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Swift 2添加协议与协议的一致性

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我可以通过快速扩展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 协议相等（只需比较ID）

extension MyData : Equatable { }

但我得到了这个可爱的错误：

“协议的扩展'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
}
swift protocols swift2

2 回答

  • 2

    正如错误消息所示：协议的扩展不能具有继承子句 . 相反，您可以在原始声明中使 MyData 协议从 Equatable 继承 .

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

    然后，您可以为符合 MyData 的类型添加 == 的实现：

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

    但是， I would highly not recommend this! 如果向符合类型添加更多属性，则不会检查其属性是否相等 . 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.

    在上面的情况中，您需要为 SomeData 覆盖 == 以获得正确的结果，从而使 == 接受 MyData 冗余 .

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  • 23

    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
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