iPhone 编程：将 Alpha 应用于父视图，但不应用于子视图

在iPhone编程中，我们经常需要为视图添加一些特效和样式来提升用户体验。其中一个常见的需求是将某种特效应用于父视图，但不希望该效果应用于其子视图。那么，该如何实现呢？

swift
let parentView = UIView(frame: CGRect(x: 0, y: 0, width: 200, height: 200))
let childView = UIButton(frame: CGRect(x: 50, y: 50, width: 100, height: 100))
parentView.backgroundColor = UIColor.red
childView.backgroundColor = UIColor.blue
parentView.addSubview(childView)

swift
parentView.alpha = 0.5

swift
childView.alpha = 1.0

swift
import UIKit
class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        let parentView = UIView(frame: CGRect(x: 0, y: 0, width: 200, height: 200))
        let childView = UIButton(frame: CGRect(x: 50, y: 50, width: 100, height: 100))
        parentView.backgroundColor = UIColor.red
        childView.backgroundColor = UIColor.blue
        parentView.addSubview(childView)
        parentView.alpha = 0.5
        childView.alpha = 1.0
        view.addSubview(parentView)
    }
}