Swift 关联类型约束

Question

我有两个协议，每个协议都定义了一个关联类型。其中一个协议需要定义另一个协议类型的变量，其中它们都具有相同类型的关联类型。是否有可能以某种方式推断关联类型的类型？

protocol A {
    associatedtype AModel
    var b: B { get }
}

protocol B {
    associatedtype BModel
    func doAnother(anotherModel: BModel)
}

这是我尝试但没有成功的方法

protocol A {
    associatedtype AModel
    associatedtype TypedB = B where B.BModel == AModel
    var another: TypedB { get }
}

protocol B {
    associatedtype BModel
    func doAnother(anotherModel: BModel)
}

Answer 1

请找到以下工作游乐场示例。您需要使用关联类型的名称，而不是约束协议的名称。原因描述为here。

import Foundation

protocol A {
    associatedtype AModel
    associatedtype TypedB: B where TypedB.BModel == AModel
    var another: TypedB { get }
}

protocol B {
    associatedtype BModel
    func doAnother(anotherModel: BModel)
}

// compiles

struct One: B {
    typealias BModel = String
    func doAnother(anotherModel: String) {}
}

struct Second: A {
    typealias AModel = String
    typealias TypedB = One
    var another: One
}

// does not compile

struct Third: B {
    typealias BModel = Int
    func doAnother(anotherModel: Int) {}
}

struct Fourth: A { // A' requires the types 'Fourth.AModel' (aka 'String') and 'Third.BModel' (aka 'Int') be equivalent
    typealias AModel = String
    typealias TypedB = Third
    var another: Third
}