iPhone, reproduce the magnifier effect

Question

I would like be able to create a movable magnifier (like the one you have when you copy and paste) in a custom view, for zooming a part of my view.

I have no idea on

Answer 1

I use this code in Swift 3 :

class MagnifyingGlassView: UIView {

    var zoom: CGFloat = 2 {
        didSet {
            setNeedsDisplay()
        }
    }

    weak var readView: UIView?

    // MARK: - UIVIew

    override init(frame: CGRect) {
        super.init(frame: frame)
        setupView()
    }

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        setupView()
    }

    override func draw(_ rect: CGRect) {
        guard let readView = readView else { return }
        let magnifiedBounds = magnifyBounds(of: readView, zoom: zoom)
        readView.drawHierarchy(in: magnifiedBounds, afterScreenUpdates: false)
    }

    // MARK: - Private

    private func setupView() {
        isOpaque = false
        backgroundColor = UIColor.clear
    }

    private func magnifyBounds(of view: UIView, zoom: CGFloat) -> CGRect {
        let transform = CGAffineTransform(scaleX: zoom, y: zoom)
        var bounds = view.bounds.applying(transform)
        bounds.center = view.bounds.center
        return view.convert(bounds, to: self)
    }
}

extension CGRect {
    var center: CGPoint {
        get {
            return CGPoint(x: origin.x + width / 2, y: origin.y + height / 2)
        }
        set {
            origin.x = newValue.x - width / 2
            origin.y = newValue.y - height / 2
        }
    }
}

You need to call setNeedsDisplay in scrollViewDidScroll: if your read view is a scrollView.