FieldNavigator.swift

class FormFieldNavigator {
    
    weak var view:UIView? {
        didSet {
            if let _ = view {
                update()
            } else {
                inputs = []
            }
        }
    }
    
    weak var firstResponder:UIView? {
        get {
            return _firstResponder()
        } set {
            _firstResponderStorage = newValue
            _firstResponderStorage?.becomeFirstResponder()
        }
    }
    private var _firstResponderStorage:UIView?
    
    ///All inputs found.
    private var inputs:[UIView] = []
    
    init(view:UIView? = nil) {
        self.view = view
    }
    
    private func allInputs(view:UIView) -> [UIView] {
        var inputs:[UIView] = []
        for view in view.subviews {
            if view is UITextField || view is UITextView {
                inputs.append(view)
            }
            let subInputs = allInputs(view: view)
            if subInputs.count > 0 {
                inputs.append(contentsOf: subInputs)
            }
        }
        return inputs
    }
    
    func update() {
        guard let view = view else { return }
        inputs = allInputs(view: view)
        inputs.sort { view1, view2 in
            let view1Frame = view1.convert(CGPoint.zero, to: nil)
            let view2Frame = view2.convert(CGPoint.zero, to: nil)
            if view1Frame.y == view2Frame.y {
                return view1Frame.x < view2Frame.x
            }
            return view1Frame.y < view2Frame.y
        }
    }
}