Ios 无法在Swift中的另一协议中将协议用作associatedtype
我有一个协议,Ios 无法在Swift中的另一协议中将协议用作associatedtype,ios,swift,swift-protocols,associated-types,Ios,Swift,Swift Protocols,Associated Types,我有一个协议,Address,它继承了另一个协议,Validator,并且Address满足扩展中的Validator要求 还有另一个协议,FromRepresentable,它有一个关联类型(ValueWrapper)要求,应该是验证器 现在,如果我尝试将地址用作关联类型,则它不会编译。上面说, 推断类型“地址”(通过匹配要求“valueForDetail”)为 无效:不符合“验证程序” 这种用法违法吗?难道我们不能用地址代替验证器,因为所有地址都是验证器 下面是我正在尝试的一段代码 enum
Address
,它继承了另一个协议,Validator
,并且Address
满足扩展中的Validator
要求
还有另一个协议,FromRepresentable
,它有一个关联类型
(ValueWrapper
)要求,应该是验证器
现在,如果我尝试将地址
用作关联类型
,则它不会编译。上面说,
推断类型“地址”(通过匹配要求“valueForDetail”)为
无效:不符合“验证程序”
这种用法违法吗?难道我们不能用地址
代替验证器
,因为所有地址
都是验证器
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:提交了一份您有几个问题: 首先,您实际上并没有声明地址实现了验证器
//Address inherits Validator
protocol Address : Validator {
var addressLine1: String {get set}
var city: String {get set}
var country: String {get set}
}
并且不声明ValueWrapper的关联类型:
typealias ValueWrapper = ShippingAddress
而且您似乎实际上希望AddressFrom.valueForDetail获取一个发货地址
:
func valueForDetail(valueWrapper: ShippingAddress) -> String {
switch self {
case .Address1:
return valueWrapper.addressLine1
case .City:
return valueWrapper.city
case .Country:
return valueWrapper.country
}
}
总之,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
}
}
}
问题是,一旦将协议的关联类型
约束到特定(非@objc
)协议,就必须使用具体类型来满足该要求
这是因为–因此,这意味着您不能使用地址
来满足符合验证程序
的类型的协议关联类型要求,因为地址
不是符合验证程序
的类型
正如我所演示的,考虑反例:
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在ValueWrapper
关联类型上放弃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
}
如果您需要关联的类型约束,并且每个AddressFrom
实例只需要一个Address
的具体实现作为输入–您可以使用泛型,以便使用方法中使用的给定具体类型的地址初始化AddressFrom
protocol FormRepresentable {
associatedtype ValueWrapper : Validator
func valueForDetail(valueWrapper: ValueWrapper) -> String
}
protocol FormRepresentable {
associatedtype ValueWrapper : Validator
func valueForDetail(valueWrapper: ValueWrapper) -> String
}
嘿,对不起,当我还在努力让它工作时,我发布了这个问题。我编辑了代码片段。地址实际上是从验证器继承的。关于typealias,我认为它应该能够从valueForDetail(93;:Address)函数推断ValueWrapper。我不希望它是concretetype。请注意,在您的实现中,实际上不必显式定义协议的
associatedtype
(通过typealias
),Swift可以从方法参数类型推断出来。好的,我明白了,但是,我们为什么必须使用concreteType作为带有协议约束的associatedType呢?若我们使用Address作为需要验证器的函数的参数,编译器不会发出任何错误。我不明白,若associatedType具有协议约束,为什么我们必须使用具体类型。它是否与不变性行为或内存分配有关?你有什么参考资料可以让我提供更多的信息吗?@VishalSingh我恐怕没有比“因为事情就是这样”更好的理由了——我当然想不出一个不可能的理由,因为无约束的版本很好用。我在swift evolution邮件列表或bug追踪器中似乎也找不到与此相关的任何信息。这可能很值得一提,看看他们怎么说。谢谢你,我会这样做,如果我得到什么,我会更新这篇文章。在那之前,我会记住这一点:)很乐意帮忙,我当然会对回复感兴趣:)
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
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))