两个UIView 左右翻转动画

点击翻转两个View, 完整代码如下：

class ViewController: UIViewController {
    
    let firstView = UIView()
    let secondView = UIView()
    var isFliped = false

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.
        
        self.view.backgroundColor = .white
        
        let bgView = UIView()
        bgView.backgroundColor = .white
        bgView.frame = .init(x: 0, y: 0, width: 280, height: 180)
        bgView.center = self.view.center
        self.view.addSubview(bgView)
        
        secondView.backgroundColor = .blue
        secondView.frame = bgView.bounds
        bgView.addSubview(secondView)
        
        firstView.backgroundColor = .red
        firstView.frame = bgView.bounds
        bgView.addSubview(firstView)
        
        let tap = UITapGestureRecognizer.init(target: self, action: #selector(tap))
        self.view.addGestureRecognizer(tap)
    }
    
    @objc func tap() {
        let fromView = isFliped ? self.secondView : self.firstView
        let toView = isFliped ? self.firstView : self.secondView
        let direction: UIView.AnimationOptions = isFliped ? .transitionFlipFromLeft : .transitionFlipFromRight
        
        UIView.transition(from: fromView, to: toView, duration: 0.5, options: direction) { finished in
            if finished {
                fromView.superview?.sendSubviewToBack(fromView)
                self.isFliped = !self.isFliped
            }
        }
    }
}