Swift嵌套枚举赋值报错：无法将子枚举存入父枚举变量如何解决？

问题代码 #

class SetupVC: UIViewController {

    var isBasic:Bool = true
    var currentStep:SetupStep?
    var currentStepIndex:Int = 0

    enum SetupStep {
        case basic(BasicStep), advanced(AdvancedStep)

        enum BasicStep: CaseIterable, Equatable {
            case one, two, three
        }
    
        enum AdvancedStep: CaseIterable, Equatable {
            case one, two, three
        }
    }

    override func viewDidLoad() {
        super.viewDidLoad()
        if isBasic == true {
            currentStep = SetupStep.BasicStep.allCases[currentStepIndex]
        } else {
            currentStep = SetupStep.AdvancedStep.allCases[currentStepIndex]
        }
        
    }
}

报错信息 #

Cannot assign value of type 'SetupVC.SetupStep.BasicStep' to type 'SetupVC.SetupStep?'

问题根源 #

你混淆了带关联值的枚举成员和嵌套枚举本身。SetupStep是一个带关联值的枚举：

• case basic(BasicStep) 是SetupStep的一个枚举成员，它需要关联一个BasicStep类型的值作为附加数据
• 直接把BasicStep类型的实例赋值给SetupStep?类型的变量，属于类型不匹配，自然会报错

修正后的代码 #

class SetupVC: UIViewController {

    var isBasic:Bool = true
    var currentStep:SetupStep?
    var currentStepIndex:Int = 0

    enum SetupStep {
        case basic(BasicStep), advanced(AdvancedStep)

        enum BasicStep: CaseIterable, Equatable {
            case one, two, three
        }
    
        enum AdvancedStep: CaseIterable, Equatable {
            case one, two, three
        }
    }

    override func viewDidLoad() {
        super.viewDidLoad()
        if isBasic {
            // 先获取BasicStep实例，再包装成SetupStep.basic成员
            let targetBasicStep = SetupStep.BasicStep.allCases[currentStepIndex]
            currentStep = .basic(targetBasicStep)
        } else {
            // 先获取AdvancedStep实例，再包装成SetupStep.advanced成员
            let targetAdvancedStep = SetupStep.AdvancedStep.allCases[currentStepIndex]
            currentStep = .advanced(targetAdvancedStep)
        }
    }
}

补充说明 #

SetupStep的设计逻辑是：用外层枚举的不同成员（basic/advanced）来区分流程类型，每个成员携带对应嵌套枚举的具体步骤值。必须通过.basic(xxx)或.advanced(xxx)的语法，把嵌套枚举的实例包装成外层枚举的实例，才能完成类型匹配的赋值。

内容的提问来源于stack exchange，提问作者Alexander Ritter