无法在Swift的另一个协议中将协议用作关联类型

Question

我有一个协议Address ，它继承自另一个协议Validator ，并且Address满足扩展名中Validator要求。

还有另一种协议， FromRepresentable ，其具有associatedType （ ValueWrapper这应该是）要求Validator 。

现在，如果我尝试使用Address作为associatedType ，那么它将无法编译。 它说，

推断的类型“地址”（通过匹配要求“ valueForDetail”）无效：不符合“ Validator”。

这种用法非法吗？ 我们不应该能够使用Address代替Validator ，因为所有Addresses都是Validator 。

下面是我正在尝试的代码。

enum ValidationResult {
    case Success
    case Failure(String)
}

protocol Validator {
    func validate() -> ValidationResult
}

//Address inherits Validator
protocol Address: Validator {
    var addressLine1: String {get set}
    var city: String {get set}
    var country: String {get set}
}

////Fulfill Validator protocol requirements in extension
extension Address {
    func validate() -> ValidationResult {
        if addressLine1.isEmpty {
            return .Failure("Address can not be empty")
        }
        return .Success
    }
}

protocol FormRepresentable {
    associatedtype ValueWrapper: Validator
    func valueForDetail(valueWrapper: ValueWrapper) -> String
}


// Shipping Address conforming to Address protocol. 
// It should also implicitly conform to Validator since
// Address inherits from Validator?
struct ShippingAddress: Address {
    var addressLine1 = "CA"
    var city = "HYD"
    var country = "India"
}


// While compiling, it says:
// Inferred type 'Address' (by matching requirement 'valueForDetail') is invalid: does not conform
// to 'Validator'.
// But Address confroms to Validator.
enum AddressFrom: Int, FormRepresentable {
    case Address1
    case City
    case Country

    func valueForDetail(valueWrapper: Address) -> String {
        switch self {
        case .Address1:
            return valueWrapper.addressLine1
        case .City:
            return valueWrapper.city
        case .Country:
            return valueWrapper.country
        }
    }
}

更新：提交了一个错误。

Answer 1

David已经提到的问题是，一旦将协议的associatedtype限制为特定的（非@objc ）协议，就必须使用具体的类型来满足该要求。

这是因为协议不符合自身 ，因此，意味着您不能使用Address来满足协议的与Validator一致的类型的关联类型要求，因为Address 不是与Validator的类型。

如我在此处的答案所示 ，请考虑以下反例：

protocol Validator {
    init()
}
protocol Address : Validator {}

protocol FormRepresentable {
    associatedtype ValueWrapper: Validator
}

extension FormRepresentable {
    static func foo() {
        // if ValueWrapper were allowed to be an Address or Validator,
        // what instance should we be constructing here?
        // we cannot create an instance of a protocol.
        print(ValueWrapper.init())
    }
}

// therefore, we cannot say:
enum AddressFrom : FormRepresentable {
    typealias ValueWrapper = Address
}

最简单的解决方案是ValueWrapper关联类型上的Validator协议约束，从而允许您在method参数中使用抽象类型。

protocol FormRepresentable {
    associatedtype ValueWrapper
    func valueForDetail(valueWrapper: ValueWrapper) -> String
}

enum AddressFrom : Int, FormRepresentable {

    // ...

    func valueForDetail(valueWrapper: Address) -> String {
        // ...
    }
}

如果您需要关联的类型约束，并且每个AddressFrom实例仅期望将Address的单个具体实现作为输入–您可以使用泛型，以便使用要在方法中使用的给定具体地址类型来初始化AddressFrom 。

protocol FormRepresentable {
    associatedtype ValueWrapper : Validator
    func valueForDetail(valueWrapper: ValueWrapper) -> String
}

enum AddressFrom<T : Address> : Int, FormRepresentable {

    // ...

    func valueForDetail(valueWrapper: T) -> String {
        // ...
    }
}

// replace ShippingAddress with whatever concrete type you want AddressFrom to use
let addressFrom = AddressFrom<ShippingAddress>.Address1

但是，如果同时需要关联的类型约束和每个AddressFrom实例必须能够处理任何类型的Address的输入，则必须使用类型擦除来将任意Address封装为具体类型。

protocol FormRepresentable {
    associatedtype ValueWrapper : Validator
    func valueForDetail(valueWrapper: ValueWrapper) -> String
}

struct AnyAddress : Address {

    private var _base: Address

    var addressLine1: String {
        get {return _base.addressLine1}
        set {_base.addressLine1 = newValue}
    }
    var country: String {
        get {return _base.country}
        set {_base.country = newValue}
    }
    var city: String {
        get {return _base.city}
        set {_base.city = newValue}
    }

    init(_ base: Address) {
        _base = base
    }
}

enum AddressFrom : Int, FormRepresentable {

    // ...

    func valueForDetail(valueWrapper: AnyAddress) -> String {
        // ...
    }
}

let addressFrom = AddressFrom.Address1

let address = ShippingAddress(addressLine1: "", city: "", country: "")

addressFrom.valueForDetail(AnyAddress(address))

Answer 2

您遇到了几个问题：

首先，您实际上并未声明Address实现了Validator

//Address inherits Validator
protocol Address : Validator {
    var addressLine1: String {get set}
    var city: String {get set}
    var country: String {get set}
}

而且您没有声明ValueWrapper的关联类型：

typealias ValueWrapper = ShippingAddress

而且您似乎实际上是希望AddressFrom.valueForDetail接受ShippingAddress ：

func valueForDetail(valueWrapper: ShippingAddress) -> String {
    switch self {
    case .Address1:
        return valueWrapper.addressLine1
    case .City:
        return valueWrapper.city
    case .Country:
        return valueWrapper.country
    }
}

总共看起来像：

enum ValidationResult {
    case Success
    case Failure(String)
}

protocol Validator {
    func validate() -> ValidationResult
}

//Address inherits Validator
protocol Address : Validator {
    var addressLine1: String {get set}
    var city: String {get set}
    var country: String {get set}
}

////Fulfill Validator protocol requirements in extension
extension Address {
    func validate() -> ValidationResult {
        if addressLine1.isEmpty {
            return .Failure("Address can not be empty")
        }
        return .Success
    }
}

protocol FormRepresentable {
    associatedtype ValueWrapper: Validator
    func valueForDetail(valueWrapper: ValueWrapper) -> String
}


// Shipping Address conforming to Address protocol.
// It should also implicity conform to Validator since
// Address inherits from Validator?
struct ShippingAddress: Address {
    var addressLine1 = "CA"
    var city = "HYD"
    var country = "India"
}


// While compiling, it says:
// Inferred type 'Address' (by matching requirement 'valueForDetail') is invalid: does not conform
// to 'Validator'.
// But Address confroms to Validator.
enum AddressFrom: Int, FormRepresentable {
    case Address1
    case City
    case Country

    // define associated type for FormRepresentable
    typealias ValueWrapper = ShippingAddress
    func valueForDetail(valueWrapper: ShippingAddress) -> String {
        switch self {
        case .Address1:
            return valueWrapper.addressLine1
        case .City:
            return valueWrapper.city
        case .Country:
            return valueWrapper.country
        }
    }
}