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具有associatedtype的Swift协议(类型查找不明确)

associated-types swift-protocols protocols generics swift

EvGeniy Ilyin · 技术社区 · 7 年前

enum : RawRepresentable 哪里 RawValue == String

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

“RawValue”不是“T”的成员类型

有什么解决办法吗

1 回复 | 直到 7 年前

dfrib 7 年前

一般来说,当在协议的上下文中覆盖泛型时,泛型类型持有者被视为可由 associatedtype 协议的一部分。在您的示例中,这将是 SectionIdentifierEnum ,它充当受约束对象的占位符 SectionIdenfierEnum 但是,它本身不是协议,因此不能将其用作泛型方法中的类型约束。你可以但是,请将其用作 test(...)

Swift 3.1

现在,目前(Swift 3.1),您不能将复杂的类型约束添加到 关联类型 但是,您可以提供仅适用于以下情况的默认实现: Self 派生自 UIViewController 并实现 RequiresEnum 通过设置 截面标识 RequiresEnumDefault RawValue String 对于此默认实现,作为 原始价值 混凝土的 要求默认值 类型为 一串

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)

上面,默认实现仅在以下情况下可用 截面标识 等于混凝土类型 RequireEnumDefault 自己 派生自 UIViewController 截面标识 是 一串 已键入 ,则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 :s,根据Swift evolution提案的实施:

SE-0142: Permit where clauses to constrain associated types

// 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,修改对默认实现的约束 测试(…) 不仅如此 截面标识 等于 要求默认值 (但对于任何枚举:在这种情况下,我们知道此类枚举将始终具有一个 一串 ,由于 关联类型

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)