ios - 开关允许两个相同的情况？

Question

测试 View :

class TestView: UIView{

    override init(frame: CGRect) {
        super.init(frame: frame)
        commonInit()
    }

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        commonInit()
    }

    func commonInit(){
        addSubview(stackView)
        setConstraints()
    }
    func setConstraints() {
        let constrains = [
            stackView.topAnchor.constraint(equalTo: topAnchor),
            stackView.leadingAnchor.constraint(equalTo: leadingAnchor),
            stackView.bottomAnchor.constraint(equalTo: bottomAnchor),
            stackView.trailingAnchor.constraint(equalTo: trailingAnchor)
        ]

        NSLayoutConstraint.activate(constrains)

    }

    public let accept: UIButton = {
        let v = UIButton()
        v.backgroundColor = .blue
        v.setTitle("Hello", for: .normal)
        v.setTitleColor(.white, for: .normal)
        v.setTitleColor(UIColor.white.withAlphaComponent(0.5), for: [.highlighted,.selected])
        return v
    }()


    public let deny: UIButton = {
        let v = UIButton()
        v.backgroundColor = .red
        v.setTitle("Deny", for: .normal)
        v.setTitleColor(.white, for: .normal)
        v.setTitleColor(UIColor.white.withAlphaComponent(0.5), for: [.highlighted,.selected])
        return v
    }()

    public lazy var stackView: UIStackView = {
        let v = UIStackView(arrangedSubviews: [accept,deny])
        v.alignment = .fill
        v.axis = .vertical
        v.spacing = 8
        v.distribution = .fillEqually
        v.translatesAutoresizingMaskIntoConstraints = false
        return v
    }()
}

测试 View Controller

class TestViewController: UIViewController{
    var myView: TestView{return view as! TestView}
    unowned var accept: UIButton {return myView.accept}
    unowned var deny: UIButton {return myView.deny}
    override func loadView() {
        view = TestView()
    }

    override func viewDidLoad() {
        super.viewDidLoad()
        accept.addTarget(self, action: #selector(didSelect), for: .touchUpInside)
        deny.addTarget(self, action: #selector(didSelect), for: .touchUpInside)
    }

    @objc func didSelect(_ sender: UIButton){
        switch sender{
        case accept:
            print("Accept button clicked")
        case deny:
            print("Deny button clicked")
        case accept:
            print("Accept button case again")
        default:
            break
        }
    }
}

上面的代码在 Xcode 10.2.1 中编译良好查看 didSelect 方法。它有一个带有两个相同外壳的开关 block 。只是好奇它不应该抛出编译时错误。该程序也成功运行，没有任何运行时错误。当我点击接受按钮时，第一个案例被执行。

问题:为什么这段代码没有编译时/运行时错误？

输出:

Answer 1

sender 的类型是 UIButton，编译器无法像对枚举类型那样进行检查。

由于 switch 的工作方式与 if/else if 语句完全相同，因此它只执行第一个。

相当于:

if sender == accept {
    print("Accept button clicked")
}
else if sender == deny {
    print("Deny button clicked")
}
else if sender == accept {
    print("Accept button case again")
}
else {
    break
}