Creating an own view subclass that can receive keyboard input.

ViewController.swift

class CustomInputView: UIView, UIKeyInput {

    private let label = UILabel()

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

        label.tintColor = .label
        label.font = .preferredFont(forTextStyle: .body, compatibleWith: nil)
        label.text = ""

        addSubview(label)

        label.translatesAutoresizingMaskIntoConstraints = false

        NSLayoutConstraint.activate([
            label.topAnchor.constraint(equalTo: topAnchor),
            label.bottomAnchor.constraint(equalTo: bottomAnchor),
            label.leadingAnchor.constraint(equalTo: leadingAnchor),
            label.trailingAnchor.constraint(equalTo: trailingAnchor)
        ])

    }

    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    // This is required. UIView returns false in `canBecomeFirstResponder` by default, but we have to meke this view become first responder.
    override var canBecomeFirstResponder: Bool {
        return true
    }

    var hasText: Bool {
        return (label.text?.count ?? 0) > 0
    }

    func insertText(_ text: String) {
        label.text = label.text?.appending(text)
    }

    func deleteBackward() {
        label.text = String(label.text!.dropLast())
    }

}

class ViewController: UIViewController {

    let customView: CustomInputView = .init()

    override func loadView() {
        let view = UIView()
        view.backgroundColor = .white
        view.isUserInteractionEnabled = true
        view.addSubview(customView)
        customView.translatesAutoresizingMaskIntoConstraints = false

        NSLayoutConstraint.activate([
            customView.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor),
            customView.bottomAnchor.constraint(equalTo: view.bottomAnchor),
            customView.leadingAnchor.constraint(equalTo: view.leadingAnchor),
            customView.trailingAnchor.constraint(equalTo: view.trailingAnchor)
        ])

        self.view = view
    }

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


    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)

        customView.becomeFirstResponder()

    }

}