Presenting a Landscape-only UIViewController on top of a Portrait-only UIViewController

landscape-on-portrait.swift

// This should not be difficult, but it is.
// If you miss the `isBeingPresented` check it breaks.
// it will force the portrait only viewcontroller into landscape (an unsupported orientation!) during dismiss

import UIKit

class ViewController: UIViewController {
    override var supportedInterfaceOrientations: UIInterfaceOrientationMask {
        .portrait
    }

    override func viewDidLoad() {
        super.viewDidLoad()

        view.backgroundColor = .lightGray
        view.layer.borderColor = UIColor.blue.cgColor
        view.layer.borderWidth = 4

        let button = UIButton()
        view.addSubview(button)
        button.translatesAutoresizingMaskIntoConstraints = false
        NSLayoutConstraint.activate([
            button.centerXAnchor.constraint(equalTo: view.centerXAnchor),
            button.centerYAnchor.constraint(equalTo: view.centerYAnchor)
        ])
        button.setTitle("tap to push to landscape", for: .normal)
        button.addTarget(self, action: #selector(didTapButton), for: .touchUpInside)
    }

    @objc func didTapButton() {
        view.window?.layer.speed = 0.25

        let vc = LandscapeOnlyVC()
        vc.modalPresentationStyle = .fullScreen
        present(vc, animated: true)
    }
}

class LandscapeOnlyVC: UIViewController {
    override var supportedInterfaceOrientations: UIInterfaceOrientationMask {
        isBeingPresented ? .landscape : .all
    }

    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = .darkGray
        view.layer.borderColor = UIColor.red.cgColor
        view.layer.borderWidth = 4

        let button = UIButton()
        view.addSubview(button)
        button.setTitle("tap to dismiss to portrait", for: .normal)
        button.translatesAutoresizingMaskIntoConstraints = false
        NSLayoutConstraint.activate([
            button.centerXAnchor.constraint(equalTo: view.centerXAnchor),
            button.centerYAnchor.constraint(equalTo: view.centerYAnchor)
        ])
        button.addTarget(self, action: #selector(didTapButton), for: .touchUpInside)
    }

    @objc func didTapButton() {
        dismiss(animated: true)
    }
}