我有一个协议(protocol)Address
,它继承自另一个协议(protocol)Validator
,并且Address
满足Validator
> 扩展中的要求。
还有另一个协议(protocol),FromRepresentable
,它有一个 linkedType
(ValueWrapper
) 要求,应该是 Validator
。
现在,如果我尝试使用 Address
作为 linkedType
,则它无法编译。它说,
Inferred type 'Address' (by matching requirement 'valueForDetail') is invalid: does not conform to 'Validator'.
这种用法违法吗?我们是否应该能够使用 Address
代替 Validator
,因为所有 Addresses
都是 Validator
。
下面是我正在尝试的代码段。
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.
最佳答案
问题,哪个David has already alluded to ,一旦您将协议(protocol)的 linkedtype
限制为特定的(非 @objc
)协议(protocol),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
来满足符合 Validator
的协议(protocol)的关联类型要求,因为 Address
是 不是符合Validator
的类型。
正如我所演示的 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对 ValueWrapper
关联类型的 Validator
协议(protocol)约束,从而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
实例仅期望 Address
的单个具体实现作为输入 - 您可以使用泛型来为您的 AddressFrom
使用要在您的方法中使用的给定具体地址类型进行初始化。
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,则每个 AddressFrom
实例必须能够处理任何类型的 Address
输入– 您将使用类型删除来将任意地址
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))
关于ios - 无法在 Swift 中使用协议(protocol)作为另一个协议(protocol)中的关联类型,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/40829051/