无法在 Swift 的另一个协议中使用协议作为关联类型

Question

我有一个协议Address，它继承自另一个协议Validator，而Address 满足扩展中的Validator 要求。

还有另一个协议FromRepresentable，它的associatedType (ValueWrapper) 要求应该是Validator。

现在如果我尝试将Address 用作associatedType，那么它不会编译。它说，

推断类型“地址”（通过匹配要求“valueForDetail”）是 无效：不符合“验证器”。

这种用法违法吗？难道我们不能使用Address 代替Validator，因为所有Addresses 都是Validator。

下面是我正在尝试的一段代码。

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bug.

Answer 1

David has already alluded to 的问题在于，一旦将协议的 associatedtype 限制为特定（非 @objc）协议，您就必须使用具体类型来满足该要求。

这是因为protocols don't conform to themselves - 因此意味着您不能使用Address 来满足协议对符合Validator 的类型的关联类型要求，因为Address 不是符合Validator的类型。

正如我演示的in my answer here，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放弃 Validator 关联类型上的 Validator 协议约束，允许您在方法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 实例只期望 Address 的单个具体实现作为输入 - 您可以使用泛型以便使用给定的具体类型初始化您的 AddressFrom在您的方法中使用的地址。

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，如果您需要关联的类型约束和，每个AddressFrom 实例必须能够处理Address 的任何类型的输入——您将在为了将任意 Address 包装在具体类型中。

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

你有几个问题：

首先，您实际上并没有声明 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}
}

并且您没有为 ValueWrapper 声明关联类型：

typealias ValueWrapper = ShippingAddress

而且您似乎实际上希望 AddressFrom.valueForDetail 采取ShippingAddress：

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

总而言之，它看起来像：

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
        }
    }
}