Question

我有以下代码（请忽略拼写错误：））

SimpleDateFormat

现在当我尝试

protocol Vehical {
    var wheelsCount:Int {get}
}

protocol LightVehical:Vehical {
    func tankWithPetrol (liters:Int)
}

protocol HeavyVehical:Vehical {
    func tankWithDisel(liters:Int)
}

protocol OwnsVehical {
    associatedtype VechicalType = Vehical
    var vehical:VechicalType {get}
}

// Here I have == for constraint  
extension OwnsVehical where VechicalType == HeavyVehical {
    func fillTankWithDisel() {

    }
}
 // Light Vehicle
struct VolVOV90 : LightVehical {

    var wheelsCount: Int = 4

    func tankWithPetrol(liters: Int) {

    }
}
     // Heavy Vehicle

struct Van : HeavyVehical {
    func tankWithDisel(liters: Int) {

    }

    var wheelsCount: Int = 6


}

struct PersonWithHeavyVehical:OwnsVehical {
    typealias VechicalType = Van
    let vehical = Van()
}

如果我更改

let personWithHeavyV = PersonWithHeavyVehical()
personWithHeavyV.fillTankWithDisel() // It is not compiling with ==

使用

extension OwnsVehical where VechicalType == HeavyVehical

代码编译成功，我没有找到==和之间的区别。：任何人都可以帮助我理解它。

Answer 1

当您这样做：

extension OwnsVehical where VechicalType == HeavyVehical

您要告诉编译器VechicalType 必须是HeavyVehical类型。这意味着方法fillTankWithDisel仅对OwnsVehical是VechicalType的{{1}}可用。这就是为什么您无法在HeavyVehical上调用fillTankWithDisel的原因，因为personWithHeavyV不是personWithHeavyV，而是HeavyVehical。

当您这样做：

Van

您要告诉编译器extension OwnsVehical where VechicalType : HeavyVehical 符合符合VechicalType协议，因此您可以调用HeavyVehical，因为{{1} }，只要遵守personWithHeavyV.fillTankWithDisel，就没有其他限制，可以调用personWithHeavyV。

如果您希望OwnsVehical进行编译，则必须将结构PersonWithHeavyVehical的实现更改为以下内容：

fillTankWithDisel

现在您有一个personWithHeavyV.fillTankWithDisel()，其VechicalType是struct PersonWithHeavyVehical: OwnsVehical { typealias VechicalType = HeavyVehical let vehical = Van() }，因此可以调用所需的方法。

Answer 2

==运算符

==运算符检查其实例值是否相等，"equal to" //因此您在这里比较VechicalType == HeavyVehical并返回false
:运算符

:是指定义变量的类型为此，您可以检查此link

带有== vs约束的Swift协议：

2 个答案: