Question

我可以通过swift扩展程序将协议一致性添加到协议中吗？

//Plain old protocol here
protocol MyData {
    var myDataID: Int { get }
}

我想默认使MyData协议等同（只比较ID）

extension MyData : Equatable { }

但我得到了这个可爱的错误：

“协议扩展'MyData'不能有继承子句”

我正在寻找的行为是符合Equatable（协议）的BananaData，因为它实现了MyData协议，它可以提供Equatable的默认实现

//This is the method to implement Equatable
func ==(lhs: MyData, rhs: MyData) -> Bool {
    return lhs.myDataID == rhs.myDataID
}

struct BananaData: MyData {
    var myDataID: Int = 1
}

func checkEquatable(bananaOne: BananaData, bananaTwo: BananaData) {
    //This compiles, verifying that BananaData can be compared
    if bananaOne == bananaTwo { }
    //But BananaData is not convertible to Equatable, which is what I want
    let equatableBanana = bananaOne as Equatable
    //I don't get the additional operations added to Equatable (!=)
    if bananaOne != bananaTwo { } //Error
}

Answer 1

正如错误消息所示：协议的扩展名不能包含继承子句。相反，您可以在原始声明中从MyData继承Equatable协议。

protocol MyData: Equatable {
    var myDataID: Int { get }
}

然后，您可以为符合==的类型扩展添加MyData的实现：

func == <T: MyData>(lhs: T, rhs: T) -> Bool {
    return lhs.myDataID == rhs.myDataID
}

但是，我强烈建议不要这样做！如果为符合类型添加更多属性，则不会检查它们的属性是否相等。以下面的例子为例：

struct SomeData: MyData {
    var myDataID: Int
    var myOtherData: String
}

let b1 = SomeData(myDataID: 1, myOtherData: "String1")
let b2 = SomeData(myDataID: 1, myOtherData: "String2")

b1 == b2 // true, although `myOtherData` properties aren't equal.

在上述情况下，您需要覆盖== SomeData以获取正确的结果，从而使接受==的{{1}}变为冗余。

Answer 2

这个游乐场能做你想做的吗？我是根据我从WWDC 2015的Protocol-Oriented Programming in Swift了解到的。

import Foundation

//Plain old protocol here

func == (lhs: MyData, rhs: MyData) -> Bool {
    return lhs.myDataID == rhs.myDataID
}

func != (lhs: MyData, rhs: MyData) -> Bool {
    return lhs.myDataID != rhs.myDataID
}

protocol MyData {
    var myDataID: Int { get }
}

extension MyData where Self: Equatable {

}


struct BananaData: MyData {
    var myDataID: Int = 1
}

func checkEquatable(bananaOne: BananaData, bananaTwo: BananaData) {
    //This compiles, verifying that BananaData can be compared
    if bananaOne == bananaTwo {
        print("Same")
    }
    //But BananaData is not convertible to Equatable, which is what I want
    //I don't get the additional operations added to Equatable (!=)
    print(bananaOne.myDataID)
    print(bananaTwo.myDataID)


    if bananaOne != bananaTwo {

    }

    //Error


}

let b1 = BananaData(myDataID: 2)
let b2 = BananaData(myDataID: 2)

checkEquatable(b1, bananaTwo: b2)

let c = b1 == b2  // Evaluates as true

Swift 2添加协议符合协议

2 个答案: