Swift 2 添加协议一致性协议

Question

我可以通过 swift 扩展将协议一致性添加到协议吗？

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

我想使 MyData 协议默认相等（只需比较 ID）

extension MyData : Equatable { }

但是我得到了这个可爱的错误：

"Extension of protocol 'MyData' cannot have an inheritance clause"

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，该协议可以提供 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

正如错误信息所说：协议的扩展不能有继承子句。相反，您可以使 MyData 协议继承自原始声明中的 Equatable。

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

然后您可以为符合 MyData:

的类型扩展添加 == 的实现

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

但是，我强烈不推荐这样做！ 如果您向符合类型添加更多属性，将不会检查它们的属性是否相等。举个例子：

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，您需要为 SomeData 覆盖 == 以获得正确的结果，从而使接受 MyData 的 == 变得多余。

Answer 2

这个游乐场符合您的要求吗？我是根据我从 WWDC 2015 Protocol-Oriented Programming in Swift 了解到的内容制作的。

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 添加协议一致性协议

Swift 2 add protocol conformance to protocols

protocols

swift

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