如何在swift中将全屏按钮添加到VLC播放器上方

import UIKit
import MobileVLCKit

class ViewController: UIViewController {
    
    var vlcPlayer: VLCMediaPlayer!
    var fullscreenButton: UIButton!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        // 创建VLC播放器实例
        vlcPlayer = VLCMediaPlayer()
        
        // 设置VLC播放器视图的位置和大小
        let playerView = UIView(frame: CGRect(x: 0, y: 0, width: 300, height: 200))
        playerView.center = view.center
        
        // 将VLC播放器视图添加到当前视图中
        view.addSubview(playerView)
        
        // 将VLC播放器关联到VLC播放器视图
        vlcPlayer.drawable = playerView
        
        // 创建全屏按钮
        fullscreenButton = UIButton(type: .system)
        fullscreenButton.setTitle("全屏", for: .normal)
        fullscreenButton.frame = CGRect(x: 0, y: 0, width: 100, height: 50)
        fullscreenButton.center.x = view.center.x
        fullscreenButton.frame.origin.y = playerView.frame.maxY + 20
        
        // 添加按钮的点击事件处理程序
        fullscreenButton.addTarget(self, action: #selector(fullscreenButtonTapped), for: .touchUpInside)
        
        // 将全屏按钮添加到当前视图中
        view.addSubview(fullscreenButton)
    }
    
    @objc func fullscreenButtonTapped() {
        // 执行全屏操作
        if let playerViewController = vlcPlayer.playerViewController {
            playerViewController.enterFullScreen(animated: true)
        }
    }
}