a.swift

import UIKit

class MyViewController: UIViewController {
    @IBOutlet weak var button: UIButton!
    var vc: UIViewController? = nil

    override func viewDidLoad() {
        super.viewDidLoad()
        
        button.addTarget(self, action: #selector(down), for: .touchDown)

        // If user generated touchDown and then didn't generate touchUpInside, we would have
        // set up a view controller that won't even be presented, which can cause problems,
        // e.g. with metrics.
        // Instead, run the .up() method for *every* possible outcome after touchDown:
        // touchUpInside, touchUpOutside, and touchCancel
        button.addTarget(self, action: #selector(up), for: .touchUpInside)
        button.addTarget(self, action: #selector(up), for: .touchUpOutside)
        button.addTarget(self, action: #selector(up), for: .touchCancel)

        // Don't let the user do anything else while they're pressing the button
        button.isExclusiveTouch = true
    }

    @objc private func up() {
        if let vc = vc {
            present(vc, animated: true, completion: nil)
        } else {
            // This else block would only happen in the (probably) impossible case when .async
            // is still enqueued from .down()
        }
        vc = nil
    }

    @objc private func down() {
        // This .async call is necessary if the button has an animation that you want to run
        // while the main thread is blocked to set up the new VC.
        // You might think that blocking the main thread, even after the animation has just
        // begun, will still block the animation. But actually, once they've been started,
        // animations keep going even when the main thread is blocked
        DispatchQueue.main.async {
            let vc = UIViewController()
            let _ = vc.view // Calling .view triggers viewDidLoad etc., so that more setup can occur
            self.vc = vc
        }
    }
}