点击按钮，使按钮进行左右翻转动画

// UIView.transition

// 1、可以设置从一个View到另一个View的转场动画
// UIView.transition(from: <#T##UIView#>, to: <#T##UIView#>, duration: <#T##TimeInterval#>, options: <#T##UIViewAnimationOptions#>, completion: <#T##((Bool) -> Void)?##((Bool) -> Void)?##(Bool) -> Void#>)

// 2、可以设置一个View的动画(比如翻转)
// UIView.transition(with: <#T##UIView#>, duration: <#T##TimeInterval#>, options: <#T##UIViewAnimationOptions#>, animations: <#T##(() -> Void)?##(() -> Void)?##() -> Void#>, completion: <#T##((Bool) -> Void)?##((Bool) -> Void)?##(Bool) -> Void#>)

import UIKit

class ViewController: UIViewController {
    
    private var isZhengMian:Bool = true
    
    override func viewDidLoad() {
        super.viewDidLoad()
    }
    
    @IBAction func btnClick(_ sender: UIButton) {
        isZhengMian = !isZhengMian
        if isZhengMian{ // 正面，带文字，从左向右翻转
            UIView.transition(with: sender, duration: 0.5, options: UIViewAnimationOptions.transitionFlipFromLeft, animations: {
                sender.setTitle("✓", for: .normal)
            }, completion: { (_) in
                
            })
        } else{ // 反面，不带文字，从右向左翻转
            UIView.transition(with: sender, duration: 0.5, options: UIViewAnimationOptions.transitionFlipFromRight, animations: {
                sender.setTitle(nil, for: .normal)
            }, completion: { (_) in
                
            })
        }
    }
}