView?.addSubview(view)
    }

}

// MARK: - UIScrollViewDelegate

extension ViewController: UIScrollViewDelegate {

    func scrollViewDidZoom(_ scrollView: UIScrollView) {
        annotations.forEach { item in
            item.transform = CGAffineTransform(scaleX: 1.0/scrollView.zoomScale, y: 1.0/scrollView.zoomScale)
        }
    }

    func viewForZooming(in scrollView: UIScrollView) -> UIView? {
        return imageView
    }

}

Simply applying CGAffineTransform(scaleX: 1.0/scrollView.zoomScale, y: 1.0/scrollView.zoomScale) does all the work. It should be OK even when using constraints.