Swift 带有关联类型的协议（类型查找不明确）

Question

我需要在协议中创建通用函数，在扩展中使用默认实现。它的功能应该与 item 一起工作，如 enum:RawRepresentable where RawValue == String always。我试过了

protocol RequiresEnum: class {
    associatedtype SectionIdentifierEnum: RawRepresentable // how this add restriction to RawValue == String

    func test<T: SectionIdentifierEnum>(identifier: T) where T.RawValue == String
}

enum RequiresEnumDefault: String {
    case `default`
}

extension RequiresEnum where Self: UIViewController, SectionIdentifierEnum.RawValue == String {

    typealias SectionIdentifierEnum = RequiresEnumDefault

    func test<T: SectionIdentifierEnum>(identifier: T) where T.RawValue == String {
        print(T.rawValue)
    }

}

但我有错误

'SectionIdentifierEnum' is ambiguous for type lookup in this context

'RawValue' is not a member type of 'T'

任何解决方案

Answer 1

通常，在协议上下文中涵盖泛型时，泛型类型持有者被视为可由协议的 associatedtype 表示。在您的示例中，这将是 SectionIdentifierEnum，它充当受约束的 type 的占位符。但是，SectionIdenfierEnum 本身不是协议，因此不能将其用作泛型方法中的类型约束。但是，您可以将其用作 test(...) 方法中的类型。

Swift3.1

现在，目前 (Swift 3.1)，您不能向 associatedtype 添加复杂的类型约束。但是，您可以提供仅适用于 Self 派生自 UIViewController 并通过将 SectionIdentifierEnum 类型设置为具体来实现 RequiresEnum 协议的情况的默认实现RequiresEnumDefault 类型。后者将确定关联的 RawValue 是此默认实现的 String，因为具体 RequiresEnumDefault 类型的 RawValue 是 String.

例如：

// Swift 3.1
// ---------
// Types that implement this protocol mustn't necessarily use a
// `SectionIdentifierEnum` type where `SectionIdentifierEnum.RawValue` 
// is constrained to equal `String`. 
protocol RequiresEnum: class {
    associatedtype SectionIdentifierEnum: RawRepresentable

    func test(identifier: SectionIdentifierEnum)
}

enum RequiresEnumDefault: String {
    case `default`
}

// This extension, however, is only available for types that use
// `RequiresEnumDefault ` as the concrete type of `SectionIdentifierEnum`
// (in which case `SectionIdentifierEnum.RawValue` is `String`).
extension RequiresEnum where Self: UIViewController, SectionIdentifierEnum == RequiresEnumDefault {

    func test(identifier: SectionIdentifierEnum) {
        print(identifier.rawValue)
    }
}

// Example usage.
class MyViewController : UIViewController, RequiresEnum {
    typealias SectionIdentifierEnum = RequiresEnumDefault
    // ...
}

let foo = MyViewController()
foo.test(identifier: RequiresEnumDefault.default) 
  // prints "default" (using extension:s default implementation)

以上，test(...) 的默认实现仅在 SectionIdentifierEnum 等于具体类型 RequireEnumDefault 时可用（并且 Self 派生自 UIViewController .. .).相反，如果您希望它仅在 SectionIdentifierEnum 是 任何 String 类型为 RawValue 的枚举时可用，您可以相应地修改扩展的类型约束:

protocol RequiresEnum: class {
    associatedtype SectionIdentifierEnum: RawRepresentable

    func test(identifier: SectionIdentifierEnum)
}

enum RequiresEnumDefault: String {
    case `default`
}

extension RequiresEnum where Self: UIViewController, SectionIdentifierEnum.RawValue == String {

    func test(identifier: SectionIdentifierEnum) {
        print(identifier.rawValue)
    }
}


// Example usage.
enum EnumWithStringRawValue: String {
    case foo
}

class MyViewController : UIViewController, RequiresEnum {
    typealias SectionIdentifierEnum = EnumWithStringRawValue
    // ...
}

let foo = MyViewController()
foo.test(identifier: EnumWithStringRawValue.foo)
    // prints "foo" (using extension:s default implementation)

一旦 Swift 4 发布，您将能够向 associatedtype:s 添加更复杂的约束，根据 Swift 进化提案的实施：

SE-0142: Permit where clauses to constrain associated types

这样的话上面可以修改成：

// Swift 4
// -------
// Here, all types that implement this protocol must use a
// `SectionIdentifierEnum` type where `SectionIdentifierEnum.RawValue` 
// is equal to `String`. 
protocol RequiresEnum: class {
    associatedtype SectionIdentifierEnum: RawRepresentable 
        where SectionIdentifierEnum.RawValue == String

    func test(identifier: SectionIdentifierEnum)
}

enum RequiresEnumDefault: String {
    case `default`
}

// For the specific case where `SectionIdentifierEnum` equals
// `RequiresEnumDefault` (and where `Self` derives from `UIViewController`), 
// this default implementation is readily available.
extension RequiresEnum where Self: UIViewController, SectionIdentifierEnum == RequiresEnumDefault {

    func test(identifier: SectionIdentifierEnum) {
        print(identifier.rawValue)
    }

}

// Example usage.
class MyViewController : UIViewController, RequiresEnum {
    typealias SectionIdentifierEnum = RequiresEnumDefault
    // ...
}

let foo = MyViewController()
foo.test(identifier: RequiresEnumDefault.default) 
  // prints "default" (using extension:s default implementation)

同样修改对 test(...) 默认实现的约束，不仅针对 SectionIdentifierEnum 等于 RequiresEnumDefault 的情况（而且针对任何枚举：在这种情况下我们知道这样的枚举将由于协议定义中对 associatedtype 的限制，总是有 RawValue String。

protocol RequiresEnum: class {
    associatedtype SectionIdentifierEnum: RawRepresentable 
        where SectionIdentifierEnum.RawValue == String

    func test(identifier: SectionIdentifierEnum)
}

enum RequiresEnumDefault: String {
    case `default`
}

// From the constraint on the `RawValue` of `SectionIdentifierEnum`
// above, we know that `RawValue` equals `String`.
extension RequiresEnum where Self: UIViewController {

    func test(identifier: SectionIdentifierEnum) {
        print(identifier.rawValue)
    }
}

// Example usage.
enum EnumWithStringRawValue: String {
    case foo
}
class MyViewController : UIViewController, RequiresEnum {
    typealias SectionIdentifierEnum = EnumWithStringRawValue
    // ...
}

let foo = MyViewController()
foo.test(identifier: EnumWithStringRawValue.foo) 
  // prints "foo" (using extension:s default implementation)

Swift 带有关联类型的协议（类型查找不明确）

Swift protocol with associatedtype (ambiguous for type lookup)

generics

protocols

associated-types

swift

swift-protocols

Swift3.1