Swift 2 向协议(protocol)添加协议(protocol)一致性

Question

我可以通过 swift 扩展将协议(protocol)一致性添加到协议(protocol)中吗？

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

我想使 MyData 协议(protocol)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(protocol))的 BananaData，因为它实现了 MyData 协议(protocol)，该协议(protocol)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

正如错误消息所说:协议(protocol)的扩展不能有继承子句。相反，您可以使 MyData 协议(protocol)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，则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 冗余。